Switching a thread's provider instance during a running turn kills the turn with "provider event stream closed unexpectedly"#22748

What happened

I run two accounts per harness as separate provider instances (personal claudeAgent / codex, work claude-work / a managed Codex instance). Turns randomly fail with "The provider event stream closed unexpectedly", on whichever provider happened to be running. It looked like a provider or account problem.

Diagnosis

The failures are caused by T3 itself: switching a thread's provider instance while the thread has a run in progress tears down the live session mid-turn. The turn then ends as a generic unknown failure instead of being refused, deferred, or reported clearly.

In my case an orchestrator agent was moving a set of worker threads from claude-work back to claudeAgent with t3_thread_configure (one call per thread). Several of those workers were mid-turn, and the orchestrator also switched its own thread. Every running thread it touched failed within ~150 ms.

Code path at v0.1.2-nightly.20261021.3457 (unchanged on main as of today):

1. t3_thread_configure (apps/server/src/mcp/toolkits/thread/handlers.ts:182) dispatches provider.switch when the instance changes (modelSelectionCommandType).
2. ProviderSwitchService (apps/server/src/orchestration-v2/ProviderSwitchService.ts:180) puts every live session from another instance in releaseProviderSessionIds for a create_with_handoff transition. It does not check whether a run is active.
3. The decider (apps/server/src/orchestration-v2/Orchestrator.ts:3640-3712) emits provider-session.detached and queues a provider-session.detach effect with detail "Provider or model selection changed.". There is no active-run guard, unlike thread.settle, which refuses with "is still running. Stop it before settling." (Orchestrator.ts:2835-2850).
4. ProviderSessionManager.detach closes the session (query.close goes to the Claude SDK while the model is still producing tool calls). The run's subscribers get a stream error, and the turn is recorded as class: "unknown" / "The provider event stream closed unexpectedly. Retry the turn; if it keeps failing, check the provider and server logs."

The detail string that explains the failure exists in the outbox, but the user never sees it.

Expected, any of:

• refuse provider.switch / thread.model-selection.set on a thread with an active run (like settle), or
• defer the switch until the current run ends, or
• at minimum, interrupt the run cleanly and report "Provider was switched while this turn was running" rather than a generic stream failure.

Steps to reproduce

1. Configure two provider instances of the same driver (e.g. claudeAgent and a second claudeAgent instance with another homePath).
2. Start a long turn on a thread using instance A (e.g. ask it to run several tool calls).
3. While it runs, switch the thread to instance B, either from another agent via the T3 MCP tool t3_thread_configure with modelSelection.instanceId = B, or by having the running agent call it on its own thread.
4. The running turn fails immediately with "The provider event stream closed unexpectedly".

Seen 8 out of 8 times in my logs: every switch issued against a running thread failed its turn. A switch in the same batch against an idle thread (07:42:15) caused no error.

Version

0.1.2-nightly.20261021.3457 (desktop app, nightly)

Environment

macOS 26 (Darwin 25.6.0) arm64, Node 26.8.2, Claude Code 2.1.306, codex-cli 0.163.0

Evidence

Related issues

#15135 is the same failure shape for thread.archive (self-archive detaches the live session without an active-run guard). This one is provider.switch / thread.model-selection.set, a different command and code branch, so a fix for archive alone won't cover it. #21369 covers the poor reporting after this error in general.

Fix applied or workaround

None applied on the machine. Workaround: only switch a thread's provider instance when it has no active run, and never let an agent switch its own thread mid-turn.

Filed by

Claude Code (claude-opus-5-5) via t3 triage