[Bug]: Imported Claude Code threads fail on their first follow-up with "Session ID … is already in use" #15243

Before submitting

• I searched existing issues and discussions. This was predicted from code reading in [Feature]: Continue a Codex or Claude Code session started in the terminal as a T3 Code thread #15234 and juliusmarminge asked for a reproduction there.
• This report includes the session details and the error text.

Area

apps/server

Steps to reproduce

1. Run a Claude Code session in a project folder from the terminal (for example claude in ~/work/<project>), so ~/.claude/projects/<encoded-cwd>/<sessionId>.jsonl exists.
2. Import that session into T3 Code through the existing session import (agentSessions.import via the welcome wizard at /welcome, or the per-session picker from [Feature]: Continue a Codex or Claude Code session started in the terminal as a T3 Code thread #15234).
3. Open the imported thread and send any follow-up message.

Expected behavior

The follow-up resumes the existing Claude Code session (resume: <sessionId>) and the turn runs.

Actual behavior

The turn fails immediately. The thread shows "Provider error / Provider turn failed." and its status becomes Failed. The server logs orchestration-v2.claude-query-stream-failed for the thread's provider turn with the cause elided.

Running the same thing the adapter does, directly against the CLI, shows the underlying error:

$ cd ~/work/being-and-becoming
$ claude -p --session-id e7f9c804-28e6-45bb-a19d-de63cd7bc206 "Reply with just the word ok."
Error: Session ID e7f9c804-28e6-45bb-a19d-de63cd7bc206 is already in use.

Cause, from reading the code: AgentSessionImporter writes the imported provider thread with a strong nativeThreadRef but nativeConversationHeadRef: null and no provider turns. In ClaudeAdapterV2 openQuery, shouldResume is true only when there is a conversation head, the process already opened the session, or providerTurnOrdinal > 1. An imported thread has none of those on its first follow-up, so makeClaudeQueryOptions passes sessionId: <imported id> instead of resume: <imported id>, and the CLI rejects the id it already owns. The adapter's own comment at that spot describes exactly this failure. The provider_resume_fallback in ProviderTurnStartService does not catch it, because the Claude resumeThread is a no-op that succeeds and the failure happens later at query open.

Codex imports are unaffected: the Codex adapter sends thread/resume { threadId } for the same ref and the turn runs.

Impact

Major degradation or frequent failure. Every imported Claude Code session (wizard import or per-session import) fails on its first follow-up, so Claude sessions cannot be continued in T3 Code.

Version or commit

main at ce90eec (2026-10-03), reproduced on a branch that adds only the per-session picker from #15234 on top of it. Claude Code CLI: 2.1.286 (Claude Code).

Environment

macOS (Darwin 27), web client in the dev server (vp run dev with an isolated --home-dir), Claude Code provider instance with its default home.

Logs or stack traces

[21:46:47.701] WARN (#8689): orchestration-v2.claude-query-stream-failed
  {
    providerSessionId: 'provider-session:provider-instance:claudeAgent:thread:import%3AclaudeAgent%3Ae7f9c804-28e6-45bb-a19d-de63cd7bc206:12ccf8a1-8a39-48d4-8f4a-1dc6129d0275',
    providerThreadId: 'provider-thread:provider:claudeAgent:native-thread:e7f9c804-28e6-45bb-a19d-de63cd7bc206',
    providerTurnId: 'provider-turn:provider:claudeAgent:native-turn:turn%3Arun-attempt%3Arun%3Arun%253Athread%253Aimport%25253AclaudeAgent%25253Ae7f9c804-28e6-45bb-a19d-de63cd7bc206%253Aordinal%253A1%3Aattempt%3A1',
    cause: { _id: 'Cause', failures: [ [Object] ] }
  }

Turn item persisted for the run: {"type":"error","title":"Provider error","failure":{"class":"transport_error","message":"Provider turn failed.","code":null,"retryable":null}}.

Workaround

None inside T3 Code. Resuming from the terminal with claude --resume <sessionId> works.

Related

• [Feature]: Continue a Codex or Claude Code session started in the terminal as a T3 Code thread #15234 (per-session continue proposal, where this was predicted)
• Discussion [Feature]: continue threads from claude code #6873

Filed by alimohammed1624. Drafted with Claude Fable 5.1 in Claude Code.