Ca Watch

---
name: ca-watch
description: Watch a PR's CI to completion — diagnose on red, notify and offer the merge on green. Never auto-merges.
argument-hint: "<PR number | url | branch>"
---

# $ca-watch — PR CI babysitter

Watch a pull request's checks to completion without babysitting them by hand. The
wait happens server-side, so it costs nothing while CI runs; arbiter wakes once, on
the verdict — diagnoses a red, or offers you the merge on a green. Arbiter never
pulls the merge trigger.

## Precondition

Requires the GitHub CLI authenticated (`gh auth status`). If `gh` is absent or
unauthenticated, report the precondition failure naming `gh` and STOP — do not start
a phantom watcher.

## Flow

1. **Resolve the PR** from `$ARGUMENTS` (number, URL, or branch; default to the
   current branch's PR).
2. **Watch to completion** — run the watch as a **detached background task** built on
   the server-side block:
   ```
   gh pr checks <PR> --watch
   ```
   This blocks until every check finishes and then exits non-zero on failure. It is
   NOT a polling loop — there is no interval-based wake-up. Arbiter is re-invoked once
   when the watch process exits.
3. **On red** — retrieve the failing job's logs (`gh run view --log-failed` /
   `gh pr checks`) and act at the configured depth. Resolve that depth with the
   canonical resolver rather than reading the env var by hand (so the accepted
   values can't drift). Resolve the interpreter once by presence —
   `PY=python3; { command -v python3 >/dev/null 2>&1 && python3 --version >/dev/null 2>&1; } || PY=python`
   — never `python3 X || python X`, which reruns X on any nonzero exit (#577):
   ```
   "$PY" "${PLUGIN_ROOT}/hooks/babysit.py" --root "<project-root>"
   ```
   It prints one JSON line; act at its `on_red` value (`CODEARBITER_BABYSIT_ONRED`,
   default `propose`):
   - **`propose`** — name the likely cause and propose a concrete fix. Do NOT edit
     any tracked file; applying the fix routes through `$ca-fix` or `$ca-feature`.
   - **`branch`** — additionally open a `spike/fix-*` branch carrying the proposed
     change for review. That branch is a spike: it can never PR or merge. The default
     branch is left untouched.
4. **On green** — notify the user and present a merge **offer** (the `gh pr merge`
   command, ready to run). The watcher itself NEVER merges. If the PR targets the
   default branch, the merge routes through the merge-to-default hard gate — the offer
   cannot bypass it.

## On/off

A global flag, `CODEARBITER_BABYSIT` (default **off**, mirrors `CODEARBITER_PRUNE`),
governs auto-attachment: when on, `$ca-pr` auto-attaches a watcher to the PR it
opens. `$ca-watch <PR>` works ad-hoc regardless of the flag. The flag is never set on
the user's behalf — enabling it is the user's explicit choice. Both the command and
the flag are dormant in a repo without `arbiter: enabled`.

## When NOT to use

- Open the PR first → `$ca-pr`.
- Apply a fix for a diagnosed red → `$ca-fix` (or `$ca-feature`).
- Review a diff without watching CI → `$ca-review`.

## Hard gate

- MUST NOT auto-merge. Green → notify + offer only; the watcher never invokes
  `gh pr merge`.
- A merge into the default branch MUST route through the merge-to-default hard gate —
  the green offer cannot and does not bypass it.
- MUST NOT implement a polling loop that re-invokes the model on a timer — the watch
  is the server-side `gh pr checks --watch` block.
- MUST surface a missing/unauthenticated `gh` as a precondition failure, not a silent
  no-op.
- At depth `propose` MUST NOT edit any tracked file; at depth `branch` the proposed
  change lands only on an unmergeable `spike/fix-*` branch.