fix(server): follow branch drift in dedicated worktrees so PRs link to their thread

[t3dotgg]

Problem

When an agent (or the user) runs a plain `git checkout -b` inside a thread's dedicated worktree and files a PR from the new branch, the PR never links to the thread. The thread's recorded `branch` only updates through T3 commands (creation, first-turn auto-rename), so it goes stale — and since #4460 the client only attributes PR state to a thread when the checked-out branch exactly equals the recorded one. Stale metadata silently orphans the PR, and (via #5151) also re-enables inactivity auto-settle for threads that actually have an open PR.

This happened in the wild: a thread recorded `t3code/ios-background-websocket-reconnect`, the agent branched off to `t3code/mobile-reconnect-hardening` mid-thread and filed #5154 from it, and the link never formed.

Solution

Keep #4460's strict matching and fix the stale data instead: after each turn completes, the CheckpointReactor already refreshes local git status for the session cwd. Use that fresh status to detect drift — if the checked-out branch differs from the thread's recorded branch and the worktree belongs to exactly that thread, dispatch a `thread.meta.update` adopting the checked-out branch. The command's existing `expectedBranch` compare-and-swap drops the update if the recorded branch moved concurrently.

Guards: shared cwds are never touched (that's where #4460's protection matters), detached HEAD and temporary `t3code/` placeholder checkouts are ignored, and threads with no recorded branch are left alone.

No schema changes, no client changes, no new polling — one server-side hook on an existing refresh path.

Tests

Three focused cases in `CheckpointReactor.test.ts`: adopts drift on a dedicated worktree, refuses when the worktree is shared, refuses temporary placeholder checkouts.

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Diagnosed and implemented by Claude Fable 5 (claude-fable-5, 1m context) running in Claude Code.

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Note

Medium Risk
Changes thread metadata on an existing turn-completion path with compare-and-swap guards; wrong adoption could mis-link PRs, but shared-worktree and temporary-branch guards limit blast radius.

Overview
When an agent or user checks out a different branch inside a thread’s dedicated worktree (outside T3’s branch commands), the thread’s stored branch can stay stale. That breaks PR↔thread linking and related behavior that only matches when checkout equals recorded branch.

CheckpointReactor now uses the local git status already refreshed on turn.completed. If the checked-out ref differs from the thread’s recorded branch, it dispatches thread.meta.update with expectedBranch so concurrent renames are ignored. Adoption only runs when the session cwd is that thread’s sole worktree; shared worktrees, detached HEAD, and temporary t3code/<hex> placeholder branches are skipped.

Tests extend the harness and add three cases: adopt on dedicated worktree, no adopt when worktree is shared, no adopt for temporary placeholder checkouts.

^{Reviewed by Cursor Bugbot for commit 2566f5c. Bugbot is set up for automated code reviews on this repo. Configure here.}

Note

Follow branch drift in dedicated worktrees so PRs link to their thread

• After a turn.completed event, CheckpointReactor now calls a new followWorktreeBranchDrift helper that compares the checked-out branch (from local git status) against the thread's recorded branch.
• When the two differ, the helper dispatches a thread.meta.update with expectedBranch (CAS semantics) to adopt the drifted branch, allowing PRs on the new branch to link back to the thread.
• The helper skips adoption if the ref is null, temporary, or shared by multiple threads, or if the worktree path doesn't match.
• New tests in CheckpointReactor.test.ts cover drift adoption, shared-worktree suppression, and temporary-ref suppression.

^{Macroscope summarized 2566f5c.}

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[cursor]

Cursor Bugbot has reviewed your changes using high effort and found 1 potential issue.

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^{Reviewed by Cursor Bugbot for commit 2566f5c. Configure here.}

[cursor]

Temp branch blocks drift adopt

Medium Severity

followWorktreeBranchDrift returns early when the thread’s recorded branch is still a temporary placeholder, even if the checkout is already a real branch. The PR only calls for ignoring temporary checkouts, and expectedBranch already drops a stale adopt if first-turn rename wins the race. When rename fails and the agent has already branched, this leave the recorded branch stuck and the PR unlinked on the server path.

 

^{Reviewed by Cursor Bugbot for commit 2566f5c. Configure here.}

[macroscopeapp]

Approvability

Verdict: Needs human review

This PR adds new runtime behavior for tracking branch drift in worktrees, with an unresolved review comment identifying a potential logic bug where temporary placeholder branches may prevent drift adoption. Both the new feature scope and the open concern warrant human review.

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