agentHost: resume failed turns without a new message

[roblourens (roblourens)]

Summary

• surface Try Again for Agent Host turns that fail with per-turn resume eligibility
• reopen the same AHP turn and preserve its original message, partial response, usage, and start time
• continue Copilot through sendMessages({ messages: [] }) and Codex through turn/start with an empty input
• preserve multi-client optimistic reconciliation and map synthetic chat request IDs back to the durable AHP turn for file changes and truncation
• coalesce Codex's physical empty-input retry turns into one logical AHP turn while retaining physical turn IDs for fork and rollback operations

Protocol dependency

Depends on microsoft/agent-host-protocol#371, which adds chat/turnResumed and per-turn resumable state for AHP 0.8.

Review notes

The highest-risk areas are:

• concurrent retries from multiple connected clients
• Copilot root-versus-subagent failure classification
• Codex logical/physical turn mapping during replay, fork, and truncation
• synthetic chat request IDs used by file changes and checkpoints

The implementation intentionally avoids a static agent capability: Turn.resumable is the precise gate, and provider resume is an optional host-side operation.

Validation

• npm run typecheck-client
• npm run valid-layers-check
• npm run precommit
• focused Agent Host unit matrix: 1,245 passing, 17 provider-gated
• latest critical resume matrix: 791 passing before the final Codex replay fix
• Codex replay mapper after latest-main merge: 15 passing
• Codex Agent Host E2E: 38 passing, 49 provider-gated

(Written by Copilot)

[Copilot]

Pull request overview

Adds AHP 0.8 failed-turn resumption while preserving logical turn state across clients and providers.

Changes:

• Adds resumable turn protocol state, dispatch, reconciliation, and Try Again UI.
• Implements zero-message resume for Copilot and Codex, including replay and turn-ID mapping.
• Expands unit coverage for retries, concurrency, restoration, file changes, forks, and truncation.

Show a summary per file

File	Description
src/vs/workbench/services/agentHost/browser/editorRemoteAgentHostServiceClient.ts	Adds sequenced action dispatch.
src/vs/workbench/contrib/terminal/test/browser/agentHostPty.test.ts	Updates connection mock.
src/vs/workbench/contrib/chat/test/browser/agentSessions/agentHostChatContribution.test.ts	Tests retry UI and reconciliation.
src/vs/workbench/contrib/chat/test/browser/agentHost/agentHostResponseFileChanges.test.ts	Tests synthetic retry IDs.
src/vs/workbench/contrib/chat/browser/agentSessions/agentHost/stateToProgressAdapter.ts	Supports custom restored errors.
src/vs/workbench/contrib/chat/browser/agentSessions/agentHost/agentHostSessionHandler.ts	Implements retry UI and observation.
src/vs/workbench/contrib/chat/browser/agentSessions/agentHost/agentHostResponseFileChanges.ts	Resolves retry IDs to turns.
src/vs/platform/agentHost/test/node/reducers.test.ts	Tests resume reducer behavior.
src/vs/platform/agentHost/test/node/mockAgent.ts	Adds mock resume support.
src/vs/platform/agentHost/test/node/mapSessionEvents.test.ts	Tests restored Copilot failures.
src/vs/platform/agentHost/test/node/copilotTestEvents.ts	Types session-error fixtures.
src/vs/platform/agentHost/test/node/copilotAgentSession.test.ts	Tests zero-message Copilot resume.
src/vs/platform/agentHost/test/node/copilotAgent.test.ts	Tests Copilot resume routing.
src/vs/platform/agentHost/test/node/codex/codexReplayMapper.test.ts	Tests retry-turn coalescing.
src/vs/platform/agentHost/test/node/codex/codexMapAppServerEvents.test.ts	Tests resumable Codex errors.
src/vs/platform/agentHost/test/node/codex/codexForkPlan.test.ts	Tests coalesced fork mapping.
src/vs/platform/agentHost/test/node/agentSideEffects.test.ts	Tests resume side effects.
src/vs/platform/agentHost/test/node/agentService.test.ts	Tests concurrent resume rejection.
src/vs/platform/agentHost/node/copilot/mapSessionEvents.ts	Restores resumable Copilot errors.
src/vs/platform/agentHost/node/copilot/copilotAgentSession.ts	Continues Copilot with no messages.
src/vs/platform/agentHost/node/copilot/copilotAgent.ts	Routes Copilot resume operations.
src/vs/platform/agentHost/node/codex/codexReplayMapper.ts	Coalesces physical retry turns.
src/vs/platform/agentHost/node/codex/codexMapAppServerEvents.ts	Marks eligible Codex failures.
src/vs/platform/agentHost/node/codex/codexForkPlan.ts	Maps logical to physical turns.
src/vs/platform/agentHost/node/codex/codexAgent.ts	Implements empty-input Codex retry.
src/vs/platform/agentHost/node/agentSideEffects.ts	Executes provider resume operations.
src/vs/platform/agentHost/node/agentService.ts	Validates resume transitions.
src/vs/platform/agentHost/electron-browser/localAgentHostService.ts	Exposes sequenced dispatch locally.
src/vs/platform/agentHost/common/state/sessionActions.ts	Re-exports resume action.
src/vs/platform/agentHost/common/state/protocol/version/registry.ts	Registers AHP 0.8 action.
src/vs/platform/agentHost/common/state/protocol/common/actions.ts	Adds resume action type.
src/vs/platform/agentHost/common/state/protocol/channels-chat/state.ts	Adds per-turn eligibility.
src/vs/platform/agentHost/common/state/protocol/channels-chat/reducer.ts	Reopens failed turns.
src/vs/platform/agentHost/common/state/protocol/channels-chat/actions.ts	Defines resume protocol data.
src/vs/platform/agentHost/common/state/protocol/action-origin.generated.ts	Updates generated action metadata.
src/vs/platform/agentHost/common/state/protocol/.ahp-version	Pins protocol dependency.
src/vs/platform/agentHost/common/agentService.ts	Extends agent and connection APIs.
src/vs/platform/agentHost/browser/remoteAgentHostProtocolClient.ts	Returns optimistic sequence IDs.
src/vs/platform/agentHost/browser/nullAgentHostService.ts	Implements unsupported dispatch stub.

Review details

• Files reviewed: 39/39 changed files
• Comments generated: 3
• Review effort level: Balanced