[juliusmarminge]

Problem

The background status poll runs git fetch about every 30s, and that fetch rewrites .git/FETCH_HEAD. Git writes that file without a lock. When the poll overlaps a user's own git pull in the same checkout, both fetches append for-merge lines, and the pull fails with fatal: Cannot fast-forward to multiple branches. Reproduced on main 125b619 with Git 2.43: the status fetch's exact arguments ran in a loop while git pull --ff-only ran against a moving remote, and 12 of 300 pulls failed that way.

Change

fetchRemoteForStatus now passes --no-write-fetch-head. The poll only needs refs/remotes/* to compute ahead/behind, and nothing reads the FETCH_HEAD it wrote. User-initiated fetches and pulls keep writing FETCH_HEAD as before. The flag needs Git 2.29 or newer. On older Git this background fetch fails and goes through the existing failure backoff.

Scope and approval

Fixes #26560

Verification

• New test leaves FETCH_HEAD to the user's own pulls in background upstream fetches in apps/server/src/vcs/GitVcsDriverCore.test.ts:

  • Fails on main's implementation: AssertionError: background status fetch wrote .git/FETCH_HEAD, which an overlapping \git pull` merges from: expected true to equal false`
  • Passes with the fix, and behindCount is still 1, so the fetch still runs.

• GitVcsDriverCore.test.ts: 172/172 pass. vp run check:changed passes.

• Runtime transcript (Git 2.43, Linux): a background loop runs the status fetch's exact command while git pull --ff-only runs 300 times against a remote that gains a commit before each pull.

  	Before (125b619)	After (97adfec)
  Cannot fast-forward to multiple branches failures	12/300	0/300

  Driver: repro2.sh. Output: transcript.txt

• Server-only, no visible surface, so there are no captures.

• Not checked: the reporter's Apple Git 2.54 on macOS, and Git older than 2.29. The changed code is platform-independent server code.

Model and harness: Claude Opus 5.5 (lead, test, implementation, review B) and GPT-6.1 Sol (review A), in Claude Code and Codex via T3 Code.

🤖 Generated with Claude Code

[macroscopeapp]

Approvability

Verdict: Approved at 97adfec

Macroscope's review found this PR approvable — Background status fetches stop overwriting FETCH_HEAD, avoiding interference with concurrent Git pulls. Remote-reference updates and explicit fetch operations remain unchanged, keeping the change narrowly contained.

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[coderabbitai]

No actionable comments were generated in the recent review.

Recent review info

Run configuration

• Source mode: Auto
• Repository: pingdotgg/t3code
• Branch: fix/issue-26560
• File: .coderabbit.config.ts
• Commit: 97adfec
• Layers: Repository TS config
• Review profile: CHILL
• Plan: Team
• Run ID: 551d88bf-941b-4c73-9e08-1a21db3e9708

Commits

Reviewing files that changed from the base of the PR and between 125b619 and 97adfec.

Files selected for processing: 2

• apps/server/src/vcs/GitVcsDriverCore.test.ts
• apps/server/src/vcs/GitVcsDriverCore.ts

Included review availability: This review used your included allowance. Your plan provides up to 10 included reviews per hour; 7 remain after this review.

Walkthrough

The background status fetch adds --no-write-fetch-head while retaining --no-auto-gc. A regression test checks that the fetch detects a remote commit without creating .git/FETCH_HEAD.

Changes: Background status fetch

Layer / File(s)	Summary
Fetch behavior and regression test apps/server/src/vcs/GitVcsDriverCore.ts, apps/server/src/vcs/GitVcsDriverCore.test.ts	The background fetch adds --no-write-fetch-head. The test verifies that remote status reports one commit behind while .git/FETCH_HEAD remains absent.

Priority: Normal

Estimated code review effort: 2 (Simple) | ~8 minutes

Change: Bug fix · Severity of issue fixed: Low

Suggested labels: size:S

Suggested reviewers: ifkumrfnl, macroscopeapp

Merge Risk: Minimal · up to 97adf

Background status fetches no longer write .git/FETCH_HEAD, which removes a race with a user's own git pull. Fetches and pulls started by the user are unchanged. Git older than 2.29 will fail the background fetch, but the repository declares no such support requirement, so nothing blocks merging.

Pre-merge checks: 4

Check name	Status	Explanation
Description check		The description includes all required sections. It explains the problem, the implementation, the issue scope, focused test results, runtime verification, untested environments, and the agent details.
Title check		The title clearly and concisely describes the main fix: preventing background status fetches from breaking concurrent Git pulls.
Linked Issues check		Issue #26560 requires the server background status fetch to avoid writing .git/FETCH_HEAD while still updating remote-tracking refs for ahead/behind status. fetchRemoteForStatus now adds `--no-wri…
Out of Scope Changes check		The changes stay within issue #26560. The source change updates the background fetch arguments and clarifies the existing --no-auto-gc comment. The added test verifies the requested behavior. No unr…

Full details: Linked Issues check

Explanation

Issue #26560 requires the server background status fetch to avoid writing .git/FETCH_HEAD while still updating remote-tracking refs for ahead/behind status. fetchRemoteForStatus now adds --no-write-fetch-head to that fetch only. The regression test verifies that statusDetailsRemote reports behindCount 1 and leaves FETCH_HEAD absent. The diff does not change user-initiated fetch or pull commands.

Full details: Out of Scope Changes check

Explanation

The changes stay within issue #26560. The source change updates the background fetch arguments and clarifies the existing --no-auto-gc comment. The added test verifies the requested behavior. No unrelated product behavior or user-facing surface changes appear in the reviewed diff.

Finishing Touches

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[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	5.0 KiB	5.0 KiB	0 B (0.0%)	6.8 KiB	✅
Codex	Thread snapshot wire	3.8 KiB	3.8 KiB	0 B (0.0%)	4.9 KiB	✅
Codex	Live turn WebSocket wire	1.3 KiB	1.3 KiB	0 B (0.0%)	2.0 KiB	✅
Codex	Live turn WebSocket decoded	21.6 KiB	21.6 KiB	0 B (0.0%)	24.4 KiB	✅
Codex	Live turn messages	2	2	0 (0.0%)	8	✅
Claude	Total thread wire	5.0 KiB	5.0 KiB	0 B (0.0%)	6.8 KiB	✅
Claude	Thread snapshot wire	3.8 KiB	3.8 KiB	0 B (0.0%)	4.9 KiB	✅
Claude	Live turn WebSocket wire	1.3 KiB	1.3 KiB	0 B (0.0%)	2.0 KiB	✅
Claude	Live turn WebSocket decoded	21.9 KiB	21.9 KiB	0 B (0.0%)	24.4 KiB	✅
Claude	Live turn messages	2	2	0 (0.0%)	8	✅

Baseline: 125b619 · PR result: 97adfec · Source CI: success

Scenario and decoded snapshot size

10 historical turns, 5 command tools per turn, 878.9 KiB retained MCP result per historical turn, and a 1.05 MiB retained result in the measured turn.

• Codex decoded thread snapshot: 108.5 KiB
• Claude decoded thread snapshot: 108.8 KiB

Updated in place by a trusted workflow. PR artifacts are strictly validated and never executed.