vercel__eve/docs/agent-config.md

title, description

title	description
Agents	Configure an eve agent's model, reasoning effort, compaction, limits, and runtime behavior in agent.ts.

An eve app has one root agent assembled from the files under agent/. Its optional agent.ts calls defineAgent (from eve) when you need to configure the model or other runtime behavior. Declared subagents have their own agent.ts and capabilities; this page covers the configuration shared by root agents and subagents.

Set the model

A typical config selects a model:

import { defineAgent } from "eve";

export default defineAgent({
  model: "anthropic/claude-opus-4.8",
});

For a static AI Gateway model ID, you can make the same source change from the project root with eve set --model anthropic/claude-opus-4.8 or from the local dev TUI with /model anthropic/claude-opus-4.8.

The root agent.ts can be omitted when no runtime config is needed. eve then selects its default agent.ts source at the same slot, configured with openai/gpt-5.6-luna-fast; authoring the file replaces that source. When agent.ts is present, model is required.

A config that selects a static Gateway model is compile-only. A config that contains a dynamic model or a direct-provider LanguageModel remains a runtime entry because eve must resolve that authored value while the agent runs. See Authored module lifecycle.

model accepts a gateway model id string, which routes through the Vercel AI Gateway. To call a provider directly and configure the model in code, pass a provider-authored LanguageModel.

Use eve's helpers for direct OpenAI or Anthropic access without installing another provider package:

import { defineAgent } from "eve";
import { anthropic } from "eve/models/anthropic";

export default defineAgent({
  model: anthropic(), // claude-sonnet-5
});

openai() from eve/models/openai defaults to gpt-5.6-luna-fast. Both helpers accept an optional native provider model ID and use OPENAI_API_KEY or ANTHROPIC_API_KEY. During local development they can also use credentials saved through /login. Deployments require their API key in the server environment.

For a local ChatGPT subscription, use chatgpt() from eve/models/openai and sign in with /login. It defaults to gpt-5.6-luna-fast and cannot run in a deployment.

/login can switch a static Gateway string to an eve helper and manage its import. /model changes the selected model and settings immediately. Custom provider SDK calls and dynamic expressions retain their authored behavior and require manual source edits. You can still install an AI SDK provider package and pass its LanguageModel when you need provider-specific configuration.

Model use is subject to the terms, data-processing commitments, retention behavior, and available controls of the selected provider and routing path. Review the AI Gateway model catalog for gateway-routed models, and review the provider's terms when you configure a direct LanguageModel.

For every OpenAI or Anthropic model call, eve fills the provider's end-user safety identifier from the active turn's auth.current principal when you have not configured it. For OpenAI, the option is providerOptions.openai.safetyIdentifier; for Anthropic, it is providerOptions.anthropic.metadata.userId. The default value is a SHA-256 fingerprint of the principal's authenticator, issuer, type, id, and subject; eve does not send the raw principal fields or attributes. The fingerprint follows the current caller when a later turn changes users. An authored value at either provider path takes precedence and is forwarded unchanged. When auth.current is null, eve does not add an identifier. The same rules apply to compaction calls.

Choose the model dynamically

To select a model from the incoming prompt with an AI SDK evaluation model, use auto from eve/models.

model also accepts defineDynamic({ events }). Each matching handler must return the concrete model for its scope; a dynamic model has no compiled default.

import { defineAgent, defineDynamic } from "eve";

export default defineAgent({
  model: defineDynamic({
    events: {
      "session.started": (_event, ctx) => {
        if (ctx.session.auth.initiator?.attributes.plan === "enterprise") {
          return "anthropic/claude-opus-4.8";
        }

        return "anthropic/claude-sonnet-5";
      },
    },
  }),
});

Handlers receive the shared dynamic resolver context and return a gateway model id, an AI SDK LanguageModel, or a selection object. Returning null or undefined fails the turn.

• Scopes. session.started (once per session), turn.started (once per turn), step.started (every model step). Precedence: step > turn > session. Prefer session.started: prompt caches are per model, so every switch re-ingests the conversation at uncached prices. If no active selection exists before model-dependent work begins, the turn fails.
• Failures stop the turn. A resolver that throws, returns no model, or returns an invalid selection fails before the provider call. A selected model without valid credentials fails at request time.
• Serialization. Session/turn selections must be model id strings; return live LanguageModel objects only from step.started.
• Selection object. { model, reasoning?, modelContextWindowTokens?, modelOptions? }. When modelContextWindowTokens is omitted, eve resolves it from the AI Gateway catalog and caches successful metadata in durable session state for 24 hours. Set it explicitly for an unlisted or custom model. Dynamic agents cannot set sibling modelContextWindowTokens or modelOptions fields; return per-model values from the handler.

The session.started runtime identity does not include a model id for a dynamic agent. Each public step.started event reports the concrete modelId selected for that model call.

Reasoning effort

Set reasoning to control the model's reasoning effort through AI SDK's provider-agnostic option:

export default defineAgent({
  model: "openai/gpt-5.5",
  reasoning: "high",
});

Supported values are "provider-default", "none", "minimal", "low", "medium", "high", and "xhigh". The selected model and provider determine which levels are available and how they map to provider-native settings. Use modelOptions.providerOptions when you need provider-specific reasoning controls. A dynamic model selection can return reasoning alongside model to override the agent-level setting for that selection. Omitting it inherits the agent setting; "provider-default" explicitly uses the provider's default.

Run eve set --reasoning high to update this field from the command line.

Compaction

Compaction summarizes older turns as you approach the context window. It's on by default, so you only tune when it kicks in. eve adds the estimated fixed checkpoint-prompt envelope to the trigger count, so compaction starts sooner than the conversation-only estimate. Lower thresholdPercent to compact sooner:

export default defineAgent({
  model: "anthropic/claude-opus-4.8",
  compaction: {
    thresholdPercent: 0.75, // default 0.9
  },
});

See Default harness for how the loop applies it.

Runtime limits

Use limits for framework-owned runtime caps. Session usage limits stop the current durable session from starting another model call after accumulated provider-reported tokens or model token cost reaches a configured limit:

export default defineAgent({
  model: "anthropic/claude-opus-4.8",
  limits: {
    maxInputTokensPerSession: 200_000,
    maxOutputTokensPerSession: 20_000,
    maxTokenCostUsdPerSession: 1.5,
    sessionTimeoutMs: 7 * 24 * 60 * 60 * 1_000,
  },
});

sessionTimeoutMs sets the lifetime for every session, including delegated sessions. It defaults to 30 days and starts at creation. Each successful deployment handoff or legacy-session import restarts the original configured duration. Process restarts, ordinary messages, and failed or skipped handoffs keep the existing deadline. At the deadline, eve lets an active turn settle, then emits session.completed and releases every continuation address; the next qualifying channel message starts fresh. Set it to false to disable the timeout. Expiration does not delete stored session data.

Input tokens, output tokens, and model token cost are checked independently. The model call that crosses a limit is allowed to finish because exact usage arrives after the call completes. Before the next model call, eve pauses the session and sends a deterministic continuation prompt with two options: Approve grants a fresh window of each configured size, and Stop cancels the in-flight turn through the standard cancellation path (turn.cancelled → session.waiting) — a user decision, not an error. The session stays resumable; because it is still over budget, the next message re-raises the prompt. Declining a delegated child's prompt cancels the root turn, which cascades to the whole delegation tree — the delegating parent never receives an error result it could retry against a fresh quota share. A reply that answers neither option is queued while the existing prompt stays pending; eve does not raise another copy. The reply is processed once the budget is granted.

Sessions that cannot reach a human — task-mode runs such as schedules and delegated runs without input proxying — skip the prompt and fail the next model call with SESSION_TOKEN_LIMIT_REACHED for token budgets or SESSION_TOKEN_COST_LIMIT_REACHED for model token cost. A delegated task with no inherited quota also fails instead of raising a continuation prompt that could only grant another zero-value window.

When maxInputTokensPerSession is omitted, root sessions apply a default input budget of 40_000_000 provider-reported input tokens. maxOutputTokensPerSession and maxTokenCostUsdPerSession are unset by default. maxTokenCostUsdPerSession is a US-dollar limit on model token cost, not tool or infrastructure spend. It uses the cost reported with each model step; AI Gateway supplies this value, while model steps without reported cost do not add to the limit. Set any usage limit to false to uncap that axis.

Delegated subagent sessions have no fixed default. Each child receives a share of the delegating parent's remaining quota at dispatch time — the remainder in the current budget window split evenly across the batch's local subagent calls — and a completed child's usage counts against the parent's quota. Token-cost budgets follow the same rules, including splitting the remaining US-dollar budget across a batch and adding completed child cost back to the parent. Approving a continuation opens a fresh parent window for later child grants without erasing lifetime usage. An authored child limit applies only when it is tighter than the parent's grant; an uncapped parent delegates uncapped children.

Workflow world

By default, eve selects the Workflow SDK world for the host: Vercel Workflow on Vercel, and the SDK's local world in local development or eve start. Advanced self-hosted deployments can select the Workflow world package to use from the root agent.ts:

import { defineAgent } from "eve";

export default defineAgent({
  model: "anthropic/claude-opus-4.8",
  experimental: {
    workflow: {
      world: "@workflow/world-postgres",
    },
  },
});

Install that package in your app. It should export a default factory or createWorld() function. Pin a version built against the same @workflow/* line as your eve release (currently the 5.0.0-beta line):

pnpm add @workflow/world-postgres@5.0.0-beta.x

The npm latest tag can lag behind that line, so an unpinned install may pull an incompatible protocol version that the Workflow SDK rejects during initialization.

Put credentials and host-specific options in runtime environment variables read by the world package, not in agent.ts. For the Postgres world, that means putting the connection string or credentials in the env vars it reads. If the installed package must stay external in hosted output, list it in build.externalDependencies.

Workflow checkpoint batching

By default, eve commits a durable Workflow step after every model call and its inline tool calls. You can experimentally let one Workflow step run several sequential model calls:

import { defineAgent } from "eve";

export default defineAgent({
  model: "anthropic/claude-opus-4.8",
  experimental: {
    workflow: {
      modelCallsPerStep: 4,
    },
  },
});

experimental.workflow.modelCallsPerStep is a positive-integer ceiling. It defaults to 1 and applies independently to each root agent or declared subagent. A higher value can reduce Workflow checkpoint overhead in sequential tool loops, but it widens the replay unit: if the Workflow step is interrupted, its earlier model calls and inline tool executions can run again. That can repeat provider costs, events, and side effects. Use stable idempotency keys for non-idempotent tools.

eve ends a batch before it waits for input, authorization, or blocking coordination, and before it acknowledges a background task. A batch can also end below the configured ceiling when the turn completes or steering arrives. Before assistant output begins, steering can interrupt pending model generation. Executing tools finish safely before the batch yields and applies the correction. This option is experimental and may change or disappear in any release. See Execution model and durability for the retry behavior.

Run data retention

The runtime keeps each run's data after the run finishes: model and tool payloads, streamed output, and the event log eve replays from. How long it keeps them is the World's decision, and on Vercel that follows your team's plan. Set experimental.workflow.retention to 0 to have it deleted as soon as the run finishes instead:

import { defineAgent } from "eve";

export default defineAgent({
  model: "anthropic/claude-opus-4.8",
  experimental: {
    workflow: {
      retention: 0,
    },
  },
});

This applies to every run that owns the session, including successor owners started after a deployment handoff, and to the run that collects session activity. Runs eve starts for other purposes keep the world's default: session timeouts, background tasks, and workflow tools.

The value applies per agent. A subagent that runs its own session uses its own value, unlike experimental.workflow.world, which is root-only.

Custom Worlds used with eve might not support this feature, in which case it falls back to the World's default retention period.

⚠️ At 0, a finished session's output is usually gone before you can read it. Since data is deleted immediately before it can be read back, results and transcripts become unreadable and a client polling for a finished session's output can see it disappear.

Other defineAgent fields

defineAgent takes a few more fields, all optional. For the exported types, see the TypeScript API Reference.

Field	Type	Default	Description
reasoning	AgentReasoningDefinition	provider default	Provider-agnostic reasoning effort forwarded to the agent's turn model calls.
modelOptions	AgentModelOptionsDefinition	none	Provider option overrides forwarded to the model call.
limits	AgentLimitsDefinition	field-specific	Framework-owned runtime limits. Sessions complete after 30 days by default; usage-limit defaults and inheritance are described above. Set a limit to false to disable it.
experimental	AgentExperimentalDefinition	unset	Unstable opt-ins. workflow.world selects the Workflow world package on the root agent; workflow.modelCallsPerStep batches sequential model calls into a wider replay unit; workflow.retention controls how long the durable runtime keeps run data.
outputSchema	Standard Schema or a JSON Schema object	none	Structured return type for function-like invocations such as a subagent turn, schedule, or remote job. Ordinary interactive turns ignore it unless the client supplies a per-message schema.
build	{ externalDependencies?: string[] }	none	Hosted-build packaging controls. externalDependencies keeps listed packages external while eve compiles authored modules such as tools and channels, and traces those packages into the hosted output.
tool	boolean	true	Exposes this agent to its parent model as a tool. On the root agent, controls the built-in agent tool. A subagent with tool: false remains callable from authored workflow tools through ctx.agent().

externalDependencies is a packaging control only. It keeps selected packages as runtime dependencies in the hosted output; it does not authorize, configure, or review any third-party service those packages may call.

During eve dev, ordinary dependencies are bundled into each retained runtime generation. Packages listed in externalDependencies keep normal Node.js resolution instead, so replacing one of those packages requires restarting the dev server.

Where adjacent settings live

Concern	Lives in
Instructions prompt	agent/instructions.md, Instructions
Per-tool approval (HITL)	agent/tools/*.ts, Tools
Inbound auth & network policy	the channel layer, Auth & route protection
Sandbox / workspace	agent/sandbox/, Sandbox
Telemetry & debugging	agent/instrumentation.ts, Instrumentation

What to read next

• Default harness for compaction and model context, and Built-in tools for the framework-provided tool set
• TypeScript API Reference for every defineAgent field and type
• Subagents for the description requirement and child-agent config