fix(server): reject thread.create for an existing id

[ScottN-PV]

Problem

A thread.create with a fresh commandId and an existing thread id is accepted. The new thread.created event overwrites the row: title, createdAt and the provider link. For a deleted thread it also clears deletedAt and revives it. Only clients that pick their own thread ids hit this.

Change

dispatchThreadCreate reads the thread projection first and rejects the command when a row exists, live or tombstoned, before any event is emitted. A retry of the original commandId still replays its receipt, and the thread.created upsert is unchanged, so projection replay stays idempotent.

The rejection is a new tagged OrchestratorThreadAlreadyExistsError. Rejected receipts keep only the error's message, and OrchestratorDispatchError's message is generic, so a string cause would leave a retry of the rejected commandId with no reason. The new error carries "Thread already exists. Use a new thread id." into the receipt and the /ws orchestration.dispatchCommand error. ThreadMessageIntake treats it as a pre-commit failure, so a launch with uploaded attachments for an existing id releases the copies it claimed.

Scope and approval

Closes #19698. Maintainer triage confirmed the bug and asked for this shape: reject a fresh-commandId create for an existing id, keep the upsert. Restoring a deleted thread stays a separate command.

Verification

• cd apps/server && vp test run src/orchestration-v2/Orchestrator.control-reads.test.ts src/orchestration-v2/ThreadLaunchService.test.ts src/orchestration-v2/ThreadMessageIntake.test.ts: 92 pass. The two new duplicate-create cases assert the typed failure, an unchanged row and event sequence, and the message on a retry of the rejected commandId. The extended attachment-intake case asserts the claimed copy is released. With both production files reverted to main (ec1313c8), exactly these three fail.
• vp run --filter t3 typecheck passes. vp fmt --check and vp lint on the changed files: no errors, no new warnings.
• Real /ws run on Linux against an isolated server: recreate a live thread and a deleted one, each with a fresh commandId. With Orchestrator.ts at main both are accepted, the title is overwritten and deletedAt is cleared. With the fix both are rejected with the message above, the rows are unchanged, and replaying the original commandId returns its sequence. Not run on macOS or Windows.
• Rebased onto main 21bedfb1 (39226f42): 93 tests pass (the earlier 92 plus a delegated-task test that main added), and with both production files reverted the same three fail.

Model: GPT-6.1-Sol, Claude Sonnet 5.5 (verification), Claude Opus 5.5 (rebase), Claude Fable 5.1 (review), GPT-6-Astra (review). Harness: Codex, Claude Code.

[macroscopeapp]

Approvability

Verdict: Approved at 3b24202

Macroscope's review found this PR approvable — This is a focused server bug fix that rejects duplicate thread creation before emitting events, including deleted-thread tombstones, while preserving normal creation and receipt replay. The accompanying tests cover projection stability and attachment cleanup, with no product-default or static-analysis changes.

^{You can add or adjust custom eligibility rules. Learn more.}

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

• Configuration used: Path: .coderabbit.config.ts
• Review profile: CHILL
• Plan: Advanced
• Run ID: fda31487-a0ec-460b-be3c-001fe4ab4e71

📥 Commits

Reviewing files that changed from the base of the PR and between 3b24202 and 39226f4.

📒 Files selected for processing: 1

• apps/server/src/orchestration-v2/Orchestrator.control-reads.test.ts

Included review availability: This review used your included allowance. Your plan provides up to 10 included reviews per hour; 9 remain after this review.

📝 Walkthrough

Thread creation now rejects IDs that already have an active or deleted thread projection. The orchestrator reports a specific error, and attachment intake treats the refusal as an unaccepted dispatch. Tests cover unchanged state, rejected-command retries, and replay of the original receipt.

Changes: Thread creation

Layer / File(s)	Summary
Reject existing thread IDs apps/server/src/orchestration-v2/Orchestrator.ts, apps/server/src/orchestration-v2/Orchestrator.control-reads.test.ts	The orchestrator checks the requested ID before creating a thread. It reports OrchestratorThreadAlreadyExistsError for existing projections, including tombstones. Tests verify rejection, unchanged projection and event sequence, retry behavior, and replay of the original receipt.
Release attachments after refusal apps/server/src/orchestration-v2/ThreadMessageIntake.ts, apps/server/src/orchestration-v2/ThreadLaunchService.test.ts	Attachment intake classifies the new error as a pre-commit refusal. The test verifies the duplicate launch error and unchanged claimed-file count and attachment bytes.

Priority: ⬇️ Low

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

Change: Bug fix · Severity of issue fixed: Low

Suggested reviewers: juliusmarminge

Merge Risk: ⚪ Minimal · up to 39226

No actionable merge-blocking issue is established; the change is mergeable after normal checks.

### Architecture Summary

Architecture risk: 🔵 Low · up to 39226

The change affects 1 system.

Changed systems: apps/server

Architecture concerns
No architecture-level concerns identified.

Review details

Systems and components

• observed — apps/server (service) was modified; 4 changed files map to changed impact.

Before / after behavior

• observed — Modified behavior in apps/server/src/orchestration-v2/Orchestrator.ts: Added OrchestratorThreadAlreadyExistsError, carrying the command and thread IDs and reporting that the existing thread ID must be replaced with a new one.
• observed — Modified behavior in apps/server/src/orchestration-v2/Orchestrator.ts: Added OrchestratorThreadAlreadyExistsError to the OrchestratorV2Error union.
• observed — Modified behavior in apps/server/src/orchestration-v2/Orchestrator.ts: Before creating a thread, dispatchThreadCreate now reads the requested ID. A missing thread is treated as available; other read failures become OrchestratorProjectionError, while any existing thread, including a tombstone, produces OrchestratorThreadAlreadyExistsError.
• observed — Modified behavior in apps/server/src/orchestration-v2/ThreadLaunchService.test.ts: The test adds the effect/Result import to inspect the duplicate launch outcome.

Pre-merge checks: 4

Passed checks: 4 passed

Check name	Status	Explanation
Linked Issues check		Issue #19698 requires thread.create to reject a fresh commandId when the thread projection exists, including live and tombstoned rows, with a clear error. dispatchThreadCreate now checks the pro…
Out of Scope Changes check		The production changes support issue #19698. The tagged error preserves the rejection message in the receipt and dispatch error. ThreadMessageIntake releases claimed attachments after the rejected c…
Title check		The title clearly and concisely describes the main change: rejecting thread.create when the requested ID already exists.
Description check		The description covers the problem, implementation, scope approval, verification results, limitations, and agent details. It matches the requested behavior and provides specific test evidence.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create a new PR

• Autofix · Keep fixing CodeRabbit findings and required CI, and resolving merge conflicts

_{Comment @coderabbitai help to get the list of available commands.}