agentHost: narrow Copilot resume fallback to true empty-session errors

[roblourens]

Symptom

Opening an existing Agent Host session sometimes shows an empty chat view, even when the same session opens fine in copilot CLI. The session contents appear lost.

Root cause

CopilotAgent._doResumeSession had a too-broad catch block: any -32603 JSON-RPC error from client.resumeSession was treated as "this session has no messages", and the agent fell back to creating a new session with the same ID via client.createSession({ sessionId }). The user then saw an empty chat instead of the real error.

In the case that prompted this investigation, the real failure was upstream schema validation: a session.compaction_complete event in the session file had tokenDetails[].batchSize: 0, and the installed @github/copilot@1.0.55-3 schema declares exclusiveMinimum: 0 for that field, so the SDK rejected the entire file with -32603 Session file is corrupted (...batchSize: Number must be greater than 0). (Upstream relaxed this to minimum: 0 in @github/copilot@1.0.56, but the underlying that the writer can emit 0 while the reader rejects is independent of our fallback.)it issue

Why the fallback exists (don't remove it)

The fallback was introduced (54caaaf3943, "finally works v.v") for the case where the on-disk session legitimately has zero most commonly after the user invokes "Start Over", which calls truncateSession and removes every turn. The SDK then refuses to resume an empty session, and the only reasonable recovery is to create a fresh session with the same ID so the user can keep typing into the same chat. This PR preserves that behavior.events

Fix

Invert the heuristic. Treat any -32603 from resumeSession as the empty-session case (preserving Start Over and any future SDK rewording of the empty-session message) unless the message clearly indicates corruption / schema validation / parse failure / malformed those propagate so the user sees the real error rather than silently getting an empty session.input

function shouldCreateEmptySessionAfterResumeError(err: unknown): boolean {
    if (getCopilotSdkErrorCode(err) !== -32603) {
        return false;
    }
    const message = getErrorMessage(err);
    return !/\b(corrupt|corrupted|invalid|validation|schema|must be|parse|malformed|unexpected token)\b/i.test(message);
}

Out of scope

1.0.56bump fixes the specificbatchSize: 0` schema issue but is a 70-file delta (CLI JS, schemas, native prebuilds, ripgrep, MCP behavior changes) that we don't want to ship the day before a release without smoke testing. If a runtime upgrade is desired, it should be its own PR.

Tests

Three regression tests in a new _resumeSession fallback suite in src/vs/platform/agentHost/test/node/copilotAgent.test.ts:

fallback fires, new session is created with the same id.
fallback fires (defensive default).
fallback does NOT fire, the error propagates.

The tests exercise the real _doResumeSession path via a small ResumePathCopilotAgent subclass (the existing TestableCopilotAgent bypasses it).

(Written by Copilot)

[Copilot]

Pull request overview

Narrows the Copilot resume-session fallback so an empty session is only recreated when the SDK error message clearly indicates "no messages"/"empty session", and never when it indicates corruption/validation/parse failures. This prevents users with a corrupted session file (e.g. batchSize: 0 schema validation rejection) from silently being shown an empty new session under the same ID instead of the real error.

Changes:

• Add getCopilotSdkErrorCode, getErrorMessage, and shouldCreateEmptySessionAfterResumeError helpers and use them in _doResumeSession's catch path.
• Replace the broad errCode !== -32603 guard with the narrower message-based check.
• Add a _resumeSession fallback test suite exercising the real resume path via a new ResumePathCopilotAgent test subclass and a useRealResumePath option.

Show a summary per file

File	Description
src/vs/platform/agentHost/node/copilot/copilotAgent.ts	Introduces the SDK-error inspection helpers and narrows the resume fallback condition.
src/vs/platform/agentHost/test/node/copilotAgent.test.ts	Adds regression tests covering both the empty-session fallback and the corrupted-session propagation, with a ResumePathCopilotAgent to drive the real _doResumeSession.

Copilot's findings

• Files reviewed: 2/2 changed files
• Comments generated: 0