# session-handoff hooks

Opt-in PostToolUse hook that detects substantive `git commit` invocations and signals the agent ("consider running session-handoff write-mode") via a system reminder. Replaces the agent-side commit-detection heuristic in the SKILL.md `When to use` section with a deterministic harness-side trigger.

## Why opt-in (not auto-installed)

`install.sh` deliberately does **not** mutate `~/.claude/settings.json`. Auto-rewriting the user's hook config on every skill install is the wrong shape — user expects `install.sh` to copy files, nothing more. The hook is shipped as scripts; user enables it once per machine.

## Enable on Windows (PowerShell)

Add to `~/.claude/settings.json`:

```jsonc
{
  "hooks": {
    "PostToolUse": [
      {
        "matcher": "Bash",
        "hooks": [
          {
            "type": "command",
            "command": "pwsh -NoProfile -ExecutionPolicy Bypass -File \"$env:USERPROFILE\\.claude\\skills\\session-handoff\\hooks\\commit-detector.ps1\""
          }
        ]
      }
    ]
  }
}
```

If `hooks.PostToolUse` already exists — append the matcher block to the array. Don't overwrite existing entries.

## Enable on Linux / macOS (bash)

```jsonc
{
  "hooks": {
    "PostToolUse": [
      {
        "matcher": "Bash",
        "hooks": [
          {
            "type": "command",
            "command": "bash ~/.claude/skills/session-handoff/hooks/commit-detector.sh"
          }
        ]
      }
    ]
  }
}
```

The bash variant needs `python3` on PATH (used to parse the PostToolUse JSON stdin).

## What gets signalled

On a successful `git commit` whose subject prefix is not in {`meta`, `docs`, `style`, `chore`} or `fix typo`, **and** whose body exceeds 200 characters or which touches more than 3 files — the hook emits a `hookSpecificOutput.additionalContext` system reminder of shape:

> Substantive commit detected on `<cwd>`: `<subject>` (N files changed, body M chars). Consider invoking session-handoff write-mode to update `.tasks/NEXT_SESSION.md`.

On any of these → silent skip (exit 0, no JSON):

- malformed PostToolUse stdin
- Bash command isn't `git commit`
- command is `git commit --amend`
- commit returned non-zero exit
- cwd isn't a git work-tree
- subject prefix is in trivial set
- body ≤ 200 chars AND files ≤ 3

## Smoke test (without enabling the hook)

Pipe a synthetic PostToolUse JSON to the script. On Windows:

```powershell
$payload = @{
  tool_input    = @{ command = 'git commit -m "subject"' }
  tool_response = @{ exit_code = 0 }
  cwd           = (Get-Location).Path
} | ConvertTo-Json -Compress

$payload | pwsh -NoProfile -File .\skills\session-handoff\hooks\commit-detector.ps1
```

If your `HEAD` is a non-trivial commit (e.g. recent `feat:` with > 200-char body or > 3 files), output is JSON containing `additionalContext`. Otherwise — empty stdout (silent skip).

## Caveats

- **Rebase / cherry-pick noise.** Every commit in a rebase or cherry-pick batch will re-fire the hook. Deferred to a follow-up if it actually annoys in practice; the hook is opt-in so the cost is bounded.
- **First-non-trivial commit of session.** The agent-side heuristic in SKILL.md treats the *first* non-trivial commit of a session as "always substantive" regardless of thresholds. The hook can't see session boundaries — uses only body/file thresholds. Slight under-detection on small first commits; acceptable trade-off for harness-side determinism.
- **No automatic write-mode invocation.** Hook only signals. The agent still decides whether to run session-handoff write-mode in response — keeps the user-agency invariant from SKILL.md `What NOT to do`.