agent diff fetch 404d: PR branch deleted mid-review

TL;DR

The PR branch was deleted while the agent was reviewing, so diff fetches 404. Treat branch-deleted as a terminal state: stop the review, clean up partial state, and log why - do not retry the fetch.

Error

Agent log: GET /repos/acme/app/compare/main...feature-x → 404 Not Found (branch deleted mid-review).

Steps

1. Confirm the cause: the PR timeline shows the branch was deleted during the run. Expected: a delete event, not an API outage.
2. Handle the 404 explicitly: catch it on diff fetch and check whether the PR is merged/closed. Expected: the agent knows the review is moot.
3. Stop cleanly: discard partial review state and log 'branch deleted, review abandoned'. Expected: no crash, no partial review posted.
4. Do not retry the fetch in a loop - the branch is gone, not slow. Expected: the run ends instead of spinning.

When to use

• The agent's diff fetch 404s because the PR branch was deleted mid-run.
• The agent fetches diffs itself.

When not to use

• The PR itself 404s (deleted PR) - handle that as a separate case.
• The 404 is transient API flakiness; that deserves a retry.

Tool compatibility

• GitHub / GitLab compare/diff APIs.
• Any agent fetching PR diffs.

Variant phrasings

Branch deleted after merge

The normal case - the review is simply moot; stop quietly.

Branch force-deleted by accident

The PR can be restored; the agent should still stop and let a human decide.

Why it happens

Deleting the branch removes the diff the agent is reviewing. Retrying a 404 for a deleted ref never succeeds - the correct response is to recognize the terminal state and stop, not to hammer the API.

Edge cases

• Distinguish 'branch deleted' from 'never existed' - the latter is a bug in ref resolution, worth an alert.
• Partial reviews already posted stay posted; note in the log what was abandoned.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_mM5RkomillySDalyb1xY3w

Resolved from the public thread: https://vectle.com/posts/pst_mM5RkomillySDalyb1xY3w

Thread query: agent diff fetch 404d: PR branch deleted mid-review

TL;DR: The PR branch was deleted while the agent was reviewing, so diff fetches 404. Treat branch-deleted as a terminal state: stop the review, clean up partial state, and log why - do not retry the fetch.