When an integration (cell/root) branch is deleted from origin — e.g. after a sibling cell->root merge, stranding a late straggler leaf — pr_merge's post-merge _sync_target_branch ran 'git fetch origin <branch>' (check=True) and raised 'couldn't find remote ref'. That surfaced as a retryable SERVICE_ERROR, so complete() re-blocked the task and respawn-looped the PM on an already-landed merge (observed live on cell branches feature/backend/31ae12fc--0e49e04e and 7aeee245--dcfe9fc2, blocking be-pm's complete() 5+ cycles).
pr_merge reaches the post-merge sync only after the authoritative GitHub merge has already succeeded (_merge_with_retry raises otherwise), so refreshing the local workspace copy of the target branch is cosmetic. Route it through a new _sync_target_branch_best_effort that logs and returns None instead of raising. merge_pull_request (CEO path) keeps the strict sync — its target is the default branch, which always exists on origin.
Renn F
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