fix-detector Hook

Detects bug fixes during a session by analyzing git diffs of edited files and arms the auto-learning pipeline

fix-detector Hook

The fix-detector hook is the first stage of Massu's auto-learning pipeline. It fires after every Edit or Write to a code file, runs a bounded git diff against the working tree, and looks for the structural fingerprint of a bug fix (issue number references, "fix" / "fixes" / "closes" keywords in the diff context, error-handling additions, etc.). When a fix-shape diff is detected, the hook sets session-level state so the rest of the auto-learning chain can trigger at session end.

Trigger Event

Fires as a PostToolUse hook on:

  • Edit tool -- after a file modification
  • Write tool -- after a file replacement

What It Does

  1. Reads the edited file path from JSON stdin (tool_input.file_path)
  2. Runs git diff --shortstat on the file (defense-in-depth via execFileSync argv form, never shell-string)
  3. Bounds the read -- skips the full diff body if estimated bytes exceed MAX_FULL_DIFF_BYTES (2 MiB, ~25k lines). Monorepos with 10MB+ working trees would otherwise blow the Stop-hook timeout.
  4. Scores fix-shape signals -- issue/PR references, "fix"/"fixes"/"closes" verbs, error-handling additions, regression-test additions
  5. Appends a signal entry to the per-session arm file in os.tmpdir() if fix-shape passes threshold
  6. Exits silently -- no stdout output. The signal is consumed downstream by classify-failure and auto-learning-pipeline.

Position in the Auto-Learning Pipeline

[fix-detector]  →  classify-failure  →  Incident Report  →  Rule  →  Enforcement
   THIS HOOK

The fix-detector is the arming step. It does not block, does not output, and does not surface to the user. Its only job is to set session-level state that the rest of the chain reads.

Example Input

json
{
  "session_id": "abc123-def456",
  "tool_name": "Edit",
  "tool_input": {
    "file_path": "/Users/dev/project/src/api/users.ts"
  }
}

Example Behavior

The hook produces no stdout. Side effects:

  • Writes a JSON line to ${tmpdir}/massu-fix-arm-<session_id>.jsonl containing the file path, diff stat summary, and fix-shape score.
  • This arm file is read at session-end by auto-learning-pipeline.

Performance

This hook must complete within 1000ms. It achieves this with:

  • git diff --shortstat bounded to a single file (sub-100ms on typical repos)
  • Full diff body read only when shortstat says the file fits under 2 MiB
  • No network calls, no LLM calls
  • All scoring is regex-based on the diff text

Tips

  • The arm file is per-session (keyed on session_id) and is cleaned up at session-end
  • Disable globally via autoLearning.enabled: false in massu.config.yaml
  • See classify-failure for the next stage that scores the detected fix against known failure classes

Related Documentation