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## Summary Fixes a regression introduced by #17203 (strict-cap atom-split) and a secondary delimiter-handling bug from #17723. **Root cause:** - #17203 added `_split_oversized_unit` / `_compute_chunk_update`, which split oversize units into ≤ token_size pieces. This collapsed `token_size=1` into 1-token chunks and set the cap at 512, mismatching the model-layer truncation boundary (embedding ~8191 / rerank 500/4096/8192/2048). Atom-split is unnecessary: oversize units stay whole and the model layer truncates. - #17723's delimiter handling dropped consecutive delimiters (`A####B` -> `A##B`), glued JSON items with `"".join`, ignored `children_delimiters`, and stripped whitespace delimiters. ## Changes - New pure helper `merge_paragraphs(paragraphs, token_size, strategy)` with a `MergeStrategy` enum (`UNDER_CAP` / `OVER_CAP`); **default `OVER_CAP`**. `UNDER_CAP` is a strict cap (never overflows `token_size`); `OVER_CAP` greedily accumulates adjacent paragraphs while the projected total stays within `token_size`, merging one boundary-overflow paragraph before closing. Oversize paragraphs stand alone. - `naive_merge` / `naive_merge_with_images` / `RAGFlowTxtParser.parser_txt` now use `merge_paragraphs`; atom-split removed. `naive_merge` / `naive_merge_with_images` always split a section on the delimiter whenever one is present (even when the section already fits `token_size`), so delimiter text never leaks into a chunk. Only the empty-delimiter (size-only) mode skips splitting. - `token_chunker`: delimiter text is dropped (not stripped); JSON flush joins buffered items with `"\n"`; `children_delimiters` and `PDF_POSITIONS_KEY` are preserved on the delimiter path. PDF positions are now attributed **per segment** — each split chunk carries only the positions of the item(s) that contributed to it — fixing a leak where page-N coordinates were attached to page-M chunks and all segments shared one preview image. - `test_txt_parser.py` rewritten to assert the new contract (not the old strict cap); `naive_merge` and delimiter-case-sensitive matrices updated. ## Contract (refs #17799) - user specified delimiter = chunk boundary; user specified delimiter text never enters a chunk. - `token_size` = soft target + merge strategy; no atom-split. - Default strategy = `OVER_CAP`; migration can switch to `UNDER_CAP` (strict cap). - `OVER_CAP` has no hard cap; the model layer truncates oversize units. `UNDER_CAP` enforces a strict cap. ## Notes - Closes the wrong-object revert in #17774 (revert #17723 would re-introduce delimiter-in-chunk and the strict cap). - Go-side alignment (`internal/ingestion/component/chunker/token.go`) is a follow-up PR. --------- Co-authored-by: CodeBuddy <noreply@tencent.com>