Agent Host: client subscribes to a session it never created on the backend (-32001 "Session not found on backend"), and the turn hangs forever

[eleanorjboyd]

Does this issue occur when all extensions are disabled?: Yes. This occurs in the core Agent Host session lifecycle. It was reproduced in automated eval containers using the eval driver extension.

• VS Code Version: Insiders commits 5a36a9c68ce6 (2026-08-10) and 22231c1c3817 (2026-08-11). It was not observed on 8eaafbf4220a (2026-08-07).
• OS Version: Linux x64, containerized
• Feature: Agents window, Agent Host / Copilot CLI provider, new-session flow

Summary

A new Agent Host chat can get stuck on Working... forever before the first model turn starts.

In the failing path, VS Code:

1. Materializes a new copilotcli:/<session-id> session.
2. Subscribes to that session in the Agent Host backend.
3. Receives -32001 Session not found on backend.
4. Never creates the missing backend session, surfaces the error, or completes the chat request.

This is a timing-dependent regression. In our automated runs it affected roughly 25-34% of attempts across OpenAI, Anthropic, Google, and other models. The model provider is not involved: affected attempts emit no model, chat, or tool spans.

Steps to Reproduce

The race reproduces probabilistically in our environment:

1. Run workbench.action.chat.openNewSessionSidebar.agent-host-copilotcli.
2. Immediately run workbench.action.chat.open with a prompt, without waiting for eager backend-session creation to settle.
3. Observe that some attempts remain on Working... indefinitely.

Our harness sends the two commands back to back. Successful attempts had only a 2-66 ms margin between backend createSession and chat.open.

Expected

The backend session is created before it is used. If session creation fails or is superseded, the chat request fails promptly with an actionable error.

Actual

VS Code subscribes to a backend session that does not exist. No model turn starts, and the request remains pending until externally cancelled. In our harness that happened after approximately 105 minutes.

Evidence at a glance

• All 4/4 hung instances sent zero createSession requests.
• All 6/6 healthy controls sent two createSession requests.
• Hung instances sent subscribe and received -32001 Session not found on backend.
• Hung instances emitted zero invoke_agent, chat, and execute_tool spans.
• Healthy and hung controls came from the same runs, builds, images, and scenarios.
• The regression appeared between 8eaafbf4220a and 5a36a9c68ce6; the evaluation harness did not change across that boundary.

Could the Agent Host owners please investigate the new-session materialization/session-creation race and ensure this path either creates a valid backend session or fails promptly? The detailed evidence below is intended to help with diagnosis; I am deliberately not prescribing the implementation because the owning team has more context.

Technical evidence and investigation notes

First divergence

Failing AHP wire log:

12:28:47.567Z c2s id=6 resolveSessionConfig
12:28:47.659Z c2s id=7 subscribe copilotcli:/24d3feef-7f28-4d16-a07b-df052d4f4c82
12:28:47.806Z s2c id=7 error -32001 Session not found on backend: copilotcli:/24d3feef-7f28-4d16-a07b-df052d4f4c82

There is no createSession request anywhere in the failing wire log.

Matched healthy instance from the same run and build:

14:17:02.618Z c2s id=10 createSession copilotcli:/6de9cc29-f86e-4ae0-809e-4c4effa362c6
14:17:02.695Z c2s id=11 subscribe     copilotcli:/6de9cc29-f86e-4ae0-809e-4c4effa362c6
14:17:02.696Z c2s id=12 createSession copilotcli:/1b9e0af5-7246-4ffc-8e6b-243344e02ecd
14:17:02.856Z c2s id=14 subscribe     copilotcli:/1b9e0af5-7246-4ffc-8e6b-243344e02ecd

Client log

12:28:47.511Z Starting command execution: workbench.action.chat.openNewSessionSidebar.agent-host-copilotcli
12:28:47.613Z Command ...openNewSessionSidebar... completed in 92ms
12:28:47.622Z Starting command execution: workbench.action.chat.open
12:28:47.646Z ChatService#materializeUntitledSession: Materialized untitled session into real session
12:28:47.787Z [AgentHost] _invokeAgent called
12:28:47.815Z [RemoteAgentHostProtocol] Request 7 failed: -32001 Session not found on backend
12:28:47.816Z [AgentHost] _readEagerlyCreatedSessionState: hydrated value=error(Session not found on backend)
12:28:47.823Z [AgentHost:stderr] SyntaxError: Unexpected end of JSON input
12:28:47.824Z [AgentHost:stderr] SyntaxError: Unexpected end of JSON input
... no agent progress ...
14:13:47.495Z Command workbench.action.chat.open completed in 6299849ms after external cancellation

Evidence matrix

Population	Instances	createSession	resolveSessionConfig	Materialize after chat.open	Model/chat/tool spans
Hung	4	0 each	1 each	24-46 ms	0
Healthy controls	6	2 each	5 each	221-537 ms	Present

The four signals separated the populations with no overlap.

At run level, invoke_agent span counts also reconcile exactly:

Run	Instances	Hung	invoke_agent spans
31312886600	120	0	120
31387967408	120	32	88
31491236537	120	38	82

Likely race location

One source path consistent with the observations is _startNewSessionBackend in baseAgentHostSessionsProvider.ts:

void newSession.trackConfigResolution(...);

const { trusted } = await this._workspaceTrustManagementService.getUriTrustInfo(workspaceUri);
if (this._newSessions.get(newSession.sessionId) !== newSession) {
    return;
}
newSession.eagerCreate(connection);

If the draft is replaced while awaiting trust information, config resolution has started but eagerCreate() is skipped. That matches the failing wire signature: one resolveSessionConfig, zero createSession, followed by materialization and subscribe.

This source-level explanation is medium confidence; the missing-session behavior itself is directly observed and high confidence.

Regression boundary

Date	VS Code commit	Result
Aug 8-9	8eaafbf4220a	Healthy
Aug 10	5a36a9c68ce6	First broken build
Aug 11	22231c1c3817	Still broken

The boundary contains 111 commits. bba5e40a34c2 (#329839) is a useful early bisect candidate because it reworked the local Agent Host transport, but this is not a causal claim. The new transport-loss branch from that change does not appear in the failing logs.

Related but not duplicates

• agentHost: add generic agent turn hang telemetry #329884 describes a similar "no model turn ever started / Working forever" symptom but adds telemetry rather than tracking this missing-createSession mechanism.
• Agent Host: Peer-chat subagent replay corrupts backend session URI #329430 produces the same -32001 error through peer-chat replay URI corruption, a different mechanism.
• agentHost: request trust before provisional rebind creates a backend #328665 covers workspace-trust ordering but not a materialized session that was never created.
• agentHost: fix lost session customizations on materialization race #330465 covers lost customizations during materialization, not missing backend-session creation.

Suggested validation

A focused regression test could repeatedly open a new Agent Host session and immediately send a prompt, then assert that every first subscribe for a copilotcli:/... channel has a preceding successful createSession, or that the request fails within a bounded time.

References

• Eval tracker: microsoft/vscode-copilot-evaluation#6298
• Run 31387967408: first broken build, 32/120 hung
• Run 31491236537: 38/120 hung
• Run 31312886600: healthy control