[Bug]: Forking a thread fails on Claude instances with a custom homePath #18260

Before submitting

• I searched existing issues and did not find a duplicate.
• I included enough detail to reproduce or investigate the problem.

Area

apps/server

Steps to reproduce

1. Add a Claude provider instance with a custom home, e.g. "config": { "homePath": "~/.claude-2" }.
2. Run at least one turn on a thread using that instance.
3. Fork the thread and send a message in the fork.

Expected behavior

The fork starts a turn from the parent's Claude session.

Actual behavior

The turn fails to start ("Provider turn failed to start: Claude Agent SDK query failed."). session.fork is retried 5 times in about 2s, and every attempt fails with:

Session <id> not found in project directory for <cwd>

The session transcript exists under the instance's home (~/.claude-2/projects/<cwd-slug>/<id>.jsonl), but the SDK searches the default ~/.claude.

Cause: ClaudeAgentSdkQueryRunner.forkSession (apps/server/src/orchestration-v2/Adapters/ClaudeAdapterV2.ts) calls the SDK's forkSession in the server process with only dir and upToMessageId. The instance's CLAUDE_CONFIG_DIR is only applied to query subprocesses, so the fork resolves sessions against the server's own environment. getSubagentMessages in the same runner is called the same way and probably has the same problem.

apps/server/src/claudeHistoryWorker.ts (and the __claude-history subcommand) exist to run these SDK history helpers in a separate process with the provider's environment, but nothing in the V2 adapter starts that process.

Default-home Claude instances are unaffected, which is why forking fails only for some Claude threads.

Impact

Major degradation or frequent failure

Version or commit

T3 Code (Nightly) desktop; main @ ee7924f

Environment

macOS (Darwin 25.6.0), desktop app, Claude provider instance with custom homePath, claude-opus-5-5

Logs or stack traces

ProviderAdapterForkThreadError: Failed to fork claudeAgent provider thread provider-thread:provider:claudeAgent:native-thread:pending%3Arun%3Athread%3A<parent>%3Aordinal%3A1.
    at ClaudeAdapterV2.forkThread
    at orchestrationV2.providerTurnStart.start
  [cause]: ClaudeAgentSdkQueryRunnerError: Claude Agent SDK query failed.
    at ClaudeAgentSdkQueryRunner.forkSession
    [cause]: Error: Session d481b7d4-0d29-4891-a74b-92cfb137b65e not found in project directory for /Users/<user>/Repos/<project>

Workaround

Keep working in the parent thread instead of forking, or fork only threads on a Claude instance that uses the default ~/.claude home.