fix(gitlab): fetch merge request heads using GitLab refs

[saphid]

Fixes #6448

Problem

Checking out a GitLab merge request in a separate worktree fails with "Could not prepare the pull request checkout" whenever the merge request's source-repository metadata is unavailable (for example, an unauthenticated or limited glab view). In that case the server falls back to fetching refs/pull/<n>/head, and GitLab does not publish that ref. For the same reason, an existing MR worktree whose head can't be resolved through its upstream can stay on old commits and report that it is not on the MR head.

Why this qualifies

This is a small, focused fix for an obvious bug, under the contribution exception. #6448 reports the same failure on self-hosted GitLab and traces it to the same hardcoded refspec. That issue has not been triaged yet. Worktree checkout is an existing feature, and this restores it for GitLab without changing behaviour for any other host.

An earlier PR, #6532, also targets #6448. It is broader: it also changes Azure DevOps and Bitbucket materialization, adds a shared helper in SourceControlProvider, and touches 11 files. It currently conflicts with main. This PR is the GitLab-only fix: it changes the two fetch sites plus the provider plumbing that feeds them. Maintainers may prefer either approach; the PRs overlap, so only one should land.

Fix

GitManager already knows which host resolved the change request (ChangeRequest.provider). It now passes that provider to both materialization fetches and to the head read used when an existing worktree is reused. GitVcsDriverCore fetches refs/merge-requests/<n>/head for GitLab and keeps refs/pull/<n>/head for every other host, so GitHub, Forgejo, Azure DevOps and Bitbucket behave as before. The public RPC contract does not change.

Tests:

• GitManager.test.ts mirrors glab: the MR has no source-repository metadata. The test creates an MR worktree from a bare remote's refs/merge-requests/90/head, advances that ref, prepares the MR again, and asserts that the same worktree moves to the new head.
• GitVcsDriverCore.test.ts runs a GitLab case and a GitHub case against a real remote. Each fetches the provider's head ref and checks that the current checkout does not move.

Evidence

Environment: macOS arm64, real web client (owned headless Chromium against the dev server on loopback), isolated T3 state with fresh databases, and fresh clones of a public GitLab repository. Verified on upstream main c5a0c78b7c; the after run uses this branch rebased onto that commit.

Reproduction:

1. Add a fresh clone of https://gitlab.com/gitlab-org/gitlab-test.git as a project.
2. Open the public, closed MR !68 "Edit README" through the Pull Requests detail view.
3. Choose Check out → In a separate worktree.

The GitLab CLI was not logged in, so the MR list shows an authentication error. The MR detail and the checkout itself work without a login. No responses were mocked.

Before (main): "Could not prepare the pull request checkout". git worktree list shows only master; no MR worktree is created.

After (this PR): "Checked out — The pull request is in its own worktree, with a thread open on it." The new worktree's HEAD is the MR head, 094f26019a639c319fc9e59382f090c1bcaeeec8. (The client's automatic theme switched to dark during this run.)

Recording of the after run: click → preparing → checked out. It plays at 3× and nothing was cut. Real-time MP4.

The public remote shows the same ref mismatch directly:

git fetch --depth=1 https://gitlab.com/gitlab-org/gitlab-test.git refs/pull/68/head
exit 128: fatal: couldn't find remote ref refs/pull/68/head

git fetch --depth=1 https://gitlab.com/gitlab-org/gitlab-test.git refs/merge-requests/68/head
exit 0: FETCH_HEAD = 094f26019a639c319fc9e59382f090c1bcaeeec8

Reopening an existing MR worktree is covered by the GitManager regression test above. An earlier real-server run of git.preparePullRequestThread with { reference: "68", mode: "worktree" }, on a worktree reset to the MR head's parent, returned isOnPullRequestHead=false before the fix and true at the MR head after it. That run predates this rebase; the test is the current proof.

Commands run at this head (TMPDIR=/private/tmp CI=true):

• vp test run apps/server/src/git/GitManager.test.ts apps/server/src/vcs/GitVcsDriverCore.test.ts -t 'materializes and refreshes GitLab|fetches .* pull request heads without moving'
  • With main's versions of the three non-test files: 2 failed (both GitLab cases), 1 passed (GitHub).
  • With this PR: 3 passed.
• vp test run apps/server/src/git/GitManager.test.ts apps/server/src/vcs/GitVcsDriverCore.test.ts: 230 passed, 1 failed. The failure is review diff previews > preserves renames, unusual paths, modes, and binary statistics, which fails the same way on unmodified main on this machine (Apple Git 2.50.1). This PR does not touch that code path, and CI on Linux passed it on the previous head.
• vp run --filter t3 typecheck: passed.
• vp lint --report-unused-disable-directives on the five changed files: passed. One existing warning remains on an unchanged line, GitVcsDriverCore.test.ts:1606.
• vp fmt --check on the five changed files: passed.
• node scripts/release-smoke.ts: passed.

Surfaces

• Web: verified, real client against the dev server.
• Desktop: desktop bundles the same server code. The fix needs no Electron shell or IPC change; the desktop shell was not exercised.
• Mobile: not affected. The mobile app does not call git.preparePullRequestThread.
• Local / remote-relay / tunnel: not affected by transport. The fix runs inside the server's git fetch, and the wire contract is unchanged. Only local loopback was exercised.
• Providers (Codex, Claude, Cursor, Grok, OpenCode, Antigravity): not affected. This is source-control host behaviour, independent of the agent provider.
• Source-control hosts: GitLab is fixed. GitHub is unchanged and covered by a test. Forgejo, Azure DevOps and Bitbucket keep their current ref selection.

Not checked

• Self-managed GitLab: [Bug]: Worktree checkout of GitLab merge requests always fails — fetchPullRequestBranch hardcodes GitHub's refs/pull/<n>/head #6448 is self-hosted; only gitlab.com was exercised. Ref selection depends on the resolved provider kind, not the hostname.
• Authenticated GitLab MR listing.
• The Electron shell, relay or tunnel clients, and mobile.
• Submodule population in the fixture. Its submodule checkout logged a warning; this proof checks the MR root's HEAD.
• A force-pushed (non-descendant) MR head on a worktree with no upstream. Existing behaviour applies: the checkout is kept and reported as not on the MR head.

GPT-6.1 Sol, Claude Opus 5.5 and GPT-6 Astra via T3 Code
🤖 Generated with Claude Code

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• apps/server/src/git/GitManager.test.ts
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• apps/server/src/vcs/GitVcsDriver.ts
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• apps/server/src/vcs/GitVcsDriverCore.ts

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📝 Walkthrough

Walkthrough

Pull-request head fetching now selects GitLab merge-request refs or pull refs for other and unspecified providers. GitManager passes provider metadata during branch materialization and reused-worktree refresh. Tests cover GitLab and GitHub fetches and GitLab worktree updates.

Changes

Provider-aware pull-request head fetching

Layer / File(s)	Summary
Select and fetch the provider-specific head ref apps/server/src/vcs/GitVcsDriver.ts, apps/server/src/vcs/GitVcsDriverCore.ts, apps/server/src/vcs/GitVcsDriverCore.test.ts	The fetch input accepts an optional provider. Fetches use refs/merge-requests/<number>/head for GitLab and refs/pull/<number>/head otherwise. Tests verify fetched commits and confirm HEAD remains unchanged.
Pass provider metadata through GitManager apps/server/src/git/GitManager.ts, apps/server/src/git/GitManager.test.ts	Pull-request head metadata includes the provider. Materialization and reused-worktree refresh pass it to fetch operations. A test checks that a reused GitLab worktree moves to an updated head.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~12 minutes

Change: Bug fix · Severity of issue fixed: Medium

Suggested reviewers: t3dotgg

Merge Risk: ⚪ Minimal · up to 96230

GitLab checkout now fetches the merge-request head. The previously flagged rewritten-head limitation predates this change, so no actionable merge-blocking regression remains.

Security Architecture Review

Security architecture risk: 🔵 Low · up to 96230

The fix is narrowly scoped and preserves remote selection and local-change protections. A conditional concurrency risk remains: another fetch in the same worktree could interfere with identifying the requested merge-request head. No authentication bypass or privilege expansion was established.

Retained concerns

• Medium · reliability · inferred: Newly successful GitLab refreshes consume FETCH_HEAD in a command separate from the fetch that writes it. If another fetch overlaps in the same worktree, the requested MR could be associated with another fetched commit, compromising checkout provenance before the existing setup invocation. The sequencing predates this PR for other providers; GitLab now reaches its successful consumption path. Actual production overlap remains unverified, and distinct MR worktrees do not by themselves establish a collision.

Security review details

Security Blast Radius

• inferred — The identified concurrency scenario is bounded to a worktree sharing transient fetch state. It does not establish cross-tenant, cross-service, credential or environment exposure. Fork worktree branches include the MR number, and a mismatched checked-out branch is not refreshed by this path.

Security Findings and Attack Paths

• inferred — A competing same-worktree fetch is required for the conditional provenance failure. Merely supplying a provider string or opening a different MR worktree does not establish that path. No unauthenticated trigger, privilege gain or reproducible security exploit was demonstrated.

Trust Boundaries and Controls

• observed — The provider registry resolves provider context from repository remotes. Checkout obtains the change request through that resolved provider, rather than accepting a provider or remote override in the checkout RPC. Git authentication remains delegated to the configured remote.

Resilience and Maintainability Implications

• observed — Added tests cover GitLab materialization without source-repository metadata, subsequent clean-worktree refresh, and GitLab/GitHub fetch ref selection without moving HEAD. These source assertions do not demonstrate concurrent, interrupted or partial-failure behavior, and tests were not executed during this review.

Hardening Proposals

• proposed — Bind each head-fetch result to its operation, through an isolated result ref or coordination covering competing fetch writers, so commit identity does not depend on shared FETCH_HEAD remaining unchanged.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main change: using GitLab-specific refs to fetch merge-request heads.
Description check	✅ Passed	The description explains the problem, fix, scope and approval rationale, verification results, and untested areas. Its “Fix” and “Why this qualifies” sections cover the template’s change and scope req…
Linked Issues check	✅ Passed	#6448 is closed and supplies historical context only. No active directly linked issue imposes coding requirements. The PR’s stated GitLab ref-selection fix matches that historical context, but it is n…
Out of Scope Changes check	✅ Passed	The change summary lists provider forwarding, GitLab-specific ref selection, and regression tests in five Git files. These changes support the PR’s stated GitLab checkout and refresh fix. The reposito…

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Approvability

Verdict: Approved at 962307b

Macroscope's review found this PR approvable — This is a small, self-contained bug fix that routes GitLab merge-request fetches to GitLab’s existing head refs while preserving other providers’ behavior. Targeted integration tests cover both initial materialization and worktree refresh without introducing schema or deployment changes.

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Dismissing prior approval to re-evaluate ee50a62

Dismissing prior approval to re-evaluate 3080acd

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Actionable comments posted: 1

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• 🪄 Fix CodeRabbit comments on this PR

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Inline comments:
Review comments at @apps/server/src/git/GitManager.ts:
- Line 2455: Update the GitManager worktree refresh flow to persist the accepted
MR head per worktree, advance it after every successful refresh, and reset a
non-descendant MR head only when the current HEAD still matches that recorded
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[saphid]

Two notes on the CodeRabbit summary for the rebased head:

• The "Merge Risk: Moderate / fix this refresh path" note restates the inline finding at GitManager.ts:2455, which CodeRabbit later withdrew in that thread. The force-push case with no upstream uses existing behaviour this PR doesn't change: the checkout is kept and reported as not on the MR head. The PR description now lists that case under "Not checked".
• Docstring coverage: the two functions counted are module-private helpers (pullRequestHeadRef, and toPullRequestHeadRemoteInfo, which has no doc comment on main either). The repo doesn't docstring private helpers. The only public method touched, GitVcsDriver.fetchPullRequestHeadCommit, has an updated doc comment. Leaving as is.

[saphid]

Review requested

Date (UTC)	Reviewer	Where
2026-10-03	Julius	Discord DM

_{Logged so this PR shows when a maintainer was asked to review it.}

Dismissing prior approval to re-evaluate 962307b

[saphid]

Review requested

Date (UTC)	Reviewer	Where
2026-10-06	Julius	Discord DM

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