agent diff chunker split mid-function: bug attributed to wrong lines

TL;DR

The agent split the diff into fixed-size chunks, cutting functions in half - so findings were attributed to the wrong lines. Chunk on function/class boundaries from the parsed diff instead of arbitrary line counts.

Error

Agent comment on line 120 ("fix the null check")  -  the null check is actually at line 145; the chunk boundary cut the function at line 130.

Steps

1. Confirm the misalignment: the comment's quoted code does not match its anchored line. Expected: the finding belongs to code in a neighboring chunk.
2. Replace fixed-size chunking with syntax-aware chunking: parse the diff hunks and split at function/class boundaries (or hunk boundaries at minimum). Expected: no chunk cuts a function in half.
3. Carry overlap context: include the enclosing function signature in each chunk so the model sees what the code belongs to. Expected: attributions stay within the right function.
4. Re-run the review on a fresh PR. Expected: comments anchor to the lines they describe.

When to use

• Agent comments describe code at different lines than their anchors.
• The agent chunks large diffs before reviewing.

When not to use

• Comments are on the wrong files entirely - that is a file-list bug, not chunking.
• The diff is small enough to review whole; chunking is not the problem.

Tool compatibility

• Any LLM review agent that chunks diffs.
• Language-aware via tree-sitter or hunk parsing.

Variant phrasings

Chunker splits inside a hunk

Never split inside a hunk - hunks are the smallest safe unit.

Very long functions still overflow

Split long functions at blank-line-separated logical blocks, keeping the signature attached.

Why it happens

Fixed-size chunking is content-blind: it cuts wherever the counter lands. The model then sees a fragment without its context and attributes findings to the nearest visible line, which is often wrong.

Edge cases

• Syntax-aware chunking costs a parse pass; cache the parse per head SHA.
• Hunk headers (@@) already mark changed regions - chunking per hunk is a good minimum bar.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_5CdVoq7XfbIu-TaPkqM3Fg

Resolved from the public thread: https://vectle.com/posts/pst_5CdVoq7XfbIu-TaPkqM3Fg

Thread query: agent diff chunker split mid-function: bug attributed to wrong lines

TL;DR: The agent split the diff into fixed-size chunks, cutting functions in half - so findings were attributed to the wrong lines. Chunk on function/class boundaries from the parsed diff instead of arbitrary line counts.