Moves the commit-body AI helper out of merger.ts into the existing
core/ai-summarize.ts module so all short-summary AI work (titles,
chat titles, fallback merge commit bodies) shares one home with
consistent dispatch semantics, error handling, and session lifecycle.
Core (ai-summarize.ts):
- New `summarizeCommitBody(diffStat, rootDir, provider, modelId, opts)`
exported alongside `summarizeTitle`. Same shape (provider/modelId
args), same get-engine-or-bail dynamic loading via `getFnAgent`,
same readonly-tools session, same disposal-in-finally pattern.
- Differs from `summarizeTitle` in three deliberate ways suited to the
commit-body job:
1. Returns null on any failure instead of throwing — the caller is
always the merger, which has a deterministic fallback chain
behind it. Throwing would force the merger to wrap every call
in try/catch.
2. Accepts an optional `signal` to forward engine-pause / shutdown
cancellation, plus a configurable `timeoutMs` (default 30s)
so a wedged AI session can't stall a merge indefinitely.
3. Larger output ceiling (2000 chars vs title's 60) and larger
input ceiling (4000 chars truncated diff) — commit bodies are
multi-line and need more room than a 60-char title.
- Exported alongside `summarizeTitle` from `@fusion/core`. Constants
(`COMMIT_BODY_SYSTEM_PROMPT`, `MAX_COMMIT_BODY_INPUT_LENGTH`,
`MAX_COMMIT_BODY_LENGTH`, `DEFAULT_COMMIT_BODY_TIMEOUT_MS`) re-exported
for callers that want to override behavior.
Engine (merger.ts):
- Dropped the local `aiGenerateCommitBody` function (~70 lines) — it
duplicated the session-creation pattern from `summarizeTitle` while
living in a place where future maintainers wouldn't think to look.
- `resolveSafeCommitBody` now imports `summarizeCommitBody` from
`@fusion/core` and delegates. The cascade behavior is unchanged
(commitLog → AI → diff stat → synthetic) and the title-summarizer
model preference is preserved (provider/modelId resolved here, then
passed through).
Tests:
- 6 new test cases in `ai-summarize.test.ts` covering:
empty input → null, missing engine → null (graceful, never throws),
missing engine + model selection → null, pre-aborted signal → null,
custom timeout (returns quickly under 1s ceiling), exposed constants.
- Core: 3136/3136 pass (was 3130 — +6 new). Engine: 2887/2887 pass.
- Typecheck clean, workspace lint clean.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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