agentHost: add diagnostic tracing for session disconnect errors

[osortega]

Problem

After a WebSocket reconnect (e.g. tab hidden → server kills socket → tab visible), sending a message to an existing agent session fails with:

Error: (sendFailed) Error: Request session.send failed with message: Session not found: <session-id>

The root cause is unclear — multiple theories exist but none are proven:

1. Stale in-memory SDK session reused after clientId change (isOutdated doesn't compare clientId)
2. SDK resumeSession failing with a non--32603 error code
3. SDK session expiring independently of the agent host lifecycle

Changes

Diagnostic logging (4 files)

Adds logging at every decision point in the session lifecycle to determine the exact failure path:

• copilotAgent.ts — sendMessage: logs cached entry status, isOutdated result, full error code/message/type on failure. setClientTools: logs clientId + whether a cached SDK session exists. _resumeSession: logs SDK resumeSession/createSession calls and results.
• agentSideEffects.ts — logs error code and type on sendMessage failure (was only logging raw error)
• protocolServerHandler.ts — logs subscription count on client disconnect
• remoteAgentHost.contribution.ts — logs old vs new clientId on reconnect

What the logs will tell us

When the error reproduces, the sequence will reveal:

[ProtocolServer] Client disconnected: <old-id>, subscriptions=N
[RemoteAgentHost] Reconnecting: oldClientId=X, newClientId=Y
[Copilot:<session>] setClientTools: clientId=Y, hasCachedSdkSession=true/false
[Copilot:<session>] sendMessage: cachedEntry=true/false, hasActiveClient=true/false
[Copilot:<session>] entry.send() failed: code=<X>, message=...

This proves whether it's path A (stale cached entry) or something else.

[Copilot]

Pull request overview

This PR adds diagnostic tracing across the agent host session lifecycle to help pinpoint the cause of “Session not found” failures after reconnects, by emitting structured logs around reconnect/disconnect, session caching, resume/create fallbacks, and send failures.

Changes:

• Add detailed lifecycle and failure-path logging in CopilotAgent around setClientTools, sendMessage, and _resumeSession.
• Improve disconnect logging to include subscription counts and reconnect logging to include old/new client IDs.
• Enhance AgentSideEffects send failure logs to include error code/type/message.

Show a summary per file

File	Description
src/vs/sessions/contrib/remoteAgentHost/browser/remoteAgentHost.contribution.ts	Logs old/new clientId and whether name changed when reconnecting a contribution.
src/vs/platform/agentHost/node/protocolServerHandler.ts	Logs subscription count when a protocol client disconnects.
src/vs/platform/agentHost/node/copilot/copilotAgent.ts	Adds detailed logs around session caching, resumeSession/createSession behavior, and send failures.
src/vs/platform/agentHost/node/agentSideEffects.ts	Logs sendMessage failures with code/type/message context.

Copilot's findings

Comments suppressed due to low confidence (1)

src/vs/platform/agentHost/node/copilot/copilotAgent.ts:972

• SDK resumeSession failed is logged without including the thrown error object, which can hide stack traces and any additional fields the SDK attaches. Pass err as an additional logger argument (as is done for getSessionMetadata failed) so the diagnostic output remains actionable.

		const snapshot = activeClient ? await activeClient.snapshot() : undefined;
		const storedMetadata = await this._readSessionMetadata(sessionUri);
		const sessionMetadata = await client.getSessionMetadata(sessionId).catch(err => {
			this._logService.warn(`[Copilot:${sessionId}] getSessionMetadata failed`, err);
			return undefined;

• Files reviewed: 4/4 changed files
• Comments generated: 5

[github-actions]

Screenshot Changes

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