fix(server): t3_thread_launch refuses a new worktree whose base ref has no commit

[tris203]

Problem

t3_thread_launch accepts a new-worktree launch whose baseRef does not resolve to a commit. The call returns success, a thread and run are created, and the failure only appears afterwards inside that thread:

Workspace preparation failed during provision worktree: Git command failed in GitVcsDriver.createWorktree (<repo>): git worktree add failed

This happened for real on 2026-10-10. An orchestrating agent called the tool nine times with {"type":"worktree","baseRef":"t3/b1ffc1cb","branch":"perf/...","startFromOrigin":false}. T3 had renamed that temporary branch, so it no longer existed. Every call returned success in about 14ms and every one of the nine threads failed about 225ms later. An agent can act on an error returned by the call; it does not easily notice nine threads failing elsewhere.

Expected: the first call fails with a message naming the ref, and nothing is created.

Change

ThreadLaunchService gains checkWorktreeBase, and the t3_thread_launch handler calls it before launching. When it fails the tool returns invalid_request naming the ref, the project title and its folder, and no thread or run is created.

The check is a separate method, not part of launch, so the other launch callers are unchanged: the WebSocket launchThread (web, desktop, mobile), scheduled tasks, and server startup. I looked at each. Scheduled tasks in particular hard-code baseRef: "main" and would stop producing a visible failed thread if launch itself refused (#16183, #16215), which is a product decision this PR does not make.

It refuses only when it is sure, and leaves everything else to provisioning as today:

• The base ref resolves to a commit (hasCommit, with - read as the previous checkout, as git worktree add reads it): accepted.
• startFromOrigin is true and the project has an origin remote: accepted without looking. Provisioning fetches the base, which may exist only on origin, and the check does not touch the network.
• Any ref in any namespace ends in that name: accepted. git worktree add starts from a remote-tracking branch of the requested name, so this needed a new GitWorkflowService.hasRefNamed (an uncached git for-each-ref 'refs/**/<name>'). It over-accepts on purpose instead of reimplementing Git's rule.
• HEAD has no commit: accepted. That is the empty repository fix(server): new worktree threads work in repos with no commits yet #15580 makes run in the project folder; the two do not fight, and this adds no textual conflict with it beyond what fix(server): new worktree threads work in repos with no commits yet #15580 already has against main.
• A missing project, a folder that is not a repository, or a failing Git: accepted, so the launch reports it as before.

Known limits, all of which still fail inside the thread exactly as they do now: a ref missing both locally and on origin with startFromOrigin: true; a base ref starting with : (the :/text commit search, which hasCommit cannot test); a remote whose fetch refspec renames the last path component of its branches.

The git worktree add failed wording itself is not touched here.

Scope and approval

No prior issue. I believe this fits the very small, focused fix for an obvious bug: a tool call that reports success for a launch that cannot work. It solves that one problem, changes behaviour only for the MCP tool, and turns a guaranteed asynchronous failure into the same failure reported up front. It adds no setting and changes no default. The three source files are one fix: the handler stays thin, the check lives in the service that owns launching, and it needs one Git question the workflow service could not yet answer.

Verification

Established the problem from the incident above and from the code: on main the handler validates only existing_worktree, and baseRef first reaches Git in createWorktree inside the forked background preparation.

Checked Git's own behaviour by hand in throwaway repositories (Git 2.43.0): git worktree add -b new <path> remote-only succeeds when only origin/remote-only exists while git rev-parse --verify 'remote-only^{commit}' fails, which is why the check also asks hasRefNamed; @{-1}^{commit} resolves the previous checkout; :/text^{commit} never resolves.

vp test run apps/server/src/orchestration-v2/ThreadLaunchService.test.ts \
  apps/server/src/mcp/toolkits/project/handlers.test.ts \
  apps/server/src/git/GitWorkflowService.test.ts \
  apps/server/src/mcp/OrchestratorMcpToolkit.integration.test.ts

4 files, 70 tests passed. What the new ones show:

• ThreadLaunchService.test.ts: the check fails with Base ref "t3/renamed" does not resolve to a commit in project "Project" (/repo). for a missing ref, for startFromOrigin: true with no origin remote, and for - with no previous checkout. It passes for a ref that resolves, a ref only origin has with startFromOrigin: true, a name a remote branch carries, a repository with no commits, - with a previous checkout, :/fix, and a folder that is not a repository.
• handlers.test.ts: a refused launch returns invalid_request with that message and ThreadLaunchService.launch is never called; the next launch with a good ref goes through.
• GitWorkflowService.test.ts: against a real repository, hasRefNamed finds a local branch, another remote's branch, a symbolic ref and a ref in a custom namespace, and finds neither a missing name nor a pattern.

vp lint on the six changed files: one warning, at ThreadLaunchService.test.ts:2043, in a test this PR does not touch. tsc --noEmit in apps/server: exit 0.

Not checked: a launch through a running server and MCP client, a real fetch from an origin, and the combined behaviour with #15580 merged. ThreadLaunchService's tests use that file's mocked Git, so Git's resolution rules are covered by the hasRefNamed test and the manual experiments, not end to end.

Claude Opus 5.5, in T3 Code through the Claude Code harness.

[macroscopeapp]

Approvability

Verdict: Approved at 5ca9102

Macroscope's review found this PR approvable — This is a focused server bug fix that preflights invalid worktree base refs before MCP thread creation, while preserving valid and non-MCP launch behavior. The added Git logic is localized and supported by targeted tests, with no schema, deployment, security, billing, default, or static-analysis configuration changes.

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[coderabbitai]

📝 Walkthrough

Walkthrough

The change adds Git ref lookup and a worktree base-ref preflight. The project handler runs the check before creating a thread and returns an invalid_request response when the base ref is unresolved.

Changes

Worktree Base Ref Validation

Layer / File(s)	Summary
Git ref lookup apps/server/src/git/GitWorkflowService.ts, apps/server/src/git/GitWorkflowService.test.ts	GitWorkflowService adds hasRefNamed to search refs. Tests cover local, remote, symbolic, and mirror refs, as well as wildcard input.
Worktree base-ref preflight apps/server/src/orchestration-v2/ThreadLaunchService.ts, apps/server/src/orchestration-v2/ThreadLaunchService.test.ts	ThreadLaunchService adds checkWorktreeBase and ThreadLaunchBaseRefError. Tests cover valid and unresolved refs, origin handling, and repository conditions.
Project launch validation apps/server/src/mcp/toolkits/project/handlers.ts, apps/server/src/mcp/toolkits/project/handlers.test.ts	The project handler checks worktree bases before thread creation. Tests verify the error response on failure and thread launch when the check succeeds.

Priority: ➖ Normal

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

Change: Bug fix

Sequence Diagram(s)

sequenceDiagram
  participant Client
  participant ProjectHandler
  participant ThreadLaunchService
  participant GitWorkflowService
  participant Git
  Client->>ProjectHandler: Request worktree launch
  ProjectHandler->>ThreadLaunchService: Check worktree base
  ThreadLaunchService->>GitWorkflowService: Check named ref when needed
  GitWorkflowService->>Git: Run for-each-ref
  Git-->>GitWorkflowService: Return matching ref output
  GitWorkflowService-->>ThreadLaunchService: Return ref result
  ThreadLaunchService-->>ProjectHandler: Return success or base-ref error
  ProjectHandler-->>Client: Return launch result or invalid_request

Loading

Suggested reviewers: juliusmarminge

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Merge Risk | 🔵 Low · up to 5ca91

Merge Risk: 🔵 Low · up to 5ca91

Some unusual Git refs can still produce a launch that fails during worktree creation rather than an immediate invalid-request response. The impact is narrow, but the preflight should validate the base that provisioning will use.

Security Architecture Review

Security architecture risk: ⚪ Minimal · up to 5ca91

The change rejects known-invalid launches earlier without expanding launch permissions or workspace access. No material security risk was found in the changed flow. The validation remains advisory, so acceptance does not guarantee successful provisioning.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

• observed — The newly reachable operations query refs in the selected project's persisted workspace root. The preflight accepts a project identifier, not an arbitrary working directory, and introduces no filesystem or database mutation.

Security Findings and Attack Paths

• inferred — No new command-injection or authority-expansion path was identified in hasRefNamed. Caller-controlled text is placed inside a fixed refs/ argument, pattern characters are rejected, and execution uses the existing structured Git argument interface.

Trust Boundaries and Controls

• observed — The handler remains behind read-only-client refusal, live-caller validation and restrictions on requested runtime and interaction modes. Base-ref validation is an advisory correctness check, not a replacement authorization control.

Resilience and Maintainability Implications

• observed — Accepted launches retain existing failure containment and resource ownership: unrecorded worktrees are removed on preparation failure or cancellation, recorded worktrees remain available for retry, preparation is reserved by command identity, and failures are persisted. Failed removal can leave filesystem state for explicit cleanup; that behavior predates this change.

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Pre-merge checks | 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check		The title clearly and concisely describes the primary fix: rejecting new worktree launches when the base ref has no commit.
Description check		The description includes complete Problem, Change, Scope and approval, and Verification sections. It explains the behavior, implementation, test results, known limits, and unchecked scenarios.
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

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[coderabbitai]

Actionable comments posted: 1

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• 🪄 Fix CodeRabbit comments on this PR

🤖 Prompt to fix review comments

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Inline comments:
Review comments at @apps/server/src/orchestration-v2/ThreadLaunchService.ts:
- Around line 276-280: Replace the git.hasRefNamed fallback in the baseRef
preflight with a worktree-specific resolver that accepts only refs Git can
resolve as a commit for git worktree add, including valid remote-tracking
branches while excluding arbitrary namespaces and non-commit refs.

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ℹ️ Review info

⚙️ Run configuration

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• Review profile: CHILL
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• Run ID: 94c96642-f8c4-4bc9-88b1-744da40f554e

📥 Commits

Reviewing files that changed from the base of the PR and between c77a7b7 and 5ca9102.

📒 Files selected for processing (6)

• apps/server/src/git/GitWorkflowService.test.ts
• apps/server/src/git/GitWorkflowService.ts
• apps/server/src/mcp/toolkits/project/handlers.test.ts
• apps/server/src/mcp/toolkits/project/handlers.ts
• apps/server/src/orchestration-v2/ThreadLaunchService.test.ts
• apps/server/src/orchestration-v2/ThreadLaunchService.ts

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