diff --git a/AGENTS.md b/AGENTS.md
index 971609f5..465b4c8a 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -223,8 +223,9 @@ During normal plan review, an Archive sidebar tab provides the same browsing via
 
 | Endpoint              | Method | Purpose                                    |
 | --------------------- | ------ | ------------------------------------------ |
-| `/api/diff`           | GET    | Returns `{ rawPatch, gitRef, origin, diffType, gitContext }` |
-| `/api/file-content`   | GET    | Returns `{ oldContent, newContent }` for expandable diff context |
+| `/api/diff`           | GET    | Returns `{ rawPatch, gitRef, origin, diffType, base, gitContext }` |
+| `/api/diff/switch`    | POST   | Switch diff type and/or base branch (body: `{ diffType, base? }`) |
+| `/api/file-content`   | GET    | Returns `{ oldContent, newContent }` for expandable diff context (`?path=&oldPath=&base=`) |
 | `/api/git-add`        | POST   | Stage/unstage a file (body: `{ filePath, undo? }`) |
 | `/api/feedback`       | POST   | Submit review (body: feedback, annotations, agentSwitch) |
 | `/api/image`          | GET    | Serve image by path query param            |
diff --git a/apps/pi-extension/plannotator-browser.ts b/apps/pi-extension/plannotator-browser.ts
index 4032959f..039609d6 100644
--- a/apps/pi-extension/plannotator-browser.ts
+++ b/apps/pi-extension/plannotator-browser.ts
@@ -181,6 +181,7 @@ export async function openCodeReview(
 	let prMetadata: Awaited<ReturnType<typeof fetchPR>>["metadata"] | undefined;
 	let diffType: DiffType | undefined;
 	let agentCwd: string | undefined;
+	let initialBase: string | undefined;
 	let worktreeCleanup: (() => void | Promise<void>) | undefined;
 	let exitHandler: (() => void) | undefined;
 
@@ -342,6 +343,10 @@ export async function openCodeReview(
 		rawPatch = result.patch;
 		gitRef = result.label;
 		diffError = result.error;
+		// Remember which base the initial diff was computed against so it can
+		// be forwarded to the server below. Only matters when the caller
+		// overrode the detected default; otherwise it matches gitCtx already.
+		initialBase = defaultBranch;
 	}
 
 	const server = await startReviewServer({
@@ -351,6 +356,7 @@ export async function openCodeReview(
 		origin: "pi",
 		diffType,
 		gitContext: gitCtx,
+		initialBase,
 		prMetadata,
 		agentCwd,
 		htmlContent: reviewHtmlContent,
diff --git a/apps/pi-extension/server.test.ts b/apps/pi-extension/server.test.ts
index 007ac539..34eb7949 100644
--- a/apps/pi-extension/server.test.ts
+++ b/apps/pi-extension/server.test.ts
@@ -305,4 +305,139 @@ describe("pi review server", () => {
       server.stop();
     }
   }, 15_000);
+
+  test("round-trips the active base branch through /api/diff and /api/diff/switch", async () => {
+    const homeDir = makeTempDir("plannotator-pi-home-");
+    const repoDir = initRepo();
+    process.env.HOME = homeDir;
+    process.chdir(repoDir);
+    process.env.PLANNOTATOR_PORT = String(await reservePort());
+
+    // Create a second branch the picker can switch to, then branch off it so
+    // currentBranch !== defaultBranch and the branch/merge-base options appear.
+    git(repoDir, ["checkout", "-b", "develop"]);
+    writeFileSync(join(repoDir, "develop-file.txt"), "develop\n", "utf-8");
+    git(repoDir, ["add", "develop-file.txt"]);
+    git(repoDir, ["commit", "-m", "develop commit"]);
+    git(repoDir, ["checkout", "-b", "feature/x"]);
+    writeFileSync(join(repoDir, "feature-file.txt"), "feature\n", "utf-8");
+    git(repoDir, ["add", "feature-file.txt"]);
+    git(repoDir, ["commit", "-m", "feature commit"]);
+
+    const gitContext = await getGitContext();
+    const diff = await runGitDiff("uncommitted", gitContext.defaultBranch);
+
+    const server = await startReviewServer({
+      rawPatch: diff.patch,
+      gitRef: diff.label,
+      error: diff.error,
+      diffType: "uncommitted",
+      gitContext,
+      origin: "pi",
+      htmlContent: "<!doctype html><html><body>review</body></html>",
+    });
+
+    try {
+      // Initial load: server echoes the detected default as the active base.
+      const initial = await fetch(`${server.url}/api/diff`).then((r) => r.json()) as {
+        base?: string;
+        gitContext?: { defaultBranch: string };
+      };
+      expect(initial.base).toBe(gitContext.defaultBranch);
+      expect(initial.base).toBe(initial.gitContext?.defaultBranch);
+
+      // Switch to a custom base — response must echo the resolved base.
+      const switchResponse = await fetch(`${server.url}/api/diff/switch`, {
+        method: "POST",
+        headers: { "Content-Type": "application/json" },
+        body: JSON.stringify({ diffType: "branch", base: "develop" }),
+      });
+      expect(switchResponse.status).toBe(200);
+      const switched = await switchResponse.json() as { base?: string; diffType: string };
+      expect(switched.base).toBe("develop");
+      expect(switched.diffType).toBe("branch");
+
+      // Subsequent /api/diff load reflects the switched base — this is what
+      // survives a page refresh / reconnect.
+      const rehydrate = await fetch(`${server.url}/api/diff`).then((r) => r.json()) as {
+        base?: string;
+      };
+      expect(rehydrate.base).toBe("develop");
+
+      // Unknown refs pass through verbatim — the resolver trusts callers so
+      // unusual-but-valid refs (tags, SHAs, non-origin remotes) work. Truly
+      // invalid refs surface via the diff error, not via a silent swap.
+      const unknownResponse = await fetch(`${server.url}/api/diff/switch`, {
+        method: "POST",
+        headers: { "Content-Type": "application/json" },
+        body: JSON.stringify({ diffType: "branch", base: "nope-does-not-exist" }),
+      });
+      const unknown = await unknownResponse.json() as { base?: string; error?: string };
+      expect(unknown.base).toBe("nope-does-not-exist");
+      expect(unknown.error).toBeTruthy();
+
+      // Feedback to clean up the waitForDecision promise.
+      await fetch(`${server.url}/api/feedback`, {
+        method: "POST",
+        headers: { "Content-Type": "application/json" },
+        body: JSON.stringify({ approved: false, feedback: "done", annotations: [] }),
+      });
+      await server.waitForDecision();
+    } finally {
+      server.stop();
+    }
+  }, 15_000);
+
+  test("initialBase overrides gitContext.defaultBranch in server state", async () => {
+    // Simulates a programmatic caller (Pi event bus, other extensions) that
+    // opens a review against a non-default base. The server's currentBase —
+    // which drives /api/diff, agent prompts, and file-content fetches — must
+    // honor that override instead of falling back to the detected default.
+    const homeDir = makeTempDir("plannotator-pi-home-");
+    const repoDir = initRepo();
+    process.env.HOME = homeDir;
+    process.chdir(repoDir);
+    process.env.PLANNOTATOR_PORT = String(await reservePort());
+
+    git(repoDir, ["checkout", "-b", "develop"]);
+    writeFileSync(join(repoDir, "develop-file.txt"), "develop\n", "utf-8");
+    git(repoDir, ["add", "develop-file.txt"]);
+    git(repoDir, ["commit", "-m", "develop commit"]);
+    git(repoDir, ["checkout", "-b", "feature/x"]);
+
+    const gitContext = await getGitContext();
+    // Detected default is "main"; caller explicitly wants "develop".
+    expect(gitContext.defaultBranch).toBe("main");
+    const diff = await runGitDiff("branch", "develop");
+
+    const server = await startReviewServer({
+      rawPatch: diff.patch,
+      gitRef: diff.label,
+      error: diff.error,
+      diffType: "branch",
+      gitContext,
+      initialBase: "develop",
+      origin: "pi",
+      htmlContent: "<!doctype html><html><body>review</body></html>",
+    });
+
+    try {
+      const payload = await fetch(`${server.url}/api/diff`).then((r) => r.json()) as {
+        base?: string;
+        gitContext?: { defaultBranch: string };
+      };
+      // The server must echo the caller's override, not the detected default.
+      expect(payload.base).toBe("develop");
+      expect(payload.gitContext?.defaultBranch).toBe("main");
+
+      await fetch(`${server.url}/api/feedback`, {
+        method: "POST",
+        headers: { "Content-Type": "application/json" },
+        body: JSON.stringify({ approved: false, feedback: "done", annotations: [] }),
+      });
+      await server.waitForDecision();
+    } finally {
+      server.stop();
+    }
+  }, 15_000);
 });
diff --git a/apps/pi-extension/server/agent-jobs.ts b/apps/pi-extension/server/agent-jobs.ts
index 4b327cb3..c4ec278b 100644
--- a/apps/pi-extension/server/agent-jobs.ts
+++ b/apps/pi-extension/server/agent-jobs.ts
@@ -73,6 +73,8 @@ export interface AgentJobHandlerOptions {
 		reasoningEffort?: string;
 		/** Whether Codex fast mode was enabled. */
 		fastMode?: boolean;
+		/** Diff context snapshot at launch (stored on AgentJobInfo for per-job "Copy All"). */
+		diffContext?: AgentJobInfo["diffContext"];
 	} | null>;
 	/** Called when a job completes successfully — parse results and push annotations. */
 	onJobComplete?: (job: AgentJobInfo, meta: { outputPath?: string; stdout?: string; cwd?: string }) => void | Promise<void>;
@@ -118,7 +120,7 @@ export function createAgentJobHandler(options: AgentJobHandlerOptions) {
 		command: string[],
 		label: string,
 		outputPath?: string,
-		spawnOptions?: { captureStdout?: boolean; stdinPrompt?: string; cwd?: string; prompt?: string; engine?: string; model?: string; effort?: string; reasoningEffort?: string; fastMode?: boolean },
+		spawnOptions?: { captureStdout?: boolean; stdinPrompt?: string; cwd?: string; prompt?: string; engine?: string; model?: string; effort?: string; reasoningEffort?: string; fastMode?: boolean; diffContext?: AgentJobInfo["diffContext"] },
 	): AgentJobInfo {
 		const id = crypto.randomUUID();
 		const source = jobSource(id);
@@ -137,6 +139,7 @@ export function createAgentJobHandler(options: AgentJobHandlerOptions) {
 			...(spawnOptions?.effort && { effort: spawnOptions.effort }),
 			...(spawnOptions?.reasoningEffort && { reasoningEffort: spawnOptions.reasoningEffort }),
 			...(spawnOptions?.fastMode && { fastMode: spawnOptions.fastMode }),
+			...(spawnOptions?.diffContext && { diffContext: spawnOptions.diffContext }),
 		};
 
 		let proc: ChildProcess | null = null;
@@ -419,6 +422,7 @@ export function createAgentJobHandler(options: AgentJobHandlerOptions) {
 					let jobEffort: string | undefined;
 					let jobReasoningEffort: string | undefined;
 					let jobFastMode: boolean | undefined;
+					let jobDiffContext: AgentJobInfo["diffContext"] | undefined;
 					if (options.buildCommand) {
 						// Thread config from POST body to buildCommand
 						const config: Record<string, unknown> = {};
@@ -441,6 +445,7 @@ export function createAgentJobHandler(options: AgentJobHandlerOptions) {
 							jobEffort = built.effort;
 							jobReasoningEffort = built.reasoningEffort;
 							jobFastMode = built.fastMode;
+							jobDiffContext = built.diffContext;
 						}
 					}
 
@@ -459,6 +464,7 @@ export function createAgentJobHandler(options: AgentJobHandlerOptions) {
 						effort: jobEffort,
 						reasoningEffort: jobReasoningEffort,
 						fastMode: jobFastMode,
+						diffContext: jobDiffContext,
 					});
 					json(res, { job }, 201);
 				} catch {
diff --git a/apps/pi-extension/server/serverReview.ts b/apps/pi-extension/server/serverReview.ts
index f5e5fc33..6ca1e35b 100644
--- a/apps/pi-extension/server/serverReview.ts
+++ b/apps/pi-extension/server/serverReview.ts
@@ -31,6 +31,7 @@ import {
 	gitAddFile as gitAddFileCore,
 	gitResetFile as gitResetFileCore,
 	parseWorktreeDiffType,
+	resolveBaseBranch,
 	type ReviewGitRuntime,
 	runGitDiff as runGitDiffCore,
 	validateFilePath,
@@ -38,6 +39,7 @@ import {
 
 import { createEditorAnnotationHandler } from "./annotations.js";
 import { createAgentJobHandler } from "./agent-jobs.js";
+import type { AgentJobInfo } from "../generated/agent-jobs.js";
 import { createExternalAnnotationHandler } from "./external-annotations.js";
 import {
 	handleDraftRequest,
@@ -145,6 +147,14 @@ export async function startReviewServer(options: {
 	origin?: string;
 	diffType?: DiffType;
 	gitContext?: GitContext;
+	/**
+	 * Initial base branch the caller used to compute `rawPatch`. When a caller
+	 * overrides the detected default (e.g. `openCodeReview({ defaultBranch })`),
+	 * this must be forwarded so the server's internal `currentBase` state, the
+	 * `/api/diff` response, and downstream agent prompts stay consistent with
+	 * the patch that's already on screen.
+	 */
+	initialBase?: string;
 	error?: string;
 	sharingEnabled?: boolean;
 	shareBaseUrl?: string;
@@ -192,6 +202,11 @@ export async function startReviewServer(options: {
 	let currentGitRef = options.gitRef;
 	let currentDiffType: DiffType = options.diffType || "uncommitted";
 	let currentError = options.error;
+	// Tracks the base branch the user picked from the UI. Agent review prompts
+	// read this (not gitContext.defaultBranch) so they analyze the same diff
+	// the reviewer is currently looking at. Honors an explicit initialBase from
+	// the caller — e.g. programmatic Pi callers can request a non-detected base.
+	let currentBase = options.initialBase || options.gitContext?.defaultBranch || "main";
 
 	// Agent jobs — background process manager (late-binds serverUrl via getter)
 	let serverUrl = "";
@@ -213,10 +228,24 @@ export async function startReviewServer(options: {
 		async buildCommand(provider, config) {
 			const cwd = resolveAgentCwd();
 			const hasAgentLocalAccess = !!options.agentCwd || !!options.gitContext;
-			const userMessageOptions = { defaultBranch: options.gitContext?.defaultBranch, hasLocalAccess: hasAgentLocalAccess };
+			const userMessageOptions = { defaultBranch: currentBase, hasLocalAccess: hasAgentLocalAccess };
+
+			// Snapshot the diff context at launch (see review.ts buildCommand
+			// for the rationale — keeps downstream "Copy All" honest across
+			// subsequent context switches).
+			const worktreeParts = currentDiffType.startsWith("worktree:")
+				? parseWorktreeDiffType(currentDiffType)
+				: null;
+			const diffContext: AgentJobInfo["diffContext"] | undefined = options.prMetadata
+				? undefined
+				: {
+						mode: (worktreeParts?.subType ?? currentDiffType) as string,
+						base: currentBase,
+						worktreePath: worktreeParts?.path ?? null,
+					};
 
 			if (provider === "tour") {
-				return tour.buildCommand({
+				const built = await tour.buildCommand({
 					cwd,
 					patch: currentPatch,
 					diffType: currentDiffType,
@@ -224,6 +253,7 @@ export async function startReviewServer(options: {
 					prMetadata: options.prMetadata,
 					config,
 				});
+				return built ? { ...built, diffContext } : built;
 			}
 
 			const userMessage = buildCodexReviewUserMessage(currentPatch, currentDiffType, userMessageOptions, options.prMetadata);
@@ -235,7 +265,7 @@ export async function startReviewServer(options: {
 				const outputPath = generateOutputPath();
 				const prompt = CODEX_REVIEW_SYSTEM_PROMPT + "\n\n---\n\n" + userMessage;
 				const command = await buildCodexCommand({ cwd, outputPath, prompt, model, reasoningEffort, fastMode });
-				return { command, outputPath, prompt, label: "Code Review", model, reasoningEffort, fastMode: fastMode || undefined };
+				return { command, outputPath, prompt, label: "Code Review", model, reasoningEffort, fastMode: fastMode || undefined, diffContext };
 			}
 
 			if (provider === "claude") {
@@ -243,7 +273,7 @@ export async function startReviewServer(options: {
 				const effort = typeof config?.effort === "string" && config.effort ? config.effort : undefined;
 				const prompt = CLAUDE_REVIEW_PROMPT + "\n\n---\n\n" + userMessage;
 				const { command, stdinPrompt } = buildClaudeCommand(prompt, model, effort);
-				return { command, stdinPrompt, prompt, cwd, label: "Code Review", captureStdout: true, model, effort };
+				return { command, stdinPrompt, prompt, cwd, label: "Code Review", captureStdout: true, model, effort, diffContext };
 			}
 
 			return null;
@@ -479,6 +509,9 @@ export async function startReviewServer(options: {
 				gitRef: currentGitRef,
 				origin: options.origin ?? "pi",
 				diffType: hasLocalAccess ? currentDiffType : undefined,
+				// Echo the active base so page refresh/reconnect rehydrates the
+				// picker to what the server is actually using, not the detected default.
+				base: hasLocalAccess ? currentBase : undefined,
 				gitContext: hasLocalAccess ? options.gitContext : undefined,
 				sharingEnabled,
 				shareBaseUrl,
@@ -503,17 +536,43 @@ export async function startReviewServer(options: {
 					json(res, { error: "Missing diffType" }, 400);
 					return;
 				}
-				const defaultBranch = options.gitContext?.defaultBranch || "main";
+				const detectedBase = options.gitContext?.defaultBranch || "main";
+				const base = resolveBaseBranch(
+					typeof body.base === "string" ? body.base : undefined,
+					detectedBase,
+				);
 				const defaultCwd = options.gitContext?.cwd;
-				const result = await runGitDiff(newType, defaultBranch, defaultCwd);
+				const result = await runGitDiff(newType, base, defaultCwd);
 				currentPatch = result.patch;
 				currentGitRef = result.label;
 				currentDiffType = newType;
+				currentBase = base;
 				currentError = result.error;
+
+				// Recompute gitContext for the effective cwd so the client's
+				// sidebar reflects the worktree we're now reviewing.
+				// Best-effort: on failure the client keeps its existing context.
+				let updatedContext: GitContext | undefined;
+				if (options.gitContext) {
+					try {
+						const worktreeParsed = parseWorktreeDiffType(newType);
+						const effectiveCwd = worktreeParsed?.path ?? options.gitContext.cwd;
+						updatedContext = await getGitContextCore(reviewRuntime, effectiveCwd);
+					} catch {
+						/* best-effort */
+					}
+				}
+
 				json(res, {
 					rawPatch: currentPatch,
 					gitRef: currentGitRef,
 					diffType: currentDiffType,
+					// Echo the base the server actually used. resolveBaseBranch
+					// trusts the caller verbatim; this echo lets the client
+					// confirm the request landed (and pick it up when the client
+					// didn't supply one and we fell back to detected default).
+					base: currentBase,
+					...(updatedContext ? { gitContext: updatedContext } : {}),
 					...(currentError ? { error: currentError } : {}),
 				});
 			} catch (err) {
@@ -613,12 +672,16 @@ export async function startReviewServer(options: {
 
 			// Local mode first (matches Bun server priority)
 			if (hasLocalAccess && !isPRMode) {
-				const defaultBranch = options.gitContext?.defaultBranch || "main";
+				const detectedBase = options.gitContext?.defaultBranch || "main";
+				const base = resolveBaseBranch(
+					url.searchParams.get("base") ?? undefined,
+					detectedBase,
+				);
 				const defaultCwd = options.gitContext?.cwd;
 				const result = await getFileContentsForDiffCore(
 					reviewRuntime,
 					currentDiffType,
-					defaultBranch,
+					base,
 					filePath,
 					oldPath,
 					defaultCwd,
diff --git a/bun.lock b/bun.lock
index 2ff0a627..0a5e4727 100644
--- a/bun.lock
+++ b/bun.lock
@@ -64,7 +64,7 @@
     },
     "apps/opencode-plugin": {
       "name": "@plannotator/opencode",
-      "version": "0.18.0",
+      "version": "0.19.0",
       "dependencies": {
         "@opencode-ai/plugin": "^1.1.10",
       },
@@ -86,7 +86,7 @@
     },
     "apps/pi-extension": {
       "name": "@plannotator/pi-extension",
-      "version": "0.18.0",
+      "version": "0.19.0",
       "dependencies": {
         "@joplin/turndown-plugin-gfm": "^1.0.64",
         "turndown": "^7.2.4",
@@ -176,6 +176,9 @@
         "@plannotator/ui": "workspace:*",
         "@radix-ui/react-checkbox": "^1.3.3",
         "@radix-ui/react-collapsible": "^1.1.12",
+        "@radix-ui/react-dialog": "^1.1.15",
+        "@radix-ui/react-dropdown-menu": "^2.1.15",
+        "@radix-ui/react-popover": "^1.1.15",
         "@radix-ui/react-tooltip": "^1.2.8",
         "highlight.js": "^11.11.1",
         "motion": "^12.38.0",
@@ -186,7 +189,7 @@
     },
     "packages/server": {
       "name": "@plannotator/server",
-      "version": "0.18.0",
+      "version": "0.19.0",
       "dependencies": {
         "@plannotator/ai": "workspace:*",
         "@plannotator/shared": "workspace:*",
@@ -210,6 +213,7 @@
         "@plannotator/shared": "workspace:*",
         "@plannotator/web-highlighter": "^0.8.1",
         "@radix-ui/react-dialog": "^1.1.15",
+        "@radix-ui/react-popover": "^1.1.15",
         "@radix-ui/react-tooltip": "^1.2.8",
         "@tanstack/react-table": "^8.21.3",
         "@viz-js/viz": "^3.25.0",
@@ -720,20 +724,30 @@
 
     "@radix-ui/react-collapsible": ["@radix-ui/react-collapsible@1.1.12", "", { "dependencies": { "@radix-ui/primitive": "1.1.3", "@radix-ui/react-compose-refs": "1.1.2", "@radix-ui/react-context": "1.1.2", "@radix-ui/react-id": "1.1.1", "@radix-ui/react-presence": "1.1.5", "@radix-ui/react-primitive": "2.1.3", "@radix-ui/react-use-controllable-state": "1.2.2", "@radix-ui/react-use-layout-effect": "1.1.1" }, "peerDependencies": { "@types/react": "*", "@types/react-dom": "*", "react": "^16.8 || ^17.0 || ^18.0 || ^19.0 || ^19.0.0-rc", "react-dom": "^16.8 || ^17.0 || ^18.0 || ^19.0 || ^19.0.0-rc" }, "optionalPeers": ["@types/react", "@types/react-dom"] }, "sha512-Uu+mSh4agx2ib1uIGPP4/CKNULyajb3p92LsVXmH2EHVMTfZWpll88XJ0j4W0z3f8NK1eYl1+Mf/szHPmcHzyA=="],
 
+    "@radix-ui/react-collection": ["@radix-ui/react-collection@1.1.7", "", { "dependencies": { "@radix-ui/react-compose-refs": "1.1.2", "@radix-ui/react-context": "1.1.2", "@radix-ui/react-primitive": "2.1.3", "@radix-ui/react-slot": "1.2.3" }, "peerDependencies": { "@types/react": "*", "@types/react-dom": "*", "react": "^16.8 || ^17.0 || ^18.0 || ^19.0 || ^19.0.0-rc", "react-dom": "^16.8 || ^17.0 || ^18.0 || ^19.0 || ^19.0.0-rc" }, "optionalPeers": ["@types/react", "@types/react-dom"] }, "sha512-Fh9rGN0MoI4ZFUNyfFVNU4y9LUz93u9/0K+yLgA2bwRojxM8JU1DyvvMBabnZPBgMWREAJvU2jjVzq+LrFUglw=="],
+
     "@radix-ui/react-compose-refs": ["@radix-ui/react-compose-refs@1.1.2", "", { "peerDependencies": { "@types/react": "*", "react": "^16.8 || ^17.0 || ^18.0 || ^19.0 || ^19.0.0-rc" }, "optionalPeers": ["@types/react"] }, "sha512-z4eqJvfiNnFMHIIvXP3CY57y2WJs5g2v3X0zm9mEJkrkNv4rDxu+sg9Jh8EkXyeqBkB7SOcboo9dMVqhyrACIg=="],
 
     "@radix-ui/react-context": ["@radix-ui/react-context@1.1.2", "", { "peerDependencies": { "@types/react": "*", "react": "^16.8 || ^17.0 || ^18.0 || ^19.0 || ^19.0.0-rc" }, "optionalPeers": ["@types/react"] }, "sha512-jCi/QKUM2r1Ju5a3J64TH2A5SpKAgh0LpknyqdQ4m6DCV0xJ2HG1xARRwNGPQfi1SLdLWZ1OJz6F4OMBBNiGJA=="],
 
     "@radix-ui/react-dialog": ["@radix-ui/react-dialog@1.1.15", "", { "dependencies": { "@radix-ui/primitive": "1.1.3", "@radix-ui/react-compose-refs": "1.1.2", "@radix-ui/react-context": "1.1.2", "@radix-ui/react-dismissable-layer": "1.1.11", "@radix-ui/react-focus-guards": "1.1.3", "@radix-ui/react-focus-scope": "1.1.7", "@radix-ui/react-id": "1.1.1", "@radix-ui/react-portal": "1.1.9", "@radix-ui/react-presence": "1.1.5", "@radix-ui/react-primitive": "2.1.3", "@radix-ui/react-slot": "1.2.3", "@radix-ui/react-use-controllable-state": "1.2.2", "aria-hidden": "^1.2.4", "react-remove-scroll": "^2.6.3" }, "peerDependencies": { "@types/react": "*", "@types/react-dom": "*", "react": "^16.8 || ^17.0 || ^18.0 || ^19.0 || ^19.0.0-rc", "react-dom": "^16.8 || ^17.0 || ^18.0 || ^19.0 || ^19.0.0-rc" }, "optionalPeers": ["@types/react", "@types/react-dom"] }, "sha512-TCglVRtzlffRNxRMEyR36DGBLJpeusFcgMVD9PZEzAKnUs1lKCgX5u9BmC2Yg+LL9MgZDugFFs1Vl+Jp4t/PGw=="],
 
+    "@radix-ui/react-direction": ["@radix-ui/react-direction@1.1.1", "", { "peerDependencies": { "@types/react": "*", "react": "^16.8 || ^17.0 || ^18.0 || ^19.0 || ^19.0.0-rc" }, "optionalPeers": ["@types/react"] }, "sha512-1UEWRX6jnOA2y4H5WczZ44gOOjTEmlqv1uNW4GAJEO5+bauCBhv8snY65Iw5/VOS/ghKN9gr2KjnLKxrsvoMVw=="],
+
     "@radix-ui/react-dismissable-layer": ["@radix-ui/react-dismissable-layer@1.1.11", "", { "dependencies": { "@radix-ui/primitive": "1.1.3", "@radix-ui/react-compose-refs": "1.1.2", "@radix-ui/react-primitive": "2.1.3", "@radix-ui/react-use-callback-ref": "1.1.1", "@radix-ui/react-use-escape-keydown": "1.1.1" }, "peerDependencies": { "@types/react": "*", "@types/react-dom": "*", "react": "^16.8 || ^17.0 || ^18.0 || ^19.0 || ^19.0.0-rc", "react-dom": "^16.8 || ^17.0 || ^18.0 || ^19.0 || ^19.0.0-rc" }, "optionalPeers": ["@types/react", "@types/react-dom"] }, "sha512-Nqcp+t5cTB8BinFkZgXiMJniQH0PsUt2k51FUhbdfeKvc4ACcG2uQniY/8+h1Yv6Kza4Q7lD7PQV0z0oicE0Mg=="],
 
+    "@radix-ui/react-dropdown-menu": ["@radix-ui/react-dropdown-menu@2.1.16", "", { "dependencies": { "@radix-ui/primitive": "1.1.3", "@radix-ui/react-compose-refs": "1.1.2", "@radix-ui/react-context": "1.1.2", "@radix-ui/react-id": "1.1.1", "@radix-ui/react-menu": "2.1.16", "@radix-ui/react-primitive": "2.1.3", "@radix-ui/react-use-controllable-state": "1.2.2" }, "peerDependencies": { "@types/react": "*", "@types/react-dom": "*", "react": "^16.8 || ^17.0 || ^18.0 || ^19.0 || ^19.0.0-rc", "react-dom": "^16.8 || ^17.0 || ^18.0 || ^19.0 || ^19.0.0-rc" }, "optionalPeers": ["@types/react", "@types/react-dom"] }, "sha512-1PLGQEynI/3OX/ftV54COn+3Sud/Mn8vALg2rWnBLnRaGtJDduNW/22XjlGgPdpcIbiQxjKtb7BkcjP00nqfJw=="],
+
     "@radix-ui/react-focus-guards": ["@radix-ui/react-focus-guards@1.1.3", "", { "peerDependencies": { "@types/react": "*", "react": "^16.8 || ^17.0 || ^18.0 || ^19.0 || ^19.0.0-rc" }, "optionalPeers": ["@types/react"] }, "sha512-0rFg/Rj2Q62NCm62jZw0QX7a3sz6QCQU0LpZdNrJX8byRGaGVTqbrW9jAoIAHyMQqsNpeZ81YgSizOt5WXq0Pw=="],
 
     "@radix-ui/react-focus-scope": ["@radix-ui/react-focus-scope@1.1.7", "", { "dependencies": { "@radix-ui/react-compose-refs": "1.1.2", "@radix-ui/react-primitive": "2.1.3", "@radix-ui/react-use-callback-ref": "1.1.1" }, "peerDependencies": { "@types/react": "*", "@types/react-dom": "*", "react": "^16.8 || ^17.0 || ^18.0 || ^19.0 || ^19.0.0-rc", "react-dom": "^16.8 || ^17.0 || ^18.0 || ^19.0 || ^19.0.0-rc" }, "optionalPeers": ["@types/react", "@types/react-dom"] }, "sha512-t2ODlkXBQyn7jkl6TNaw/MtVEVvIGelJDCG41Okq/KwUsJBwQ4XVZsHAVUkK4mBv3ewiAS3PGuUWuY2BoK4ZUw=="],
 
     "@radix-ui/react-id": ["@radix-ui/react-id@1.1.1", "", { "dependencies": { "@radix-ui/react-use-layout-effect": "1.1.1" }, "peerDependencies": { "@types/react": "*", "react": "^16.8 || ^17.0 || ^18.0 || ^19.0 || ^19.0.0-rc" }, "optionalPeers": ["@types/react"] }, "sha512-kGkGegYIdQsOb4XjsfM97rXsiHaBwco+hFI66oO4s9LU+PLAC5oJ7khdOVFxkhsmlbpUqDAvXw11CluXP+jkHg=="],
 
+    "@radix-ui/react-menu": ["@radix-ui/react-menu@2.1.16", "", { "dependencies": { "@radix-ui/primitive": "1.1.3", "@radix-ui/react-collection": "1.1.7", "@radix-ui/react-compose-refs": "1.1.2", "@radix-ui/react-context": "1.1.2", "@radix-ui/react-direction": "1.1.1", "@radix-ui/react-dismissable-layer": "1.1.11", "@radix-ui/react-focus-guards": "1.1.3", "@radix-ui/react-focus-scope": "1.1.7", "@radix-ui/react-id": "1.1.1", "@radix-ui/react-popper": "1.2.8", "@radix-ui/react-portal": "1.1.9", "@radix-ui/react-presence": "1.1.5", "@radix-ui/react-primitive": "2.1.3", "@radix-ui/react-roving-focus": "1.1.11", "@radix-ui/react-slot": "1.2.3", "@radix-ui/react-use-callback-ref": "1.1.1", "aria-hidden": "^1.2.4", "react-remove-scroll": "^2.6.3" }, "peerDependencies": { "@types/react": "*", "@types/react-dom": "*", "react": "^16.8 || ^17.0 || ^18.0 || ^19.0 || ^19.0.0-rc", "react-dom": "^16.8 || ^17.0 || ^18.0 || ^19.0 || ^19.0.0-rc" }, "optionalPeers": ["@types/react", "@types/react-dom"] }, "sha512-72F2T+PLlphrqLcAotYPp0uJMr5SjP5SL01wfEspJbru5Zs5vQaSHb4VB3ZMJPimgHHCHG7gMOeOB9H3Hdmtxg=="],
+
+    "@radix-ui/react-popover": ["@radix-ui/react-popover@1.1.15", "", { "dependencies": { "@radix-ui/primitive": "1.1.3", "@radix-ui/react-compose-refs": "1.1.2", "@radix-ui/react-context": "1.1.2", "@radix-ui/react-dismissable-layer": "1.1.11", "@radix-ui/react-focus-guards": "1.1.3", "@radix-ui/react-focus-scope": "1.1.7", "@radix-ui/react-id": "1.1.1", "@radix-ui/react-popper": "1.2.8", "@radix-ui/react-portal": "1.1.9", "@radix-ui/react-presence": "1.1.5", "@radix-ui/react-primitive": "2.1.3", "@radix-ui/react-slot": "1.2.3", "@radix-ui/react-use-controllable-state": "1.2.2", "aria-hidden": "^1.2.4", "react-remove-scroll": "^2.6.3" }, "peerDependencies": { "@types/react": "*", "@types/react-dom": "*", "react": "^16.8 || ^17.0 || ^18.0 || ^19.0 || ^19.0.0-rc", "react-dom": "^16.8 || ^17.0 || ^18.0 || ^19.0 || ^19.0.0-rc" }, "optionalPeers": ["@types/react", "@types/react-dom"] }, "sha512-kr0X2+6Yy/vJzLYJUPCZEc8SfQcf+1COFoAqauJm74umQhta9M7lNJHP7QQS3vkvcGLQUbWpMzwrXYwrYztHKA=="],
+
     "@radix-ui/react-popper": ["@radix-ui/react-popper@1.2.8", "", { "dependencies": { "@floating-ui/react-dom": "^2.0.0", "@radix-ui/react-arrow": "1.1.7", "@radix-ui/react-compose-refs": "1.1.2", "@radix-ui/react-context": "1.1.2", "@radix-ui/react-primitive": "2.1.3", "@radix-ui/react-use-callback-ref": "1.1.1", "@radix-ui/react-use-layout-effect": "1.1.1", "@radix-ui/react-use-rect": "1.1.1", "@radix-ui/react-use-size": "1.1.1", "@radix-ui/rect": "1.1.1" }, "peerDependencies": { "@types/react": "*", "@types/react-dom": "*", "react": "^16.8 || ^17.0 || ^18.0 || ^19.0 || ^19.0.0-rc", "react-dom": "^16.8 || ^17.0 || ^18.0 || ^19.0 || ^19.0.0-rc" }, "optionalPeers": ["@types/react", "@types/react-dom"] }, "sha512-0NJQ4LFFUuWkE7Oxf0htBKS6zLkkjBH+hM1uk7Ng705ReR8m/uelduy1DBo0PyBXPKVnBA6YBlU94MBGXrSBCw=="],
 
     "@radix-ui/react-portal": ["@radix-ui/react-portal@1.1.9", "", { "dependencies": { "@radix-ui/react-primitive": "2.1.3", "@radix-ui/react-use-layout-effect": "1.1.1" }, "peerDependencies": { "@types/react": "*", "@types/react-dom": "*", "react": "^16.8 || ^17.0 || ^18.0 || ^19.0 || ^19.0.0-rc", "react-dom": "^16.8 || ^17.0 || ^18.0 || ^19.0 || ^19.0.0-rc" }, "optionalPeers": ["@types/react", "@types/react-dom"] }, "sha512-bpIxvq03if6UNwXZ+HTK71JLh4APvnXntDc6XOX8UVq4XQOVl7lwok0AvIl+b8zgCw3fSaVTZMpAPPagXbKmHQ=="],
@@ -742,6 +756,8 @@
 
     "@radix-ui/react-primitive": ["@radix-ui/react-primitive@2.1.3", "", { "dependencies": { "@radix-ui/react-slot": "1.2.3" }, "peerDependencies": { "@types/react": "*", "@types/react-dom": "*", "react": "^16.8 || ^17.0 || ^18.0 || ^19.0 || ^19.0.0-rc", "react-dom": "^16.8 || ^17.0 || ^18.0 || ^19.0 || ^19.0.0-rc" }, "optionalPeers": ["@types/react", "@types/react-dom"] }, "sha512-m9gTwRkhy2lvCPe6QJp4d3G1TYEUHn/FzJUtq9MjH46an1wJU+GdoGC5VLof8RX8Ft/DlpshApkhswDLZzHIcQ=="],
 
+    "@radix-ui/react-roving-focus": ["@radix-ui/react-roving-focus@1.1.11", "", { "dependencies": { "@radix-ui/primitive": "1.1.3", "@radix-ui/react-collection": "1.1.7", "@radix-ui/react-compose-refs": "1.1.2", "@radix-ui/react-context": "1.1.2", "@radix-ui/react-direction": "1.1.1", "@radix-ui/react-id": "1.1.1", "@radix-ui/react-primitive": "2.1.3", "@radix-ui/react-use-callback-ref": "1.1.1", "@radix-ui/react-use-controllable-state": "1.2.2" }, "peerDependencies": { "@types/react": "*", "@types/react-dom": "*", "react": "^16.8 || ^17.0 || ^18.0 || ^19.0 || ^19.0.0-rc", "react-dom": "^16.8 || ^17.0 || ^18.0 || ^19.0 || ^19.0.0-rc" }, "optionalPeers": ["@types/react", "@types/react-dom"] }, "sha512-7A6S9jSgm/S+7MdtNDSb+IU859vQqJ/QAtcYQcfFC6W8RS4IxIZDldLR0xqCFZ6DCyrQLjLPsxtTNch5jVA4lA=="],
+
     "@radix-ui/react-slot": ["@radix-ui/react-slot@1.2.3", "", { "dependencies": { "@radix-ui/react-compose-refs": "1.1.2" }, "peerDependencies": { "@types/react": "*", "react": "^16.8 || ^17.0 || ^18.0 || ^19.0 || ^19.0.0-rc" }, "optionalPeers": ["@types/react"] }, "sha512-aeNmHnBxbi2St0au6VBVC7JXFlhLlOnvIIlePNniyUNAClzmtAUEY8/pBiK3iHjufOlwA+c20/8jngo7xcrg8A=="],
 
     "@radix-ui/react-tooltip": ["@radix-ui/react-tooltip@1.2.8", "", { "dependencies": { "@radix-ui/primitive": "1.1.3", "@radix-ui/react-compose-refs": "1.1.2", "@radix-ui/react-context": "1.1.2", "@radix-ui/react-dismissable-layer": "1.1.11", "@radix-ui/react-id": "1.1.1", "@radix-ui/react-popper": "1.2.8", "@radix-ui/react-portal": "1.1.9", "@radix-ui/react-presence": "1.1.5", "@radix-ui/react-primitive": "2.1.3", "@radix-ui/react-slot": "1.2.3", "@radix-ui/react-use-controllable-state": "1.2.2", "@radix-ui/react-visually-hidden": "1.2.3" }, "peerDependencies": { "@types/react": "*", "@types/react-dom": "*", "react": "^16.8 || ^17.0 || ^18.0 || ^19.0 || ^19.0.0-rc", "react-dom": "^16.8 || ^17.0 || ^18.0 || ^19.0 || ^19.0.0-rc" }, "optionalPeers": ["@types/react", "@types/react-dom"] }, "sha512-tY7sVt1yL9ozIxvmbtN5qtmH2krXcBCfjEiCgKGLqunJHvgvZG2Pcl2oQ3kbcZARb1BGEHdkLzcYGO8ynVlieg=="],
diff --git a/packages/ai/providers/codex-sdk.ts b/packages/ai/providers/codex-sdk.ts
index 0c6da35c..1088be57 100644
--- a/packages/ai/providers/codex-sdk.ts
+++ b/packages/ai/providers/codex-sdk.ts
@@ -41,6 +41,7 @@ export class CodexSDKProvider implements AIProvider {
     tools: true,
   };
   readonly models = [
+    { id: 'gpt-5.5', label: 'GPT-5.5' },
     { id: 'gpt-5.4', label: 'GPT-5.4', default: true },
     { id: 'gpt-5.4-mini', label: 'GPT-5.4 Mini' },
     { id: 'gpt-5.3-codex', label: 'GPT-5.3 Codex' },
diff --git a/packages/review-editor/App.tsx b/packages/review-editor/App.tsx
index bfa17f80..cc20e42b 100644
--- a/packages/review-editor/App.tsx
+++ b/packages/review-editor/App.tsx
@@ -1,6 +1,7 @@
 import React, { useState, useEffect, useCallback, useMemo, useRef } from 'react';
 import { type Origin, getAgentName } from '@plannotator/shared/agents';
 import { ThemeProvider, useTheme } from '@plannotator/ui/components/ThemeProvider';
+import { TooltipProvider } from '@plannotator/ui/components/Tooltip';
 import { ConfirmDialog } from '@plannotator/ui/components/ConfirmDialog';
 import { Settings } from '@plannotator/ui/components/Settings';
 import { FeedbackButton, ApproveButton, ExitButton } from '@plannotator/ui/components/ToolbarButtons';
@@ -158,6 +159,17 @@ const ReviewApp: React.FC = () => {
   const [isWSL, setIsWSL] = useState(false);
   const [diffType, setDiffType] = useState<string>('uncommitted');
   const [gitContext, setGitContext] = useState<GitContext | null>(null);
+  // Two bases:
+  //   selectedBase  — what the picker is currently showing (UI intent).
+  //                   Updates immediately when the user picks, so the chip
+  //                   feels responsive.
+  //   committedBase — the base the server last computed the patch against.
+  //                   Drives file-content fetches. Only updates after
+  //                   /api/diff/switch returns, so we never pair an old
+  //                   patch with a new base's file contents (race that
+  //                   produced "trailing context mismatch" warnings).
+  const [selectedBase, setSelectedBase] = useState<string | null>(null);
+  const [committedBase, setCommittedBase] = useState<string | null>(null);
   const [agentCwd, setAgentCwd] = useState<string | null>(null);
   const [isLoadingDiff, setIsLoadingDiff] = useState(false);
   const [diffError, setDiffError] = useState<string | null>(null);
@@ -666,6 +678,7 @@ const ReviewApp: React.FC = () => {
         gitRef: string;
         origin?: Origin;
         diffType?: string;
+        base?: string;
         gitContext?: GitContext;
         agentCwd?: string;
         sharingEnabled?: boolean;
@@ -694,7 +707,15 @@ const ReviewApp: React.FC = () => {
         setFiles(apiFiles);
         if (data.origin) setOrigin(data.origin);
         if (data.diffType) setDiffType(data.diffType);
-        if (data.gitContext) setGitContext(data.gitContext);
+        if (data.gitContext) {
+          setGitContext(data.gitContext);
+          // Prefer the server's active base (survives page refresh / reconnect)
+          // over the detected default, so the picker rehydrates to what the
+          // server is actually using.
+          const initial = data.base || data.gitContext.defaultBranch || null;
+          setSelectedBase(initial);
+          setCommittedBase(initial);
+        }
         if (data.agentCwd) setAgentCwd(data.agentCwd);
         if (data.sharingEnabled !== undefined) setSharingEnabled(data.sharingEnabled);
         if (data.repoInfo) setRepoInfo(data.repoInfo);
@@ -904,7 +925,7 @@ const ReviewApp: React.FC = () => {
       const lastColon = rest.lastIndexOf(':');
       if (lastColon !== -1) {
         const sub = rest.slice(lastColon + 1);
-        if (['uncommitted', 'staged', 'unstaged', 'last-commit', 'branch'].includes(sub)) {
+        if (['uncommitted', 'staged', 'unstaged', 'last-commit', 'branch', 'merge-base'].includes(sub)) {
           return { activeWorktreePath: rest.slice(0, lastColon), activeDiffBase: sub };
         }
       }
@@ -925,14 +946,21 @@ const ReviewApp: React.FC = () => {
   // Staging is never available in PR review mode — the server rejects it and the UI shouldn't offer it.
   const canStageFiles = canStageRaw && !prMetadata;
 
-  // Shared helper: fetch a diff switch and update state
-  const fetchDiffSwitch = useCallback(async (fullDiffType: string) => {
+  // Shared helper: fetch a diff switch and update state.
+  // Returns true on success, false on failure — callers that optimistically
+  // updated UI state (e.g. the base picker) can use this to revert.
+  const fetchDiffSwitch = useCallback(async (fullDiffType: string, baseOverride?: string): Promise<boolean> => {
     setIsLoadingDiff(true);
     try {
       const res = await fetch('/api/diff/switch', {
         method: 'POST',
         headers: { 'Content-Type': 'application/json' },
-        body: JSON.stringify({ diffType: fullDiffType }),
+        body: JSON.stringify({
+          diffType: fullDiffType,
+          // Server ignores base for modes that don't use it (uncommitted/staged/etc),
+          // so forwarding unconditionally is safe and keeps the request shape uniform.
+          ...((baseOverride ?? selectedBase) && { base: baseOverride ?? selectedBase }),
+        }),
       });
 
       if (!res.ok) throw new Error('Failed to switch diff');
@@ -941,6 +969,8 @@ const ReviewApp: React.FC = () => {
         rawPatch: string;
         gitRef: string;
         diffType: string;
+        base?: string;
+        gitContext?: GitContext;
         error?: string;
       };
 
@@ -950,17 +980,67 @@ const ReviewApp: React.FC = () => {
       setDiffData(prev => prev ? { ...prev, rawPatch: data.rawPatch, gitRef: data.gitRef, diffType: data.diffType } : prev);
       setFiles(nextFiles);
       setDiffType(data.diffType);
+      // Adopt the server's echoed base. The server trusts whatever we sent
+      // verbatim — this sync just makes sure selectedBase and committedBase
+      // match the server's view (important when the caller didn't send a
+      // base and the server used the detected default instead).
+      if (data.base) {
+        setSelectedBase(data.base);
+        setCommittedBase(data.base);
+      }
+      // Merge only the per-cwd fields so the sidebar reflects the worktree
+      // we're now in. Keep the original `worktrees` list (already filtered to
+      // exclude the server's startup cwd — replacing it with the new context's
+      // list would duplicate the "Main repo" entry) and `availableBranches`
+      // (shared across worktrees of the same repo).
+      //
+      // IMPORTANT: we deliberately do NOT overwrite `currentBranch`. The
+      // WorktreePicker's top "launch" row uses it as a label, and that row
+      // represents the cwd plannotator was launched in — not whichever
+      // worktree is currently active. Freezing `currentBranch` at its
+      // initial-load value keeps that label truthful. `defaultBranch` and
+      // `diffOptions` update because they describe the active diff, which
+      // other UI (empty-state text, diff-type picker) should see fresh.
+      if (data.gitContext) {
+        setGitContext((prev) => {
+          if (!prev) return data.gitContext!;
+          return {
+            ...prev,
+            defaultBranch: data.gitContext!.defaultBranch,
+            diffOptions: data.gitContext!.diffOptions,
+          };
+        });
+      }
       setActiveFileIndex(0);
       setPendingSelection(null);
       setDiffError(data.error || null);
       resetStagedFiles();
+      return true;
     } catch (err) {
       console.error('Failed to switch diff:', err);
       setDiffError(err instanceof Error ? err.message : 'Failed to switch diff');
+      return false;
     } finally {
       setIsLoadingDiff(false);
     }
-  }, [dockApi, resetStagedFiles]);
+  }, [dockApi, resetStagedFiles, selectedBase]);
+
+  // Switch the base branch the current diff compares against.
+  // Only triggers a refetch when the active mode actually uses a base.
+  // Optimistically updates the picker; reverts if the server-side switch
+  // fails so the chip doesn't lie about what the viewer is actually showing.
+  const handleBaseSelect = useCallback(
+    async (branch: string) => {
+      if (branch === selectedBase) return;
+      const previous = selectedBase;
+      setSelectedBase(branch);
+      if (activeDiffBase === 'branch' || activeDiffBase === 'merge-base') {
+        const ok = await fetchDiffSwitch(diffType, branch);
+        if (!ok) setSelectedBase(previous);
+      }
+    },
+    [selectedBase, activeDiffBase, diffType, fetchDiffSwitch],
+  );
 
   // Switch diff type (uncommitted, last-commit, branch) — composes worktree prefix if active
   const handleDiffSwitch = useCallback(async (baseDiffType: string) => {
@@ -971,14 +1051,17 @@ const ReviewApp: React.FC = () => {
     await fetchDiffSwitch(fullDiffType);
   }, [diffType, activeWorktreePath, fetchDiffSwitch]);
 
-  // Switch worktree context (or back to main repo)
+  // Switch worktree context (or back to main repo). Preserves the current
+  // diff mode across the switch — if the reviewer was looking at "PR Diff"
+  // in the main repo, they should keep looking at "PR Diff" in the target
+  // worktree rather than being silently snapped back to "Uncommitted".
   const handleWorktreeSwitch = useCallback(async (worktreePath: string | null) => {
     if (worktreePath === activeWorktreePath) return;
     const fullDiffType = worktreePath
-      ? `worktree:${worktreePath}:uncommitted`
-      : 'uncommitted';
+      ? `worktree:${worktreePath}:${activeDiffBase}`
+      : activeDiffBase;
     await fetchDiffSwitch(fullDiffType);
-  }, [activeWorktreePath, fetchDiffSwitch]);
+  }, [activeWorktreePath, activeDiffBase, fetchDiffSwitch]);
 
   // Select annotation - switches file if needed and scrolls to it
   const handleSelectAnnotation = useCallback((id: string | null) => {
@@ -1003,6 +1086,26 @@ const ReviewApp: React.FC = () => {
     setSelectedAnnotationId(id);
   }, [allAnnotations, files, handleFileSwitch]);
 
+  // Diff context bundled into local-mode feedback headers so the receiving
+  // agent knows which diff the annotations are anchored to. Uses committedBase
+  // (what the server actually computed) and activeDiffBase/activeWorktreePath
+  // (derived from the committed diffType). Skipped in PR mode — the PR header
+  // already carries the relevant context.
+  // Declared before reviewStateValue because both reviewStateValue and the
+  // feedbackMarkdown memo below read it; moving it below either would put it
+  // in the TDZ when those memos run on first render.
+  const feedbackDiffContext = useMemo(
+    () =>
+      prMetadata || !activeDiffBase
+        ? undefined
+        : {
+            mode: activeDiffBase,
+            base: committedBase ?? undefined,
+            worktreePath: activeWorktreePath,
+          },
+    [prMetadata, activeDiffBase, committedBase, activeWorktreePath],
+  );
+
   // Build ReviewState value for dock panel context
   const reviewStateValue = useMemo<ReviewState>(() => ({
     files,
@@ -1016,6 +1119,17 @@ const ReviewApp: React.FC = () => {
     disableBackground: !diffShowBackground,
     fontFamily: diffFontFamily || undefined,
     fontSize: diffFontSize || undefined,
+    // Only propagate base for modes where it affects old/new content. Avoids
+    // needless file-content re-fetches when switching to uncommitted/staged/etc.
+    // Uses committedBase (not selectedBase) so file-content queries wait for
+    // the new patch to arrive before refetching — otherwise the viewer can
+    // briefly pair an old patch with the new base's content.
+    reviewBase:
+      (activeDiffBase === 'branch' || activeDiffBase === 'merge-base')
+        ? committedBase ?? undefined
+        : undefined,
+    activeDiffBase,
+    feedbackDiffContext,
     allAnnotations,
     externalAnnotations,
     selectedAnnotationId,
@@ -1058,7 +1172,8 @@ const ReviewApp: React.FC = () => {
   }), [
     files, activeFileIndex, diffStyle, diffOverflow, diffIndicators,
     diffLineDiffType, diffShowLineNumbers, diffShowBackground,
-    diffFontFamily, diffFontSize, allAnnotations, externalAnnotations,
+    diffFontFamily, diffFontSize, activeDiffBase, committedBase, feedbackDiffContext,
+    allAnnotations, externalAnnotations,
     selectedAnnotationId, pendingSelection, handleLineSelection,
     handleAddAnnotation, handleAddFileComment, handleEditAnnotation,
     handleSelectAnnotation, handleDeleteAnnotation, viewedFiles,
@@ -1096,7 +1211,7 @@ const ReviewApp: React.FC = () => {
       return;
     }
     try {
-      const feedback = exportReviewFeedback(allAnnotations, prMetadata);
+      const feedback = exportReviewFeedback(allAnnotations, prMetadata, feedbackDiffContext);
       await navigator.clipboard.writeText(feedback);
       setCopyFeedback('Feedback copied!');
       setTimeout(() => setCopyFeedback(null), 2000);
@@ -1105,15 +1220,15 @@ const ReviewApp: React.FC = () => {
       setCopyFeedback('Failed to copy');
       setTimeout(() => setCopyFeedback(null), 2000);
     }
-  }, [allAnnotations, prMetadata]);
+  }, [allAnnotations, prMetadata, feedbackDiffContext]);
 
   const feedbackMarkdown = useMemo(() => {
-    let output = exportReviewFeedback(allAnnotations, prMetadata);
+    let output = exportReviewFeedback(allAnnotations, prMetadata, feedbackDiffContext);
     if (editorAnnotations.length > 0) {
       output += exportEditorAnnotations(editorAnnotations);
     }
     return output;
-  }, [allAnnotations, prMetadata, editorAnnotations]);
+  }, [allAnnotations, prMetadata, feedbackDiffContext, editorAnnotations]);
 
   const totalAnnotationCount = allAnnotations.length + editorAnnotations.length;
 
@@ -1404,6 +1519,7 @@ const ReviewApp: React.FC = () => {
 
   return (
     <ThemeProvider defaultTheme="dark">
+      <TooltipProvider delayDuration={200} skipDelayDuration={100}>
       <ReviewStateProvider value={reviewStateValue}>
       <JobLogsProvider value={jobLogsValue}>
       <div className="h-screen flex flex-col bg-background overflow-hidden">
@@ -1702,6 +1818,10 @@ const ReviewApp: React.FC = () => {
                 activeWorktreePath={activeWorktreePath}
                 onSelectWorktree={handleWorktreeSwitch}
                 currentBranch={gitContext?.currentBranch}
+                availableBranches={prMetadata ? undefined : gitContext?.availableBranches}
+                selectedBase={prMetadata ? undefined : selectedBase ?? undefined}
+                detectedBase={prMetadata ? undefined : gitContext?.defaultBranch}
+                onSelectBase={prMetadata ? undefined : handleBaseSelect}
                 stagedFiles={stagedFiles}
                 onCopyRawDiff={handleCopyDiff}
                 canCopyRawDiff={!!diffData?.rawPatch}
@@ -1777,7 +1897,8 @@ const ReviewApp: React.FC = () => {
                           {activeDiffBase === 'staged' && "No staged changes. Stage some files with git add."}
                           {activeDiffBase === 'unstaged' && "No unstaged changes. All changes are staged."}
                           {activeDiffBase === 'last-commit' && `No changes in the last commit${activeWorktreePath ? ' in this worktree' : ''}.`}
-                          {activeDiffBase === 'branch' && `No changes vs ${gitContext?.defaultBranch || 'main'}${activeWorktreePath ? ' in this worktree' : ''}.`}
+                          {activeDiffBase === 'branch' && `No changes vs ${selectedBase || gitContext?.defaultBranch || 'main'}${activeWorktreePath ? ' in this worktree' : ''}.`}
+                          {activeDiffBase === 'merge-base' && `No changes vs ${selectedBase || gitContext?.defaultBranch || 'main'}${activeWorktreePath ? ' in this worktree' : ''}.`}
                         </p>
                       </>
                     )}
@@ -2078,6 +2199,7 @@ const ReviewApp: React.FC = () => {
 
     </JobLogsProvider>
     </ReviewStateProvider>
+    </TooltipProvider>
     </ThemeProvider>
   );
 };
diff --git a/packages/review-editor/components/BaseBranchPicker.tsx b/packages/review-editor/components/BaseBranchPicker.tsx
new file mode 100644
index 00000000..5810d3f8
--- /dev/null
+++ b/packages/review-editor/components/BaseBranchPicker.tsx
@@ -0,0 +1,193 @@
+import React, { useMemo, useRef, useState } from 'react';
+import * as Popover from '@radix-ui/react-popover';
+import type { AvailableBranches } from '@plannotator/shared/types';
+
+interface BaseBranchPickerProps {
+  availableBranches: AvailableBranches;
+  selectedBase: string;
+  detectedBase: string;
+  onSelectBase: (branch: string) => void;
+  disabled?: boolean;
+}
+
+export const BaseBranchPicker: React.FC<BaseBranchPickerProps> = ({
+  availableBranches,
+  selectedBase,
+  detectedBase,
+  onSelectBase,
+  disabled,
+}) => {
+  const [open, setOpen] = useState(false);
+  const [query, setQuery] = useState('');
+  const searchRef = useRef<HTMLInputElement>(null);
+
+  const { local, remote } = availableBranches;
+  const filtered = useMemo(() => {
+    const q = query.trim().toLowerCase();
+    if (!q) return { local, remote };
+    return {
+      local: local.filter((b) => b.toLowerCase().includes(q)),
+      remote: remote.filter((b) => b.toLowerCase().includes(q)),
+    };
+  }, [local, remote, query]);
+
+  const handleSelect = (branch: string) => {
+    onSelectBase(branch);
+    setOpen(false);
+    setQuery('');
+  };
+
+  const handleReset = () => {
+    onSelectBase(detectedBase);
+    setOpen(false);
+    setQuery('');
+  };
+
+  const isCustom = selectedBase !== detectedBase;
+
+  return (
+    <Popover.Root
+      open={open}
+      onOpenChange={(v) => {
+        setOpen(v);
+        if (!v) setQuery('');
+      }}
+    >
+      <Popover.Trigger asChild>
+        <button
+          type="button"
+          disabled={disabled}
+          title={`Review base: ${selectedBase}`}
+          className={`w-full flex items-center gap-1.5 px-2.5 py-1.5 rounded text-xs font-medium transition-colors focus:outline-none focus:ring-1 focus:ring-primary/50 disabled:opacity-50 disabled:cursor-not-allowed ${
+            isCustom
+              ? 'bg-primary/10 border border-primary/30 text-foreground'
+              : 'bg-muted border border-transparent text-foreground'
+          }`}
+        >
+          <span className="text-[10px] uppercase tracking-wide opacity-60 flex-shrink-0">
+            base
+          </span>
+          <span className="truncate flex-1 text-left">{selectedBase}</span>
+          <svg
+            className="w-3.5 h-3.5 text-muted-foreground flex-shrink-0"
+            fill="none"
+            viewBox="0 0 24 24"
+            stroke="currentColor"
+            strokeWidth={2}
+          >
+            <path strokeLinecap="round" strokeLinejoin="round" d="M19 9l-7 7-7-7" />
+          </svg>
+        </button>
+      </Popover.Trigger>
+      <Popover.Portal>
+        <Popover.Content
+          side="bottom"
+          align="start"
+          sideOffset={4}
+          className="z-50 w-72 bg-popover text-popover-foreground border border-border rounded shadow-lg overflow-hidden origin-[var(--radix-popover-content-transform-origin)] data-[state=open]:animate-in data-[state=closed]:animate-out data-[state=closed]:fade-out-0 data-[state=open]:fade-in-0"
+          onOpenAutoFocus={(e) => {
+            e.preventDefault();
+            searchRef.current?.focus();
+          }}
+        >
+          <div className="p-2 border-b border-border/50">
+            <input
+              ref={searchRef}
+              type="text"
+              value={query}
+              onChange={(e) => setQuery(e.target.value)}
+              placeholder="Search branches…"
+              className="w-full px-2 py-1.5 bg-muted rounded text-xs text-foreground placeholder:text-muted-foreground focus:outline-none focus:ring-1 focus:ring-primary/50"
+            />
+          </div>
+          <div className="max-h-72 overflow-y-auto py-1">
+            {filtered.local.length === 0 && filtered.remote.length === 0 && (
+              <div className="px-3 py-2 text-xs text-muted-foreground">
+                No branches match.
+              </div>
+            )}
+            {filtered.local.length > 0 && (
+              <BranchGroup
+                title="Local"
+                branches={filtered.local}
+                selectedBase={selectedBase}
+                detectedBase={detectedBase}
+                onSelect={handleSelect}
+              />
+            )}
+            {filtered.remote.length > 0 && (
+              <BranchGroup
+                title="Remote"
+                branches={filtered.remote}
+                selectedBase={selectedBase}
+                detectedBase={detectedBase}
+                onSelect={handleSelect}
+              />
+            )}
+          </div>
+          {isCustom && (
+            <div className="border-t border-border/50 p-1">
+              <button
+                type="button"
+                onClick={handleReset}
+                className="w-full text-left px-2 py-1.5 text-xs text-muted-foreground hover:text-foreground hover:bg-muted rounded"
+              >
+                Reset to detected ({detectedBase})
+              </button>
+            </div>
+          )}
+        </Popover.Content>
+      </Popover.Portal>
+    </Popover.Root>
+  );
+};
+
+interface BranchGroupProps {
+  title: string;
+  branches: string[];
+  selectedBase: string;
+  detectedBase: string;
+  onSelect: (branch: string) => void;
+}
+
+const BranchGroup: React.FC<BranchGroupProps> = ({
+  title,
+  branches,
+  selectedBase,
+  detectedBase,
+  onSelect,
+}) => (
+  <div className="py-1">
+    <div className="px-3 pb-1 text-[10px] uppercase tracking-wide text-muted-foreground">
+      {title}
+    </div>
+    {branches.map((branch) => {
+      const isSelected = branch === selectedBase;
+      const isDetected = branch === detectedBase;
+      return (
+        <button
+          key={branch}
+          type="button"
+          onClick={() => onSelect(branch)}
+          className={`w-full flex items-center gap-2 px-3 py-1.5 text-xs text-left hover:bg-muted focus:outline-none focus:bg-muted ${
+            isSelected ? 'text-foreground font-medium' : 'text-foreground/80'
+          }`}
+        >
+          <span className="w-3 flex-shrink-0">
+            {isSelected && (
+              <svg className="w-3 h-3" fill="none" viewBox="0 0 24 24" stroke="currentColor" strokeWidth={3}>
+                <path strokeLinecap="round" strokeLinejoin="round" d="M5 13l4 4L19 7" />
+              </svg>
+            )}
+          </span>
+          <span className="truncate flex-1">{branch}</span>
+          {isDetected && (
+            <span className="text-[10px] uppercase tracking-wide text-muted-foreground px-1 py-0.5 rounded bg-muted">
+              detected
+            </span>
+          )}
+        </button>
+      );
+    })}
+  </div>
+);
diff --git a/packages/review-editor/components/DiffTypePicker.tsx b/packages/review-editor/components/DiffTypePicker.tsx
new file mode 100644
index 00000000..3547c138
--- /dev/null
+++ b/packages/review-editor/components/DiffTypePicker.tsx
@@ -0,0 +1,117 @@
+import React, { useState } from 'react';
+import * as DropdownMenu from '@radix-ui/react-dropdown-menu';
+import { Tooltip } from '@plannotator/ui/components/Tooltip';
+import type { DiffOption } from '@plannotator/shared/types';
+
+interface DiffTypePickerProps {
+  options: DiffOption[];
+  activeDiffType: string;
+  onSelect: (diffType: string) => void;
+  isLoading?: boolean;
+  hasBasePicker: boolean;
+}
+
+/**
+ * Plain-English explanations shown in a tooltip next to each option.
+ * Keep these short — they're hover hints, not docs.
+ */
+const OPTION_HINTS: Record<string, string> = {
+  uncommitted: "All your local changes — anything you haven't committed yet.",
+  staged: "Only what you've run `git add` on.",
+  unstaged: "What `git diff` shows with no arguments.",
+  'last-commit': "Just your most recent commit.",
+  branch: "Straight compare against the base branch (picked below). Can show commits that aren't yours if the base has new commits.",
+  'merge-base': "Only what you've added on top of the base branch (picked below). Same as GitHub's PR view.",
+};
+
+export const DiffTypePicker: React.FC<DiffTypePickerProps> = ({
+  options,
+  activeDiffType,
+  onSelect,
+  isLoading,
+  hasBasePicker,
+}) => {
+  const [open, setOpen] = useState(false);
+
+  // When a base picker is wired up, strip branch names from the
+  // base-dependent labels — the branch belongs in the picker.
+  const displayLabel = (opt: DiffOption) => {
+    if (!hasBasePicker) return opt.label;
+    if (opt.id === 'branch') return 'Branch diff';
+    if (opt.id === 'merge-base') return 'PR Diff';
+    return opt.label;
+  };
+
+  const active = options.find((o) => o.id === activeDiffType);
+  const activeLabel = active ? displayLabel(active) : 'Select diff';
+
+  return (
+    <DropdownMenu.Root open={open} onOpenChange={setOpen}>
+      <DropdownMenu.Trigger asChild>
+        <button
+          type="button"
+          disabled={isLoading}
+          className="w-full flex items-center gap-1.5 px-2.5 py-1.5 bg-muted rounded text-xs text-foreground focus:outline-none focus:ring-1 focus:ring-primary/50 disabled:opacity-50 disabled:cursor-wait"
+        >
+          <span className="truncate flex-1 text-left">{activeLabel}</span>
+          {isLoading ? (
+            <svg className="w-3.5 h-3.5 text-muted-foreground animate-spin flex-shrink-0" fill="none" viewBox="0 0 24 24">
+              <circle className="opacity-25" cx="12" cy="12" r="10" stroke="currentColor" strokeWidth="4" />
+              <path className="opacity-75" fill="currentColor" d="M4 12a8 8 0 018-8V0C5.373 0 0 5.373 0 12h4zm2 5.291A7.962 7.962 0 014 12H0c0 3.042 1.135 5.824 3 7.938l3-2.647z" />
+            </svg>
+          ) : (
+            <svg className="w-3.5 h-3.5 text-muted-foreground flex-shrink-0" fill="none" viewBox="0 0 24 24" stroke="currentColor" strokeWidth={2}>
+              <path strokeLinecap="round" strokeLinejoin="round" d="M19 9l-7 7-7-7" />
+            </svg>
+          )}
+        </button>
+      </DropdownMenu.Trigger>
+      <DropdownMenu.Portal>
+        <DropdownMenu.Content
+          side="bottom"
+          align="start"
+          sideOffset={4}
+          className="z-50 min-w-[var(--radix-dropdown-menu-trigger-width)] bg-popover text-popover-foreground border border-border rounded shadow-lg overflow-hidden py-1 origin-[var(--radix-dropdown-menu-content-transform-origin)] data-[state=open]:animate-in data-[state=closed]:animate-out data-[state=closed]:fade-out-0 data-[state=open]:fade-in-0"
+        >
+          {options.map((opt) => {
+            const hint = OPTION_HINTS[opt.id];
+            const isActive = opt.id === activeDiffType;
+            return (
+              <DropdownMenu.Item
+                key={opt.id}
+                onSelect={() => onSelect(opt.id)}
+                className={`flex items-center gap-2 mx-1 px-2 py-1.5 text-xs rounded cursor-pointer outline-none data-[highlighted]:bg-muted ${
+                  isActive ? 'text-foreground font-medium' : 'text-foreground/80'
+                }`}
+              >
+                <span className="w-3 flex-shrink-0">
+                  {isActive && (
+                    <svg className="w-3 h-3" fill="none" viewBox="0 0 24 24" stroke="currentColor" strokeWidth={3}>
+                      <path strokeLinecap="round" strokeLinejoin="round" d="M5 13l4 4L19 7" />
+                    </svg>
+                  )}
+                </span>
+                <span className="truncate flex-1">{displayLabel(opt)}</span>
+                {hint && (
+                  <Tooltip content={hint} side="right" delayDuration={200} wide>
+                    <span
+                      className="flex-shrink-0 text-muted-foreground hover:text-foreground"
+                      onPointerDown={(e) => e.stopPropagation()}
+                      onPointerUp={(e) => e.stopPropagation()}
+                      onClick={(e) => e.stopPropagation()}
+                    >
+                      {/* Heroicons information-circle (outline) */}
+                      <svg className="w-3.5 h-3.5" fill="none" viewBox="0 0 24 24" stroke="currentColor" strokeWidth={1.5}>
+                        <path strokeLinecap="round" strokeLinejoin="round" d="M11.25 11.25l.041-.02a.75.75 0 011.063.852l-.708 2.836a.75.75 0 001.063.853l.041-.021M21 12a9 9 0 11-18 0 9 9 0 0118 0zm-9-3.75h.008v.008H12V8.25z" />
+                      </svg>
+                    </span>
+                  </Tooltip>
+                )}
+              </DropdownMenu.Item>
+            );
+          })}
+        </DropdownMenu.Content>
+      </DropdownMenu.Portal>
+    </DropdownMenu.Root>
+  );
+};
diff --git a/packages/review-editor/components/DiffViewer.tsx b/packages/review-editor/components/DiffViewer.tsx
index 5a4aaf65..bd5cfba1 100644
--- a/packages/review-editor/components/DiffViewer.tsx
+++ b/packages/review-editor/components/DiffViewer.tsx
@@ -118,6 +118,8 @@ interface DiffViewerProps {
   patch: string;
   filePath: string;
   oldPath?: string;
+  /** Base branch override used for file-content lookups (branch / merge-base modes only). */
+  reviewBase?: string;
   isFocused?: boolean;
   diffStyle: 'split' | 'unified';
   diffOverflow?: 'scroll' | 'wrap';
@@ -162,6 +164,7 @@ export const DiffViewer: React.FC<DiffViewerProps> = ({
   patch,
   filePath,
   oldPath,
+  reviewBase,
   isFocused = false,
   diffStyle,
   diffOverflow,
@@ -275,6 +278,7 @@ export const DiffViewer: React.FC<DiffViewerProps> = ({
     setFileContents(null);
     const params = new URLSearchParams({ path: filePath });
     if (oldPath) params.set('oldPath', oldPath);
+    if (reviewBase) params.set('base', reviewBase);
     fetch(`/api/file-content?${params}`, { signal: controller.signal })
       .then(res => res.ok ? res.json() : null)
       .then((data: { oldContent: string | null; newContent: string | null } | null) => {
@@ -284,7 +288,7 @@ export const DiffViewer: React.FC<DiffViewerProps> = ({
       })
       .catch(() => {}); // Silent fallback — no expansion in demo mode
     return () => controller.abort();
-  }, [filePath, oldPath]);
+  }, [filePath, oldPath, reviewBase]);
 
   // Re-parse the patch with full file contents so hunk indices are computed
   // against the complete file (isPartial: false), enabling expansion.
diff --git a/packages/review-editor/components/FileTree.tsx b/packages/review-editor/components/FileTree.tsx
index 4abbc1f7..284ff044 100644
--- a/packages/review-editor/components/FileTree.tsx
+++ b/packages/review-editor/components/FileTree.tsx
@@ -1,8 +1,11 @@
 import React, { useEffect, useCallback, useState, useMemo } from 'react';
 import { CodeAnnotation } from '@plannotator/ui/types';
-import type { DiffOption, WorktreeInfo } from '@plannotator/shared/types';
+import type { AvailableBranches, DiffOption, WorktreeInfo } from '@plannotator/shared/types';
 import { buildFileTree, getAncestorPaths, getAllFolderPaths } from '../utils/buildFileTree';
 import { FileTreeNodeItem } from './FileTreeNode';
+import { BaseBranchPicker } from './BaseBranchPicker';
+import { DiffTypePicker } from './DiffTypePicker';
+import { WorktreePicker } from './WorktreePicker';
 import { getReviewSearchSideLabel, type ReviewSearchFileGroup, type ReviewSearchMatch } from '../utils/reviewSearch';
 import type { DiffFile } from '../types';
 import { OverlayScrollArea } from '@plannotator/ui/components/OverlayScrollArea';
@@ -27,6 +30,11 @@ interface FileTreeProps {
   activeWorktreePath?: string | null;
   onSelectWorktree?: (path: string | null) => void;
   currentBranch?: string;
+  /** Base-branch picker — only meaningful when activeDiffType is "branch" or "merge-base". */
+  availableBranches?: AvailableBranches;
+  selectedBase?: string;
+  detectedBase?: string;
+  onSelectBase?: (branch: string) => void;
   stagedFiles?: Set<string>;
   onCopyRawDiff?: () => void;
   canCopyRawDiff?: boolean;
@@ -66,6 +74,10 @@ export const FileTree: React.FC<FileTreeProps> = ({
   activeWorktreePath,
   onSelectWorktree,
   currentBranch,
+  availableBranches,
+  selectedBase,
+  detectedBase,
+  onSelectBase,
   stagedFiles,
   onCopyRawDiff,
   canCopyRawDiff = false,
@@ -95,6 +107,20 @@ export const FileTree: React.FC<FileTreeProps> = ({
       return;
     }
 
+    // Yield keyboard nav when a floating overlay owns the focus — Radix
+    // DropdownMenu / Popover / Dialog handle arrow keys themselves, and the
+    // old native <select> used to absorb these natively. `data-radix-popper-
+    // content-wrapper` is Radix's shared wrapper for every floating primitive
+    // (Popover, DropdownMenu, Tooltip, HoverCard), so it catches the base
+    // picker and worktree picker in addition to role-based dialogs/menus.
+    const active = document.activeElement;
+    if (
+      active instanceof HTMLElement &&
+      active.closest('[role="menu"], [role="dialog"], [role="listbox"], [data-radix-popper-content-wrapper]')
+    ) {
+      return;
+    }
+
     if (e.key === 'j' || e.key === 'ArrowDown') {
       e.preventDefault();
       const nextIndex = Math.min(activeFileIndex + 1, files.length - 1);
@@ -297,62 +323,52 @@ export const FileTree: React.FC<FileTreeProps> = ({
       {((worktrees && worktrees.length > 0 && onSelectWorktree) || (diffOptions && diffOptions.length > 0 && onSelectDiff)) && (
         <div className="px-2 py-1.5 border-b border-border/30 flex gap-2">
           {worktrees && worktrees.length > 0 && onSelectWorktree && (
-            <div className="relative flex-1 min-w-0">
-              <select
-                value={activeWorktreePath || ''}
-                onChange={(e) => onSelectWorktree(e.target.value || null)}
+            <div className="flex-1 min-w-0">
+              <WorktreePicker
+                worktrees={worktrees}
+                activeWorktreePath={activeWorktreePath ?? null}
+                currentBranch={currentBranch}
+                onSelect={onSelectWorktree}
                 disabled={isLoadingDiff}
-                className={`w-full px-2.5 py-1.5 rounded text-xs text-foreground focus:outline-none focus:ring-1 focus:ring-primary/50 cursor-pointer disabled:opacity-50 disabled:cursor-wait appearance-none pr-7 ${
-                  activeWorktreePath
-                    ? 'bg-primary/10 border border-primary/30'
-                    : 'bg-muted'
-                }`}
-              >
-                <option value="">{currentBranch || 'Main repo'}</option>
-                {worktrees.map(wt => (
-                  <option key={wt.path} value={wt.path}>
-                    {(wt.branch || wt.path.split('/').pop()) + ' (worktree)'}
-                  </option>
-                ))}
-              </select>
-              <div className="absolute right-2 top-1/2 -translate-y-1/2 pointer-events-none">
-                <svg className="w-3.5 h-3.5 text-muted-foreground" fill="none" viewBox="0 0 24 24" stroke="currentColor" strokeWidth={2}>
-                  <path strokeLinecap="round" strokeLinejoin="round" d="M19 9l-7 7-7-7" />
-                </svg>
-              </div>
+              />
             </div>
           )}
           {diffOptions && diffOptions.length > 0 && onSelectDiff && (
-            <div className="relative flex-1 min-w-0">
-              <select
-                value={activeDiffType || 'uncommitted'}
-                onChange={(e) => onSelectDiff(e.target.value)}
-                disabled={isLoadingDiff}
-                className="w-full px-2.5 py-1.5 bg-muted rounded text-xs text-foreground focus:outline-none focus:ring-1 focus:ring-primary/50 cursor-pointer disabled:opacity-50 disabled:cursor-wait appearance-none pr-7"
-              >
-                {diffOptions.map((option) => (
-                  <option key={option.id} value={option.id}>
-                    {option.label}
-                  </option>
-                ))}
-              </select>
-              <div className="absolute right-2 top-1/2 -translate-y-1/2 pointer-events-none">
-                {isLoadingDiff ? (
-                  <svg className="w-3.5 h-3.5 text-muted-foreground animate-spin" fill="none" viewBox="0 0 24 24">
-                    <circle className="opacity-25" cx="12" cy="12" r="10" stroke="currentColor" strokeWidth="4" />
-                    <path className="opacity-75" fill="currentColor" d="M4 12a8 8 0 018-8V0C5.373 0 0 5.373 0 12h4zm2 5.291A7.962 7.962 0 014 12H0c0 3.042 1.135 5.824 3 7.938l3-2.647z" />
-                  </svg>
-                ) : (
-                  <svg className="w-3.5 h-3.5 text-muted-foreground" fill="none" viewBox="0 0 24 24" stroke="currentColor" strokeWidth={2}>
-                    <path strokeLinecap="round" strokeLinejoin="round" d="M19 9l-7 7-7-7" />
-                  </svg>
-                )}
-              </div>
+            <div className="flex-1 min-w-0">
+              <DiffTypePicker
+                options={diffOptions}
+                activeDiffType={activeDiffType || 'uncommitted'}
+                onSelect={onSelectDiff}
+                isLoading={isLoadingDiff}
+                hasBasePicker={!!onSelectBase && !!availableBranches}
+              />
             </div>
           )}
         </div>
       )}
 
+      {/* Base-branch picker — only relevant for branch / merge-base diff types */}
+      {onSelectBase &&
+        selectedBase &&
+        detectedBase &&
+        availableBranches &&
+        (activeDiffType === 'branch' || activeDiffType === 'merge-base') && (
+          <div className="px-2 py-1.5 border-b border-border/30 flex items-center gap-2">
+            <span className="text-[10px] uppercase tracking-wide text-muted-foreground flex-shrink-0">
+              compare against
+            </span>
+            <div className="flex-1 min-w-0">
+              <BaseBranchPicker
+                availableBranches={availableBranches}
+                selectedBase={selectedBase}
+                detectedBase={detectedBase}
+                onSelectBase={onSelectBase}
+                disabled={isLoadingDiff}
+              />
+            </div>
+          </div>
+        )}
+
       {/* File tree or search results */}
       <OverlayScrollArea className="flex-1 min-h-0">
       <div className="px-1 py-1">
diff --git a/packages/review-editor/components/WorktreePicker.tsx b/packages/review-editor/components/WorktreePicker.tsx
new file mode 100644
index 00000000..5c589d5b
--- /dev/null
+++ b/packages/review-editor/components/WorktreePicker.tsx
@@ -0,0 +1,191 @@
+import React, { useMemo, useRef, useState } from 'react';
+import * as Popover from '@radix-ui/react-popover';
+import type { WorktreeInfo } from '@plannotator/shared/types';
+
+interface WorktreePickerProps {
+  worktrees: WorktreeInfo[];
+  activeWorktreePath: string | null;
+  currentBranch?: string;
+  onSelect: (path: string | null) => void;
+  disabled?: boolean;
+}
+
+const MAIN_REPO = null;
+
+export const WorktreePicker: React.FC<WorktreePickerProps> = ({
+  worktrees,
+  activeWorktreePath,
+  currentBranch,
+  onSelect,
+  disabled,
+}) => {
+  const [open, setOpen] = useState(false);
+  const [query, setQuery] = useState('');
+  const searchRef = useRef<HTMLInputElement>(null);
+
+  const mainLabel = currentBranch || 'Main repo';
+
+  const filtered = useMemo(() => {
+    const q = query.trim().toLowerCase();
+    if (!q) return worktrees;
+    return worktrees.filter((wt) => {
+      const branch = (wt.branch || '').toLowerCase();
+      const path = wt.path.toLowerCase();
+      return branch.includes(q) || path.includes(q);
+    });
+  }, [worktrees, query]);
+
+  const mainMatchesQuery = !query.trim() || mainLabel.toLowerCase().includes(query.trim().toLowerCase());
+
+  const handleSelect = (path: string | null) => {
+    onSelect(path);
+    setOpen(false);
+    setQuery('');
+  };
+
+  const active = activeWorktreePath
+    ? worktrees.find((wt) => wt.path === activeWorktreePath)
+    : null;
+  const activeLabel = active
+    ? (active.branch || active.path.split('/').pop() || 'worktree')
+    : mainLabel;
+  const isCustom = activeWorktreePath !== null;
+
+  return (
+    <Popover.Root
+      open={open}
+      onOpenChange={(v) => {
+        setOpen(v);
+        if (!v) setQuery('');
+      }}
+    >
+      <Popover.Trigger asChild>
+        <button
+          type="button"
+          disabled={disabled}
+          title={active ? `Worktree: ${active.path}` : 'Main repository'}
+          className={`w-full flex items-center gap-1.5 px-2.5 py-1.5 rounded text-xs font-medium transition-colors focus:outline-none focus:ring-1 focus:ring-primary/50 disabled:opacity-50 disabled:cursor-not-allowed ${
+            isCustom
+              ? 'bg-primary/10 border border-primary/30 text-foreground'
+              : 'bg-muted border border-transparent text-foreground'
+          }`}
+        >
+          <span className="truncate flex-1 text-left">{activeLabel}</span>
+          {isCustom && (
+            <span className="text-[10px] uppercase tracking-wide opacity-60 flex-shrink-0">
+              worktree
+            </span>
+          )}
+          <svg
+            className="w-3.5 h-3.5 text-muted-foreground flex-shrink-0"
+            fill="none"
+            viewBox="0 0 24 24"
+            stroke="currentColor"
+            strokeWidth={2}
+          >
+            <path strokeLinecap="round" strokeLinejoin="round" d="M19 9l-7 7-7-7" />
+          </svg>
+        </button>
+      </Popover.Trigger>
+      <Popover.Portal>
+        <Popover.Content
+          side="bottom"
+          align="start"
+          sideOffset={4}
+          className="z-50 w-72 bg-popover text-popover-foreground border border-border rounded shadow-lg overflow-hidden origin-[var(--radix-popover-content-transform-origin)] data-[state=open]:animate-in data-[state=closed]:animate-out data-[state=closed]:fade-out-0 data-[state=open]:fade-in-0"
+          onOpenAutoFocus={(e) => {
+            // Only override Radix's default focus when the search input is
+            // actually rendered — otherwise the preventDefault() would leave
+            // focus on the trigger, and arrow keys would bubble out to the
+            // file-tree nav. For short worktree lists we let Radix focus the
+            // first picker button itself.
+            if (searchRef.current) {
+              e.preventDefault();
+              searchRef.current.focus();
+            }
+          }}
+        >
+          {worktrees.length > 3 && (
+            <div className="p-2 border-b border-border/50">
+              <input
+                ref={searchRef}
+                type="text"
+                value={query}
+                onChange={(e) => setQuery(e.target.value)}
+                placeholder="Search worktrees…"
+                className="w-full px-2 py-1.5 bg-muted rounded text-xs text-foreground placeholder:text-muted-foreground focus:outline-none focus:ring-1 focus:ring-primary/50"
+              />
+            </div>
+          )}
+          <div className="max-h-72 overflow-y-auto py-1">
+            {mainMatchesQuery && (
+              <WorktreeRow
+                label={mainLabel}
+                sublabel={null}
+                isSelected={activeWorktreePath === MAIN_REPO}
+                onClick={() => handleSelect(MAIN_REPO)}
+              />
+            )}
+
+            {filtered.length > 0 && (
+              <>
+                {mainMatchesQuery && (
+                  <div className="h-px bg-border/50 mx-2 my-1" />
+                )}
+                <div className="px-3 pb-1 text-[10px] uppercase tracking-wide text-muted-foreground">
+                  Worktrees
+                </div>
+                {filtered.map((wt) => (
+                  <WorktreeRow
+                    key={wt.path}
+                    label={wt.branch || wt.path.split('/').pop() || 'worktree'}
+                    sublabel={wt.path}
+                    isSelected={wt.path === activeWorktreePath}
+                    onClick={() => handleSelect(wt.path)}
+                  />
+                ))}
+              </>
+            )}
+
+            {!mainMatchesQuery && filtered.length === 0 && (
+              <div className="px-3 py-2 text-xs text-muted-foreground">
+                No worktrees match.
+              </div>
+            )}
+          </div>
+        </Popover.Content>
+      </Popover.Portal>
+    </Popover.Root>
+  );
+};
+
+interface WorktreeRowProps {
+  label: string;
+  sublabel: string | null;
+  isSelected: boolean;
+  onClick: () => void;
+}
+
+const WorktreeRow: React.FC<WorktreeRowProps> = ({ label, sublabel, isSelected, onClick }) => (
+  <button
+    type="button"
+    onClick={onClick}
+    className={`w-full flex items-center gap-2 mx-1 px-2 py-1.5 text-xs text-left rounded hover:bg-muted focus:outline-none focus:bg-muted ${
+      isSelected ? 'text-foreground font-medium' : 'text-foreground/80'
+    }`}
+  >
+    <span className="w-3 flex-shrink-0">
+      {isSelected && (
+        <svg className="w-3 h-3" fill="none" viewBox="0 0 24 24" stroke="currentColor" strokeWidth={3}>
+          <path strokeLinecap="round" strokeLinejoin="round" d="M5 13l4 4L19 7" />
+        </svg>
+      )}
+    </span>
+    <div className="min-w-0 flex-1">
+      <div className="truncate">{label}</div>
+      {sublabel && (
+        <div className="truncate text-[10px] text-muted-foreground">{sublabel}</div>
+      )}
+    </div>
+  </button>
+);
diff --git a/packages/review-editor/dock/ReviewStateContext.tsx b/packages/review-editor/dock/ReviewStateContext.tsx
index 4aa75ba9..14110ae7 100644
--- a/packages/review-editor/dock/ReviewStateContext.tsx
+++ b/packages/review-editor/dock/ReviewStateContext.tsx
@@ -5,6 +5,7 @@ import type { DiffFile } from '../types';
 import type { AIChatEntry } from '../hooks/useAIChat';
 import type { ReviewSearchMatch } from '../utils/reviewSearch';
 import type { PRMetadata, PRContext } from '@plannotator/shared/pr-provider';
+import type { FeedbackDiffContext } from '../utils/exportFeedback';
 
 /**
  * Shared review state consumed by dockview panel wrappers.
@@ -26,6 +27,15 @@ export interface ReviewState {
   disableBackground?: boolean;
   fontFamily?: string;
   fontSize?: string;
+  /** User-selected base branch; feeds the `base` query param on file-content fetches. */
+  reviewBase?: string;
+  /** Active diff mode (e.g. "branch", "merge-base", "uncommitted"). Used as
+   *  part of the DiffViewer remount key so mode switches invalidate cached
+   *  file content — branch and merge-base compute different "old" sides. */
+  activeDiffBase?: string;
+  /** Diff context baked into exported feedback so downstream panels (agent job
+   * detail, etc.) produce the same markdown the main feedback path sends. */
+  feedbackDiffContext?: FeedbackDiffContext;
 
   // Annotations
   allAnnotations: CodeAnnotation[];
diff --git a/packages/review-editor/dock/panels/ReviewAgentJobDetailPanel.tsx b/packages/review-editor/dock/panels/ReviewAgentJobDetailPanel.tsx
index b0045f69..c775b689 100644
--- a/packages/review-editor/dock/panels/ReviewAgentJobDetailPanel.tsx
+++ b/packages/review-editor/dock/panels/ReviewAgentJobDetailPanel.tsx
@@ -87,9 +87,21 @@ export const ReviewAgentJobDetailPanel: React.FC<IDockviewPanelProps> = (props)
     state.onSelectAnnotation(ann.id);
   }, [state.openDiffFile, state.onSelectAnnotation]);
 
+  // Copy All uses the diff context snapshotted on the JOB at launch, not the
+  // current UI state — so if the reviewer switches modes/bases after the job
+  // ran, the exported markdown still describes the diff the agent actually
+  // analyzed. Falls back to current UI state only if the job predates the
+  // snapshotting (older jobs without diffContext).
   const copyAllText = useMemo(
-    () => activeAnnotations.length > 0 ? exportReviewFeedback(activeAnnotations, state.prMetadata) : '',
-    [activeAnnotations, state.prMetadata]
+    () =>
+      activeAnnotations.length > 0
+        ? exportReviewFeedback(
+            activeAnnotations,
+            state.prMetadata,
+            job?.diffContext ?? state.feedbackDiffContext,
+          )
+        : '',
+    [activeAnnotations, state.prMetadata, job?.diffContext, state.feedbackDiffContext],
   );
 
   const { jobLogs } = useJobLogs();
diff --git a/packages/review-editor/dock/panels/ReviewDiffPanel.tsx b/packages/review-editor/dock/panels/ReviewDiffPanel.tsx
index 75817db5..a5458b66 100644
--- a/packages/review-editor/dock/panels/ReviewDiffPanel.tsx
+++ b/packages/review-editor/dock/panels/ReviewDiffPanel.tsx
@@ -51,12 +51,17 @@ export const ReviewDiffPanel: React.FC<IDockviewPanelProps> = (props) => {
     );
   }
 
+  // Keying on reviewBase forces a remount when the user picks a new base.
+  // Otherwise the file-content fetch for the new base can land before the new
+  // patch, and Pierre briefly reconciles old-patch + new-content → "trailing
+  // context mismatch" warnings in the console.
   return (
-    <div key={file.path} className="h-full relative">
+    <div key={`${file.path}:${state.reviewBase ?? ''}:${state.activeDiffBase ?? ''}`} className="h-full relative">
       <DiffViewer
         patch={file.patch}
         filePath={file.path}
         oldPath={file.oldPath}
+        reviewBase={state.reviewBase}
         isFocused={isFocusedFile}
         diffStyle={state.diffStyle}
         diffOverflow={state.diffOverflow}
diff --git a/packages/review-editor/package.json b/packages/review-editor/package.json
index 5ab3d63a..005b7f02 100644
--- a/packages/review-editor/package.json
+++ b/packages/review-editor/package.json
@@ -12,6 +12,9 @@
     "@plannotator/ui": "workspace:*",
     "@radix-ui/react-checkbox": "^1.3.3",
     "@radix-ui/react-collapsible": "^1.1.12",
+    "@radix-ui/react-dialog": "^1.1.15",
+    "@radix-ui/react-dropdown-menu": "^2.1.15",
+    "@radix-ui/react-popover": "^1.1.15",
     "@radix-ui/react-tooltip": "^1.2.8",
     "highlight.js": "^11.11.1",
     "motion": "^12.38.0",
diff --git a/packages/review-editor/utils/exportFeedback.test.ts b/packages/review-editor/utils/exportFeedback.test.ts
index 442a4b5f..c7037adc 100644
--- a/packages/review-editor/utils/exportFeedback.test.ts
+++ b/packages/review-editor/utils/exportFeedback.test.ts
@@ -52,6 +52,41 @@ describe("exportReviewFeedback", () => {
     expect(result).not.toContain("PR Review");
   });
 
+  it("local mode with diff context: describes mode + base in the header", () => {
+    const result = exportReviewFeedback([ann()], undefined, {
+      mode: "branch",
+      base: "develop",
+    });
+    expect(result).toContain("**Diff:** Branch diff vs `develop`");
+  });
+
+  it("local mode with merge-base: labels PR Diff with the base", () => {
+    const result = exportReviewFeedback([ann()], undefined, {
+      mode: "merge-base",
+      base: "release/v2",
+    });
+    expect(result).toContain("**Diff:** PR Diff vs `release/v2`");
+  });
+
+  it("local mode with worktree path: appends worktree info", () => {
+    const result = exportReviewFeedback([ann()], undefined, {
+      mode: "uncommitted",
+      worktreePath: "/tmp/feature-wt",
+    });
+    expect(result).toContain("**Diff:** Uncommitted changes _(worktree: /tmp/feature-wt)_");
+  });
+
+  it("PR mode ignores diff context (PR header already carries branches)", () => {
+    const result = exportReviewFeedback([ann()], prMeta, {
+      mode: "branch",
+      base: "develop",
+    });
+    // The PR-style branches line must appear.
+    expect(result).toContain("Branch: `fix/widget` → `main`");
+    // The local-mode Diff line must not.
+    expect(result).not.toContain("**Diff:**");
+  });
+
   it("PR mode: includes all PR context fields", () => {
     const result = exportReviewFeedback([ann()], prMeta);
     expect(result).toStartWith("# PR Review: acme/widgets#42\n\n");
diff --git a/packages/review-editor/utils/exportFeedback.ts b/packages/review-editor/utils/exportFeedback.ts
index 7e298626..ed94df41 100644
--- a/packages/review-editor/utils/exportFeedback.ts
+++ b/packages/review-editor/utils/exportFeedback.ts
@@ -16,15 +16,47 @@ export function formatConventionalPrefix(
   return `**${label}${decs}:** `;
 }
 
+/**
+ * Describes what the reviewer was looking at in local-review mode — diff mode,
+ * optional base branch, optional worktree. Threaded into the feedback header so
+ * the receiving agent knows which diff the annotations are anchored to. Ignored
+ * in PR mode, where `prMeta` already carries equivalent context.
+ */
+export interface FeedbackDiffContext {
+  mode: string;
+  base?: string;
+  worktreePath?: string | null;
+}
+
+function describeDiff(ctx: FeedbackDiffContext): string {
+  const { mode, base, worktreePath } = ctx;
+  let label: string;
+  switch (mode) {
+    case "uncommitted":  label = "Uncommitted changes"; break;
+    case "staged":       label = "Staged changes"; break;
+    case "unstaged":     label = "Unstaged changes"; break;
+    case "last-commit":  label = "Last commit"; break;
+    case "branch":       label = base ? `Branch diff vs \`${base}\`` : "Branch diff"; break;
+    case "merge-base":   label = base ? `PR Diff vs \`${base}\`` : "PR Diff"; break;
+    default:             label = mode; // p4-* or anything else — show raw
+  }
+  return worktreePath ? `${label} _(worktree: ${worktreePath})_` : label;
+}
+
 /**
  * Build markdown feedback from code review annotations.
  *
  * In PR mode (prMeta provided), the header includes repo, PR number,
  * title, branches, and URL so the receiving agent has full context.
+ *
+ * In local mode, an optional diffContext adds one line describing which
+ * diff the reviewer was looking at — otherwise the agent only sees file
+ * paths and line numbers and has to guess which diff those anchor to.
  */
 export function exportReviewFeedback(
   annotations: CodeAnnotation[],
   prMeta?: PRMetadata | null,
+  diffContext?: FeedbackDiffContext,
 ): string {
   if (annotations.length === 0) {
     return '# Code Review\n\nNo feedback provided.';
@@ -42,7 +74,7 @@ export function exportReviewFeedback(
       `**${prMeta.title}**\n` +
       `Branch: \`${prMeta.headBranch}\` → \`${prMeta.baseBranch}\`\n` +
       `${prMeta.url}\n\n`
-    : '# Code Review Feedback\n\n';
+    : `# Code Review Feedback\n\n${diffContext ? `**Diff:** ${describeDiff(diffContext)}\n\n` : ''}`;
 
   for (const [filePath, fileAnnotations] of grouped) {
     output += `## ${filePath}\n\n`;
diff --git a/packages/server/agent-jobs.ts b/packages/server/agent-jobs.ts
index 584491e0..c4917c7d 100644
--- a/packages/server/agent-jobs.ts
+++ b/packages/server/agent-jobs.ts
@@ -81,6 +81,8 @@ export interface AgentJobHandlerOptions {
     reasoningEffort?: string;
     /** Whether Codex fast mode was enabled. */
     fastMode?: boolean;
+    /** Diff context snapshot at launch (stored on AgentJobInfo for per-job "Copy All"). */
+    diffContext?: AgentJobInfo["diffContext"];
   } | null>;
   /**
    * Called after a job process exits with exit code 0.
@@ -130,7 +132,7 @@ export function createAgentJobHandler(options: AgentJobHandlerOptions): AgentJob
     command: string[],
     label: string,
     outputPath?: string,
-    spawnOptions?: { captureStdout?: boolean; stdinPrompt?: string; cwd?: string; prompt?: string; engine?: string; model?: string; effort?: string; reasoningEffort?: string; fastMode?: boolean },
+    spawnOptions?: { captureStdout?: boolean; stdinPrompt?: string; cwd?: string; prompt?: string; engine?: string; model?: string; effort?: string; reasoningEffort?: string; fastMode?: boolean; diffContext?: AgentJobInfo["diffContext"] },
   ): AgentJobInfo {
     const id = crypto.randomUUID();
     const source = jobSource(id);
@@ -149,6 +151,7 @@ export function createAgentJobHandler(options: AgentJobHandlerOptions): AgentJob
       ...(spawnOptions?.effort && { effort: spawnOptions.effort }),
       ...(spawnOptions?.reasoningEffort && { reasoningEffort: spawnOptions.reasoningEffort }),
       ...(spawnOptions?.fastMode && { fastMode: spawnOptions.fastMode }),
+      ...(spawnOptions?.diffContext && { diffContext: spawnOptions.diffContext }),
     };
 
     let proc: ReturnType<typeof Bun.spawn> | null = null;
@@ -445,6 +448,7 @@ export function createAgentJobHandler(options: AgentJobHandlerOptions): AgentJob
           let jobEffort: string | undefined;
           let jobReasoningEffort: string | undefined;
           let jobFastMode: boolean | undefined;
+          let jobDiffContext: AgentJobInfo["diffContext"] | undefined;
           if (options.buildCommand) {
             // Thread config from POST body to buildCommand
             const config: Record<string, unknown> = {};
@@ -467,6 +471,7 @@ export function createAgentJobHandler(options: AgentJobHandlerOptions): AgentJob
               jobEffort = built.effort;
               jobReasoningEffort = built.reasoningEffort;
               jobFastMode = built.fastMode;
+              jobDiffContext = built.diffContext;
             }
           }
 
@@ -487,6 +492,7 @@ export function createAgentJobHandler(options: AgentJobHandlerOptions): AgentJob
             effort: jobEffort,
             reasoningEffort: jobReasoningEffort,
             fastMode: jobFastMode,
+            diffContext: jobDiffContext,
           });
           return Response.json({ job }, { status: 201 });
         } catch {
diff --git a/packages/server/codex-review.ts b/packages/server/codex-review.ts
index 559f044c..abef52f6 100644
--- a/packages/server/codex-review.ts
+++ b/packages/server/codex-review.ts
@@ -206,6 +206,11 @@ export function buildCodexReviewUserMessage(
       return `Review the code changes against the base branch '${base}'. Run \`git diff ${base}..HEAD\` to inspect the changes. Provide prioritized, actionable findings.`;
     }
 
+    case "merge-base": {
+      const base = options?.defaultBranch || "main";
+      return `Review the PR-style diff against base '${base}'. First find the common ancestor with \`git merge-base ${base} HEAD\`, then run \`git diff <merge-base>..HEAD\` using that commit to inspect only the changes introduced on this branch (matches GitHub's PR view). Provide prioritized, actionable findings.`;
+    }
+
     default:
       // p4 or unknown — fall back to generic with inlined diff
       return [
diff --git a/packages/server/p4.ts b/packages/server/p4.ts
index 6b84a371..c9a0b199 100644
--- a/packages/server/p4.ts
+++ b/packages/server/p4.ts
@@ -125,6 +125,7 @@ export async function getP4Context(cwd?: string): Promise<GitContext> {
       defaultBranch: "",
       diffOptions: [],
       worktrees: [],
+      availableBranches: { local: [], remote: [] },
       cwd,
       vcsType: "p4",
     };
@@ -174,6 +175,7 @@ export async function getP4Context(cwd?: string): Promise<GitContext> {
     defaultBranch: "",
     diffOptions,
     worktrees: [],
+    availableBranches: { local: [], remote: [] },
     cwd: cwd ?? workspace.clientRoot,
     vcsType: "p4",
   };
diff --git a/packages/server/review.ts b/packages/server/review.ts
index 2b9cbb04..f14306c0 100644
--- a/packages/server/review.ts
+++ b/packages/server/review.ts
@@ -11,7 +11,10 @@
 
 import { isRemoteSession, getServerHostname, getServerPort } from "./remote";
 import type { Origin } from "@plannotator/shared/agents";
-import { type DiffType, type GitContext, runVcsDiff, getVcsFileContentsForDiff, canStageFiles, stageFile, unstageFile, resolveVcsCwd, validateFilePath } from "./vcs";
+import { type DiffType, type GitContext, runVcsDiff, getVcsFileContentsForDiff, canStageFiles, stageFile, unstageFile, resolveVcsCwd, validateFilePath, getVcsContext } from "./vcs";
+import { parseWorktreeDiffType } from "@plannotator/shared/review-core";
+import type { AgentJobInfo } from "@plannotator/shared/agent-jobs";
+import { resolveBaseBranch } from "@plannotator/shared/review-core";
 import { getRepoInfo } from "./repo";
 import { handleImage, handleUpload, handleAgents, handleServerReady, handleDraftSave, handleDraftLoad, handleDraftDelete, handleFavicon, type OpencodeClient } from "./shared-handlers";
 import { contentHash, deleteDraft } from "./draft";
@@ -62,6 +65,14 @@ export interface ReviewServerOptions {
   diffType?: DiffType;
   /** Git context with branch info and available diff options */
   gitContext?: GitContext;
+  /**
+   * Initial base branch the caller used to compute `rawPatch`. When a caller
+   * overrides the detected default (e.g. Pi's `openCodeReview` accepting a
+   * custom `defaultBranch`), this must be forwarded so the server's internal
+   * `currentBase` state, the `/api/diff` response, and downstream agent
+   * prompts stay consistent with the patch that's already on screen.
+   */
+  initialBase?: string;
   /** Whether URL sharing is enabled (default: true) */
   sharingEnabled?: boolean;
   /** Custom base URL for share links (default: https://share.plannotator.ai) */
@@ -128,6 +139,11 @@ export async function startReviewServer(
   let currentGitRef = options.gitRef;
   let currentDiffType: DiffType = options.diffType || "uncommitted";
   let currentError = options.error;
+  // Tracks the base branch the user picked from the UI. Agent review prompts
+  // read this (not gitContext.defaultBranch) so they analyze the same diff
+  // the reviewer is currently looking at. Honors an explicit initialBase from
+  // the caller — e.g. programmatic Pi callers can request a non-detected base.
+  let currentBase = options.initialBase || gitContext?.defaultBranch || "main";
 
   // Agent jobs — background process manager (late-binds serverUrl via getter)
   let serverUrl = "";
@@ -141,10 +157,25 @@ export async function startReviewServer(
     async buildCommand(provider, config) {
       const cwd = resolveAgentCwd();
       const hasAgentLocalAccess = !!options.agentCwd || !!gitContext;
-      const userMessageOptions = { defaultBranch: gitContext?.defaultBranch, hasLocalAccess: hasAgentLocalAccess };
+      const userMessageOptions = { defaultBranch: currentBase, hasLocalAccess: hasAgentLocalAccess };
+
+      // Snapshot the diff context at launch — stored on the job so
+      // downstream "Copy All" produces the same markdown as /api/feedback
+      // would right now, even if the reviewer switches modes/bases later.
+      // Skipped in PR mode (prMetadata carries equivalent context).
+      const worktreeParts = currentDiffType.startsWith("worktree:")
+        ? parseWorktreeDiffType(currentDiffType)
+        : null;
+      const diffContext: AgentJobInfo["diffContext"] | undefined = prMetadata
+        ? undefined
+        : {
+            mode: (worktreeParts?.subType ?? currentDiffType) as string,
+            base: currentBase,
+            worktreePath: worktreeParts?.path ?? null,
+          };
 
       if (provider === "tour") {
-        return tour.buildCommand({
+        const built = await tour.buildCommand({
           cwd,
           patch: currentPatch,
           diffType: currentDiffType,
@@ -152,6 +183,7 @@ export async function startReviewServer(
           prMetadata,
           config,
         });
+        return built ? { ...built, diffContext } : built;
       }
 
       const userMessage = buildCodexReviewUserMessage(currentPatch, currentDiffType, userMessageOptions, prMetadata);
@@ -163,7 +195,7 @@ export async function startReviewServer(
         const outputPath = generateOutputPath();
         const prompt = CODEX_REVIEW_SYSTEM_PROMPT + "\n\n---\n\n" + userMessage;
         const command = await buildCodexCommand({ cwd, outputPath, prompt, model, reasoningEffort, fastMode });
-        return { command, outputPath, prompt, label: "Code Review", model, reasoningEffort, fastMode: fastMode || undefined };
+        return { command, outputPath, prompt, label: "Code Review", model, reasoningEffort, fastMode: fastMode || undefined, diffContext };
       }
 
       if (provider === "claude") {
@@ -171,7 +203,7 @@ export async function startReviewServer(
         const effort = typeof config?.effort === "string" && config.effort ? config.effort : undefined;
         const prompt = CLAUDE_REVIEW_PROMPT + "\n\n---\n\n" + userMessage;
         const { command, stdinPrompt } = buildClaudeCommand(prompt, model, effort);
-        return { command, stdinPrompt, prompt, cwd, label: "Code Review", captureStdout: true, model, effort };
+        return { command, stdinPrompt, prompt, cwd, label: "Code Review", captureStdout: true, model, effort, diffContext };
       }
 
       return null;
@@ -412,6 +444,10 @@ export async function startReviewServer(
               gitRef: currentGitRef,
               origin,
               diffType: hasLocalAccess ? currentDiffType : undefined,
+              // Echo the active base so a page refresh or reconnect rehydrates
+              // the picker to what the server is actually using — not the
+              // detected default.
+              base: hasLocalAccess ? currentBase : undefined,
               gitContext: hasLocalAccess ? gitContext : undefined,
               sharingEnabled,
               shareBaseUrl,
@@ -434,7 +470,7 @@ export async function startReviewServer(
               );
             }
             try {
-              const body = (await req.json()) as { diffType: DiffType };
+              const body = (await req.json()) as { diffType: DiffType; base?: string };
               let newDiffType = body.diffType;
 
               if (!newDiffType) {
@@ -444,22 +480,49 @@ export async function startReviewServer(
                 );
               }
 
-              const defaultBranch = gitContext?.defaultBranch || "main";
+              const detectedBase = gitContext?.defaultBranch || "main";
+              // Guard against non-string payloads — resolveBaseBranch calls
+              // string methods and would throw a TypeError otherwise. Mirrors
+              // Pi's guard so both runtimes validate identically.
+              const requestedBase = typeof body.base === "string" ? body.base : undefined;
+              const base = resolveBaseBranch(requestedBase, detectedBase);
               const defaultCwd = gitContext?.cwd;
 
               // Run the new diff
-              const result = await runVcsDiff(newDiffType, defaultBranch, defaultCwd);
+              const result = await runVcsDiff(newDiffType, base, defaultCwd);
 
               // Update state
               currentPatch = result.patch;
               currentGitRef = result.label;
               currentDiffType = newDiffType;
+              currentBase = base;
               currentError = result.error;
 
+              // Recompute gitContext for the effective cwd so the client's
+              // sidebar (current branch, default branch, diff-mode options)
+              // reflects the worktree we're now reviewing — not the main
+              // repo's startup state. Best-effort: on failure the client
+              // keeps its existing context.
+              let updatedContext: GitContext | undefined;
+              if (gitContext) {
+                try {
+                  const effectiveCwd = resolveVcsCwd(newDiffType, gitContext.cwd);
+                  updatedContext = await getVcsContext(effectiveCwd);
+                } catch {
+                  /* best-effort */
+                }
+              }
+
               return Response.json({
                 rawPatch: currentPatch,
                 gitRef: currentGitRef,
                 diffType: currentDiffType,
+                // Echo the base the server actually used. resolveBaseBranch
+                // trusts the caller verbatim; this echo lets the client
+                // confirm the request landed (and pick it up when the client
+                // didn't supply one and we fell back to detected default).
+                base: currentBase,
+                ...(updatedContext && { gitContext: updatedContext }),
                 ...(currentError && { error: currentError }),
               });
             } catch (err) {
@@ -505,11 +568,15 @@ export async function startReviewServer(
 
             // Local review: read file contents from local git
             if (hasLocalAccess) {
-              const defaultBranch = gitContext?.defaultBranch || "main";
+              const detectedBase = gitContext?.defaultBranch || "main";
+              const base = resolveBaseBranch(
+                url.searchParams.get("base") ?? undefined,
+                detectedBase,
+              );
               const defaultCwd = gitContext?.cwd;
               const result = await getVcsFileContentsForDiff(
                 currentDiffType,
-                defaultBranch,
+                base,
                 filePath,
                 oldPath,
                 defaultCwd,
diff --git a/packages/server/tour/tour-review.ts b/packages/server/tour/tour-review.ts
index bb71e12b..36d45128 100644
--- a/packages/server/tour/tour-review.ts
+++ b/packages/server/tour/tour-review.ts
@@ -322,6 +322,10 @@ function buildTourUserMessage(
       const base = options?.defaultBranch || "main";
       return `Walk the reviewer through the code changes against the base branch '${base}' as a guided tour. Run \`git diff ${base}..HEAD\` to inspect the changes.`;
     }
+    case "merge-base": {
+      const base = options?.defaultBranch || "main";
+      return `Walk the reviewer through the PR-style diff against base '${base}' as a guided tour. First find the common ancestor with \`git merge-base ${base} HEAD\`, then run \`git diff <merge-base>..HEAD\` using that commit to inspect only the changes introduced on this branch (matches GitHub's PR view).`;
+    }
     default:
       return [
         "Walk the reviewer through the following code changes as a guided tour.",
diff --git a/packages/shared/agent-jobs.ts b/packages/shared/agent-jobs.ts
index 20817e70..84283e83 100644
--- a/packages/shared/agent-jobs.ts
+++ b/packages/shared/agent-jobs.ts
@@ -14,6 +14,20 @@
 
 export type AgentJobStatus = "starting" | "running" | "done" | "failed" | "killed";
 
+/**
+ * Snapshot of the diff the reviewer was looking at when this job was launched.
+ * Carried on the job so downstream UIs (agent-result panel "Copy All") export
+ * the same `**Diff:** ...` header the job was actually run against — if the
+ * reviewer switches the UI to a different diff afterwards, the job's snapshot
+ * still reflects truth. Structurally compatible with the UI-side
+ * `FeedbackDiffContext` in `packages/review-editor/utils/exportFeedback.ts`.
+ */
+export interface AgentJobDiffContext {
+  mode: string;
+  base?: string;
+  worktreePath?: string | null;
+}
+
 export interface AgentJobInfo {
   /** Unique job identifier (UUID). */
   id: string;
@@ -55,6 +69,8 @@ export interface AgentJobInfo {
     explanation: string;
     confidence: number;
   };
+  /** Diff context at launch time (see AgentJobDiffContext). */
+  diffContext?: AgentJobDiffContext;
 }
 
 export interface AgentCapability {
diff --git a/packages/shared/review-core.test.ts b/packages/shared/review-core.test.ts
index f08927e7..082552e1 100644
--- a/packages/shared/review-core.test.ts
+++ b/packages/shared/review-core.test.ts
@@ -4,9 +4,12 @@ import { mkdirSync, mkdtempSync, readFileSync, rmSync, writeFileSync } from "nod
 import { tmpdir } from "node:os";
 import { join, resolve as resolvePath } from "node:path";
 import {
+  getDefaultBranch,
   getFileContentsForDiff,
   getGitContext,
+  parseWorktreeDiffType,
   runGitDiff,
+  type DiffType,
   type ReviewGitRuntime,
 } from "./review-core";
 
@@ -159,7 +162,11 @@ describe("review-core", () => {
 
     expect(result.patch).toBe("");
     expect(result.label).toBe("Error: branch");
-    expect(result.error).toContain("git diff --no-ext-diff master..HEAD");
+    // Error is derived from the argv — assert the meaningful parts rather
+    // than the exact string so harmless argv reorders (e.g. --end-of-options)
+    // don't break it.
+    expect(result.error).toContain("git diff");
+    expect(result.error).toContain("master..HEAD");
   });
 
   test("git context lists worktrees and file content lookup returns old/new content", async () => {
@@ -199,4 +206,41 @@ describe("review-core", () => {
     expect(newFileContents.oldContent).toBeNull();
     expect(newFileContents.newContent).toBe("brand new\n");
   });
+
+  test("getDefaultBranch falls back to local when origin/HEAD points at an unfetched ref", () => {
+    // Simulates a narrow / partial clone where origin/HEAD is configured but
+    // the target ref was never fetched. Before the verify step, the server
+    // would return "origin/phantom" and every branch/merge-base diff would
+    // fail with "unknown revision". With the verify step we fall back to
+    // local main.
+    const repoDir = initRepo();
+
+    // Manually set origin/HEAD → origin/phantom without ever fetching it.
+    git(repoDir, ["symbolic-ref", "refs/remotes/origin/HEAD", "refs/remotes/origin/phantom"]);
+
+    const runtime = makeRuntime(repoDir);
+    return getDefaultBranch(runtime).then((result) => {
+      expect(result).toBe("main");
+    });
+  });
+
+  test("parseWorktreeDiffType recognises every DiffType suffix, including merge-base", () => {
+    // Regression guard: every local diff type must round-trip through the
+    // worktree-prefixed form. Missing `merge-base` here previously routed
+    // "worktree:/path:merge-base" to { path: "/path:merge-base", subType: "uncommitted" }
+    // which pointed git at a non-existent cwd and silently collapsed the diff mode.
+    const subTypes = [
+      "uncommitted",
+      "staged",
+      "unstaged",
+      "last-commit",
+      "branch",
+      "merge-base",
+    ] as const;
+    for (const sub of subTypes) {
+      const composite = `worktree:/tmp/my-worktree:${sub}` as DiffType;
+      const parsed = parseWorktreeDiffType(composite);
+      expect(parsed).toEqual({ path: "/tmp/my-worktree", subType: sub });
+    }
+  });
 });
diff --git a/packages/shared/review-core.ts b/packages/shared/review-core.ts
index 1a719513..c9b4c3a5 100644
--- a/packages/shared/review-core.ts
+++ b/packages/shared/review-core.ts
@@ -29,11 +29,17 @@ export interface WorktreeInfo {
   head: string;
 }
 
+export interface AvailableBranches {
+  local: string[];
+  remote: string[];
+}
+
 export interface GitContext {
   currentBranch: string;
   defaultBranch: string;
   diffOptions: DiffOption[];
   worktrees: WorktreeInfo[];
+  availableBranches: AvailableBranches;
   cwd?: string;
   vcsType?: "git" | "p4";
 }
@@ -73,13 +79,27 @@ export async function getDefaultBranch(
   runtime: ReviewGitRuntime,
   cwd?: string,
 ): Promise<string> {
+  // Prefer the remote tracking ref (e.g. `origin/main`) so diffs run against
+  // the upstream tip, not a potentially stale local copy. Only fall back to
+  // a local ref when there's no remote configured at all.
   const remoteHead = await runtime.runGit(
     ["symbolic-ref", "refs/remotes/origin/HEAD"],
     { cwd },
   );
   if (remoteHead.exitCode === 0) {
     const ref = remoteHead.stdout.trim();
-    if (ref) return ref.replace("refs/remotes/origin/", "");
+    if (ref) {
+      // `symbolic-ref` only tells us what origin/HEAD *points at* — it does
+      // not guarantee that the target ref was actually fetched. In narrow
+      // or partial clones the pointer can be set while the target is
+      // missing, in which case a later `git diff origin/main..HEAD` would
+      // error. Verify the target exists before trusting it.
+      const verify = await runtime.runGit(
+        ["show-ref", "--verify", "--quiet", ref],
+        { cwd },
+      );
+      if (verify.exitCode === 0) return ref.replace("refs/remotes/", "");
+    }
   }
 
   const mainBranch = await runtime.runGit(
@@ -91,6 +111,66 @@ export async function getDefaultBranch(
   return "master";
 }
 
+export async function listBranches(
+  runtime: ReviewGitRuntime,
+  cwd?: string,
+): Promise<AvailableBranches> {
+  // Emit `<full-refname>\t<short-name>` so we can classify by ref prefix
+  // without guessing from the short form — local branches can contain `/`
+  // (e.g. `feature/foo`), so `name.includes("/")` would misclassify them.
+  const result = await runtime.runGit(
+    [
+      "for-each-ref",
+      "--format=%(refname)\t%(refname:short)",
+      "refs/heads",
+      "refs/remotes",
+    ],
+    { cwd },
+  );
+  if (result.exitCode !== 0) return { local: [], remote: [] };
+
+  const local: string[] = [];
+  const remote: string[] = [];
+
+  for (const line of result.stdout.split("\n")) {
+    const [fullRef, shortName] = line.split("\t");
+    if (!fullRef || !shortName) continue;
+    if (shortName.endsWith("/HEAD")) continue;
+    if (fullRef.startsWith("refs/heads/")) {
+      local.push(shortName);
+    } else if (fullRef.startsWith("refs/remotes/")) {
+      remote.push(shortName);
+    }
+  }
+
+  // Keep both local and remote refs — they can point to different commits
+  // (stale local tracking branches are common) and users need to be able to
+  // pick either explicitly. The picker groups them separately for clarity.
+  local.sort();
+  remote.sort();
+
+  return { local, remote };
+}
+
+/**
+ * Pick a safe base branch. Trusts the caller verbatim if they supplied one,
+ * otherwise falls back to the detected default. Shared by Bun (`review.ts`)
+ * and Pi (`serverReview.ts`) so both runtimes behave identically.
+ *
+ * Why trust the caller: the UI picker only ever sends refs from the known
+ * list, and external/programmatic callers may pass tags, SHAs, or refs under
+ * non-`origin` remotes that we must not silently rewrite (a tag `release` is
+ * not the same commit as a branch `origin/release`). Invalid refs surface as
+ * git errors on the next diff call, which is better than silently producing
+ * a patch against the wrong commit.
+ */
+export function resolveBaseBranch(
+  requested: string | undefined,
+  detected: string,
+): string {
+  return requested || detected;
+}
+
 export async function getWorktrees(
   runtime: ReviewGitRuntime,
   cwd?: string,
@@ -137,9 +217,10 @@ export async function getGitContext(
   runtime: ReviewGitRuntime,
   cwd?: string,
 ): Promise<GitContext> {
-  const [currentBranch, defaultBranch] = await Promise.all([
+  const [currentBranch, defaultBranch, availableBranches] = await Promise.all([
     getCurrentBranch(runtime, cwd),
     getDefaultBranch(runtime, cwd),
+    listBranches(runtime, cwd),
   ]);
 
   const diffOptions: DiffOption[] = [
@@ -149,7 +230,16 @@ export async function getGitContext(
     { id: "last-commit", label: "Last commit" },
   ];
 
-  if (currentBranch !== defaultBranch) {
+  // Always offer Branch diff / PR Diff when a default branch exists. The
+  // older guard hid them when the reviewer was on the default branch (the
+  // `vs <default>` diff from the default branch itself is always empty), but
+  // the base picker now lets reviewers compare against any branch from any
+  // branch, so there's no meaningless-by-construction option. Also: preserving
+  // diff mode across worktree switches and Pi's `initialBase` can land the
+  // reviewer on the default branch with branch/merge-base already active — the
+  // old guard hid the active mode's option, trapping them. Unconditional
+  // emission keeps the active option reachable in every flow.
+  if (defaultBranch) {
     diffOptions.push({ id: "branch", label: `vs ${defaultBranch}` });
     diffOptions.push({ id: "merge-base", label: `Current PR Diff` });
   }
@@ -169,6 +259,7 @@ export async function getGitContext(
     defaultBranch,
     diffOptions,
     worktrees: worktrees.filter((wt) => wt.path !== currentTreePath),
+    availableBranches,
     cwd,
   };
 }
@@ -245,6 +336,7 @@ const WORKTREE_SUB_TYPES = new Set([
   "unstaged",
   "last-commit",
   "branch",
+  "merge-base",
 ]);
 
 export function parseWorktreeDiffType(
@@ -371,12 +463,17 @@ export async function runGitDiff(
       }
 
       case "branch": {
+        // `--end-of-options` hardens against a caller-supplied `defaultBranch`
+        // that starts with `-` being parsed as a git flag (e.g. `--output=...`
+        // would redirect diff output to an attacker-chosen path). Same pattern
+        // applied wherever user-controlled refs flow into a git argv.
         const branchDiffArgs = [
           "diff",
           "--no-ext-diff",
-          `${defaultBranch}..HEAD`,
           "--src-prefix=a/",
           "--dst-prefix=b/",
+          "--end-of-options",
+          `${defaultBranch}..HEAD`,
         ];
         const branchDiff = assertGitSuccess(
           await runtime.runGit(branchDiffArgs, { cwd }),
@@ -388,17 +485,19 @@ export async function runGitDiff(
       }
 
       case "merge-base": {
+        const mergeBaseLookupArgs = ["merge-base", "--end-of-options", defaultBranch, "HEAD"];
         const mergeBaseResult = assertGitSuccess(
-          await runtime.runGit(["merge-base", defaultBranch, "HEAD"], { cwd }),
-          ["merge-base", defaultBranch, "HEAD"],
+          await runtime.runGit(mergeBaseLookupArgs, { cwd }),
+          mergeBaseLookupArgs,
         );
         const mergeBase = mergeBaseResult.stdout.trim();
         const mergeBaseDiffArgs = [
           "diff",
           "--no-ext-diff",
-          `${mergeBase}..HEAD`,
           "--src-prefix=a/",
           "--dst-prefix=b/",
+          "--end-of-options",
+          `${mergeBase}..HEAD`,
         ];
         const mergeBaseDiff = assertGitSuccess(
           await runtime.runGit(mergeBaseDiffArgs, { cwd }),
@@ -463,7 +562,8 @@ export async function getFileContentsForDiff(
   }
 
   async function gitShow(ref: string, path: string): Promise<string | null> {
-    const result = await runtime.runGit(["show", `${ref}:${path}`], { cwd });
+    // `--end-of-options` hardens against user-supplied refs starting with `-`.
+    const result = await runtime.runGit(["show", "--end-of-options", `${ref}:${path}`], { cwd });
     return result.exitCode === 0 ? result.stdout : null;
   }
 
@@ -499,7 +599,7 @@ export async function getFileContentsForDiff(
         newContent: await gitShow("HEAD", filePath),
       };
     case "merge-base": {
-      const mbResult = await runtime.runGit(["merge-base", defaultBranch, "HEAD"], { cwd });
+      const mbResult = await runtime.runGit(["merge-base", "--end-of-options", defaultBranch, "HEAD"], { cwd });
       const mb = mbResult.exitCode === 0 ? mbResult.stdout.trim() : defaultBranch;
       return {
         oldContent: await gitShow(mb, oldFilePath),
diff --git a/packages/shared/types.ts b/packages/shared/types.ts
index d12a50d0..e6f24164 100644
--- a/packages/shared/types.ts
+++ b/packages/shared/types.ts
@@ -14,4 +14,5 @@ export type {
   DiffOption,
   WorktreeInfo,
   GitContext,
+  AvailableBranches,
 } from "./review-core";
diff --git a/packages/ui/components/AgentsTab.tsx b/packages/ui/components/AgentsTab.tsx
index d82c2c7e..1f9c1cd5 100644
--- a/packages/ui/components/AgentsTab.tsx
+++ b/packages/ui/components/AgentsTab.tsx
@@ -21,6 +21,7 @@ const CLAUDE_EFFORT: Array<{ value: string; label: string }> = [
 ];
 
 const CODEX_MODELS: Array<{ value: string; label: string }> = [
+  { value: 'gpt-5.5', label: 'GPT-5.5' },
   { value: 'gpt-5.4', label: 'GPT-5.4' },
   { value: 'gpt-5.3-codex', label: 'GPT-5.3 Codex' },
   { value: 'gpt-5.3-codex-spark', label: 'GPT-5.3 Codex Spark' },
diff --git a/packages/ui/components/Tooltip.tsx b/packages/ui/components/Tooltip.tsx
index 782fbf2b..89d28bb6 100644
--- a/packages/ui/components/Tooltip.tsx
+++ b/packages/ui/components/Tooltip.tsx
@@ -10,6 +10,12 @@ interface TooltipProps {
   align?: 'start' | 'center' | 'end';
   delayDuration?: number;
   sideOffset?: number;
+  /**
+   * When true, allow the tooltip to wrap onto multiple lines with a reasonable
+   * max width. Default is single-line (nowrap) — matches the original callsites
+   * that use it for short button labels.
+   */
+  wide?: boolean;
 }
 
 export const Tooltip: React.FC<TooltipProps> = ({
@@ -19,6 +25,7 @@ export const Tooltip: React.FC<TooltipProps> = ({
   align = 'center',
   delayDuration,
   sideOffset = 8,
+  wide = false,
 }) => (
   <RadixTooltip.Root delayDuration={delayDuration}>
     <RadixTooltip.Trigger asChild>{children}</RadixTooltip.Trigger>
@@ -27,7 +34,9 @@ export const Tooltip: React.FC<TooltipProps> = ({
         side={side}
         align={align}
         sideOffset={sideOffset}
-        className="z-50 px-2 py-1 text-xs bg-popover text-popover-foreground border border-border rounded shadow-md whitespace-nowrap origin-[var(--radix-tooltip-content-transform-origin)] transition-[opacity,transform] duration-150 ease-out data-[state=closed]:opacity-0 data-[state=closed]:scale-95 data-[state=delayed-open]:opacity-100 data-[state=delayed-open]:scale-100 data-[state=instant-open]:opacity-100 data-[state=instant-open]:scale-100"
+        className={`z-50 px-2 py-1 text-xs bg-popover text-popover-foreground border border-border rounded shadow-md origin-[var(--radix-tooltip-content-transform-origin)] transition-[opacity,transform] duration-150 ease-out data-[state=closed]:opacity-0 data-[state=closed]:scale-95 data-[state=delayed-open]:opacity-100 data-[state=delayed-open]:scale-100 data-[state=instant-open]:opacity-100 data-[state=instant-open]:scale-100 ${
+          wide ? 'max-w-[260px] leading-snug whitespace-normal' : 'whitespace-nowrap'
+        }`}
       >
         {content}
       </RadixTooltip.Content>
diff --git a/packages/ui/package.json b/packages/ui/package.json
index c994e8ba..da9a201e 100644
--- a/packages/ui/package.json
+++ b/packages/ui/package.json
@@ -17,6 +17,7 @@
     "@plannotator/shared": "workspace:*",
     "@plannotator/web-highlighter": "^0.8.1",
     "@radix-ui/react-dialog": "^1.1.15",
+    "@radix-ui/react-popover": "^1.1.15",
     "@radix-ui/react-tooltip": "^1.2.8",
     "@tanstack/react-table": "^8.21.3",
     "@viz-js/viz": "^3.25.0",