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

[saphid]

Model: GPT-6.1 Sol, Claude Opus 5.5 and GPT-6 Astra via T3 Code. Rebased onto current main with Claude Opus 5.5 on 2026-10-12.

Rebased 2026-10-10 onto main 57b3780770; head 693310f6a8. Rebased onto the latest main. Main now falls back to fetching a same-repository pull request head by branch name when the host publishes no refs/pull/<n>/head (added for Azure DevOps). This PR keeps that fallback and passes the provider's head ref into the same fetch, so GitLab merge requests are fetched from refs/merge-requests/<n>/head first and only hosts without a head ref of their own rely on the fallback. The design is unchanged: SourceControlProvider has an optional changeRequestHeadRef, the GitLab provider implements it, and GitManager passes the ref to the git driver. The new tests fail on main and pass on this head. GPT-6.1 Sol (high) reviewed this port: SHIP. At this head these pass: focused tests (2 files, 260 tests), typecheck (server and source-control packages), lint and fmt on the changed files, and knip. One unrelated driver test also fails on plain main in this environment and was excluded. The real-client captures below predate this port.

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; a user report shows authenticated glab 1.102.0 and 1.121.0 also omit source_project for same-project MRs on self-managed GitLab, so every same-project MR takes this path). 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

Each source-control provider can now say which ref holds a change request's head (SourceControlProvider.changeRequestHeadRef, optional). The GitLab provider returns refs/merge-requests/<n>/head; providers without it keep refs/pull/<n>/head, so GitHub, Forgejo, Azure DevOps and Bitbucket behave as before. GitManager reads the ref from the provider that resolved the change request and passes it to both materialization fetches and to the head read used when an existing worktree is reused. 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, by us. A user reproduced the failure with authenticated glab on self-managed GitLab (see the PR comments); the fix was not run on their instance.
• 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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📥 Commits

Reviewing files that changed from the base of the PR and between 857e308 and 940c1bd.

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📒 Files selected for processing (7)

• apps/server/src/git/GitManager.test.ts
• apps/server/src/git/GitManager.ts
• apps/server/src/vcs/GitVcsDriver.ts
• apps/server/src/vcs/GitVcsDriverCore.test.ts
• apps/server/src/vcs/GitVcsDriverCore.ts
• packages/source-control-core/src/server/SourceControlProvider.ts
• packages/source-control-gitlab/src/server/GitLabSourceControlProvider.ts

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📝 Walkthrough

📝 Walkthrough

📝 Walkthrough

📝 Walkthrough

Walkthrough

Source-control providers can supply a host-specific pull-request head ref. Git fetch operations use that ref or default to refs/pull/<number>/head. GitManager passes the provider’s ref during pull-request preparation and worktree refresh.

Changes

Host-specific pull-request head fetching

Layer / File(s)	Summary
Define and fetch the head ref packages/source-control-core/src/server/SourceControlProvider.ts, packages/source-control-gitlab/src/server/GitLabSourceControlProvider.ts, apps/server/src/vcs/GitVcsDriver.ts, apps/server/src/vcs/GitVcsDriverCore.ts, apps/server/src/vcs/GitVcsDriverCore.test.ts	The provider interface adds an optional callback for the change-request head ref. GitLab returns refs/merge-requests/<number>/head. Git fetch operations use a supplied headRef or default to refs/pull/<number>/head. Tests cover both ref formats and verify that fetches do not move HEAD.
Pass the head ref through preparation apps/server/src/git/GitManager.ts, apps/server/src/git/GitManager.test.ts	GitManager passes the provider-supplied ref through local-checkout and worktree preparation, including reused-worktree refresh. A test checks initial GitLab worktree preparation and refresh after the merge-request head changes.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Bug fix · Severity of issue fixed: Medium

Sequence Diagram(s)

sequenceDiagram
  participant GitManager
  participant SourceControlProvider
  participant GitVcsDriverCore
  participant GitRemote
  GitManager->>SourceControlProvider: Get change-request head ref
  GitManager->>GitVcsDriverCore: Fetch branch or head commit using headRef
  GitVcsDriverCore->>GitRemote: Fetch selected head ref

Loading

Possibly related PRs

• pingdotgg/t3code#6532: Both changes pass host-specific head refs through GitManager to Git fetch operations. That PR also covers Bitbucket and Azure DevOps fallback cases.

Suggested reviewers: t3dotgg

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Merge Risk: ⚪ Minimal · up to 940c1

GitLab merge requests can now be checked out and refreshed using their published head ref, and other providers keep their existing behavior. No merge-blocking risk remains.

Security Architecture Review

Security architecture risk: 🔵 Low · up to 940c1

This is a bounded change to repository-head selection. The implemented GitLab path retains existing remote, credential and worktree safeguards. No introduced security issue was identified, but custom-provider callers and interrupted or concurrent operations were not fully validated.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

• inferred — The reviewed path affects branch materialization and refresh in the selected repository/worktree under its existing Git credential context. The new capability does not select a remote URL or confer additional credentials. MR-controlled commits remain repository content; no new execution or service-wide authority path was identified in the compared changes.

Trust Boundaries and Controls

• observed — Git execution uses argv arrays rather than shell-command interpolation. With the inspected GitLab ref producer, branch fetching retains the constructed local branch destination, while head-commit fetching uses FETCH_HEAD without directly updating the checked-out branch. The low-level headRef interface itself has no runtime plain-ref validation, so argv separation alone does not establish safety for arbitrary future refspec or option-like inputs.

Resilience and Maintainability Implications

• observed — A reused worktree without an upstream still refuses a non-descendant target when its current commit cannot match the captured upstream commit. This can leave the checkout stale after a force-push, but the same guard and null-upstream behavior exist at the merge base; it is not an introduced PR concern.

Hardening Proposals

• proposed — Before allowing externally configured ref producers, document this capability as a trusted plain source-ref contract and validate it at the Git boundary to reject option-like values and destination-bearing refspecs. This is future hardening, not an observed exploit in the GitLab implementation.

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Pre-merge checks | 6

✅ Passed checks (6 passed)

Check name	Status	Explanation
Linked Issues check		The description links issue #6448, explains the matching failure, and documents why this focused fix addresses the issue despite the issue being closed.
Out of Scope Changes check		The changes are limited to GitLab head-ref resolution, shared provider plumbing, Git fetch behavior, and focused regression tests. The described scope matches the implementation.
Linked Issues check		Issue #6448 is closed and completed. It supplies historical context only, so no active coding requirements apply. The reported changes address the historical GitLab checkout and refresh failure, but t…
Out of Scope Changes check		The changed files add provider-specific ref selection, pass that ref through GitManager and the Git driver, and add focused regression tests. These changes support GitLab merge-request checkout and re…
Title check		The title clearly and concisely describes the primary change: fetching GitLab merge request heads from GitLab-specific refs.
Description check		The description covers the problem, fix, scope rationale, linked issue, verification results, evidence, and untested areas. It uses alternative headings for some template sections, but it provides the…

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[macroscopeapp]

Approvability

Verdict: Approved at 693310f

Macroscope's review found this PR approvable — This is a narrowly scoped GitLab bug fix that routes existing merge-request checkout and refresh operations to GitLab’s published head ref, while preserving the prior default for other providers. Regression tests cover both the GitLab-specific and fallback paths, with no schema, deployment, security, billing, or product-default 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
baseline.

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📒 Files selected for processing (5)

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

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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

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

[w3z315]

Data point for the "Not checked" items: self-managed GitLab with an authenticated glab 1.102.0, same-project MR, nightly 0.0.46-nightly.20261008. Worktree checkout fails with Failed to materialize pull request #285 …; the trace shows both the primary and the fallback attempt fetching refs/pull/285/head, while the remote only has refs/merge-requests/285/head.

So this isn't limited to unauthenticated or limited views: glab mr view --output json didn't include source_project with an authenticated 1.102.0 or 1.121.0 either, so every same-project MR takes this path.

Side note: #6448 was closed as completed via #7174, which was closed unmerged.

Workaround until this lands: create the worktree for the MR branch with git worktree add. preparePullRequestThread then reuses it.

[saphid]

Thanks, this is very useful. It confirms the failure isn't limited to unauthenticated or limited views: with authenticated glab 1.102.0 and 1.121.0, mr view --output json still omits source_project for same-project MRs, so every same-project MR falls back to refs/pull/<n>/head, which GitLab doesn't publish. This PR fetches refs/merge-requests/<n>/head in that case. I've updated the description to cite your report.

Your git worktree add workaround matches what the fix relies on: preparePullRequestThread reuses an existing worktree for the MR branch.

Noted on #6448/#7174, too. This PR is the open fix for that path.

Dismissing prior approval to re-evaluate 940c1bd

Dismissing prior approval to re-evaluate 693310f