agent-host-chat-contributions

name	agent-host-chat-contributions
description	Build and review cross-cutting agent-host chat behavior through lifecycle contributions. Use when adding turn lifecycle side effects, prompt or context injection, restored-history transformation, protocol-action observation, or when reviewing changes that add code to AgentSideEffects or AgentService.

Agent Host Chat Contributions

Use the contribution model for a self-contained behavior that crosses an agent-host chat lifecycle boundary. Read these files before changing the model:

• src/vs/platform/agentHost/common/agentHostChatContributionsService.ts
• src/vs/platform/agentHost/node/agentHostChatContributionsService.ts
• src/vs/platform/agentHost/node/chatContributions/TODO.md
• src/vs/platform/agentHost/node/chatContributions/builtInChatContributions.ts

The rule that prevents the old failure mode

New cross-cutting features go in contributions, not in AgentSideEffects or AgentService.

AgentSideEffects has shed well over 700 lines by extracting seventeen behaviors. Do not put another lifecycle side effect, outgoing-prompt addition, restored-turn enrichment, admission check, or action observer back into either large orchestration class. AgentSideEffects dispatches the lifecycle and retains routing and send mechanics; AgentService owns session lifecycle and invokes hydration and memento eviction. See agentSideEffects.ts and agentService.ts.

This is also a review rule: code added to either file needs a specific reason that it is routing, a correctness invariant, provider-shaped behavior, or a dispatcher call site. "It was convenient to add here" is not a reason.

Decide before you code

Write a contribution when the behavior answers yes to this test:

1. Does it react to a lifecycle moment the hooks already name — a turn request asking to proceed, a completed turn, a dispatched action, a turn about to be sent, a restored turn list, or a chat being restored?
2. Can an explicitly registered, dependency-injected unit own it without changing protocol routing or provider mapping?
3. Does it compose with other behavior in a defined order?

Examples are admission gating, local-command interception, checkpoint and changeset work, queue draining, GitHub-reference attachment, title handling, persisted usage, session input-needed aggregation, session flag persistence, chat drafts, worktree announcements, unread state, rich-link guidance, artifact-tool guidance, and chat-surface guidance. Their directories under node/chatContributions/ are the reference implementations.

Do not use a contribution for the following:

• Subagent signal routing and buffering in node/agentSideEffects.ts are routing fabric and correctness invariants.
• Turn-id remapping in node/agentSideEffects.ts is also routing fabric.
• The three SDK event mappers and three attachment serializers are provider-shaped.
• Provider-shaped overlays applied during restore, such as seeding a draft's model from IAgent-supplied session metadata in _doRestoreSession. Moving one would push provider metadata into IHydrationContext for one consumer.

If the answer is "this must route every signal correctly before state changes" or "this differs by SDK," it is not a contribution. Open work and standing caveats are tracked in node/chatContributions/TODO.md.

Keep the hook surface small

IAgentHostChatContribution has seven hooks in common/agentHostChatContributionsService.ts. Only add another method when no existing payload can express the behavior.

The set was once reduced from six to four:

• onTurnConsumable became onTurnEnd with TurnEndReason.kind === 'localCommand'.
• onUserMessage became onOutgoingTurn.

Prefer adding a discriminant or payload to an existing type, such as a new TurnEndReason variant, over adding a hook. A proposed single-purpose hook with one caller and one implementer is the smell that produced both removals. Do not create an abstraction merely to move one direct call.

The three hooks added since clear a specific bar: no existing payload could express them because they fire at a different moment or from a different source, not merely with different data.

• onHydrateChat restores host-owned chat state before a chat enters the catalog. onHydrateTurns runs lazily, after a provider returns turns, so reusing it would leave a restored peer chat untitled until it was opened.
• onIncomingRequest gates a turn before it reaches a provider, and can reject it. No observer hook can refuse.
• onDidDispatchAction observes server-dispatched and rejected actions, which onDidApplyClientAction structurally cannot see.

Adding a { kind: ... } discriminant to an existing hook was considered and rejected for onHydrateChat: the discriminant guidance covers payload variants within one moment, the way TurnEndReason discriminates outcomes of a single turn ending.

Anatomy and registration

Give every contribution its own directory under node/chatContributions/<feature>/. It owns its implementation, helpers, and tests as it grows. A contribution must have:

• static readonly id, unique across registrations.
• An explicit order, even when it currently appears independent.
• A constructor whose first parameter is IAgentHostChatContributionContext, followed by injected services.
• A single registration in node/chatContributions/builtInChatContributions.ts.

This is the complete shape of the existing artifact-tools contribution, adapted from artifactTools/artifactToolsContribution.ts:

export class ArtifactToolsContribution extends Disposable implements IAgentHostChatContribution {
	static readonly id = 'artifactTools';
	readonly order = 200;

	constructor(
		protected readonly _context: IAgentHostChatContributionContext,
		@IAgentConfigurationService private readonly _agentConfigService: IAgentConfigurationService,
	) {
		super();
	}

	onOutgoingTurn(): ISendContribution | undefined {
		return this._agentConfigService.getRootValue(platformRootSchema, AgentHostArtifactToolsConfigKey)
			? { instructions: [ARTIFACT_TOOLS_INSTRUCTION] }
			: undefined;
	}
}

Add its constructor to the one built-in list with contributions.registerContribution(ArtifactToolsContribution). That list is activated by activateAgentHostContributions in node/agentHostContributions.ts; do not add a competing registration site. The dispatcher constructs each contribution through IInstantiationService, rejects duplicate ids, and disposes it on unregistration.

The seven hooks

Hook	Fires when	Must not assume	Existing example
onIncomingRequest	A turn request asks to proceed to a provider, before any side effect. Synchronous, and fails closed.	It decides admission, so a throw rejects rather than being skipped. Return accept, handled, or reject; the first non-accept wins and later contributions are not consulted.	localCommand 50, turnAdmission 100
onTurnEnd	Any terminal outcome a started turn can reach, plus a request refused before its turn started.	Discriminate rejected from error: the former has no started turn to finalize and no checkpoint to capture. Do not throw. It does not fire for an agent-emitted terminal action arriving with no active turn, because the reducer no-ops for those.	checkpointAndChangeset, queueDrain, githubReferences, sessionTitle, markUnread
onDidApplyClientAction	A client-dispatched action was applied to host state.	It never sees server-dispatched or rejected actions. Anything it sees definitely reduced.	queueDrain tracks queued senders; sessionTitle persists user title changes.
onDidDispatchAction	Any action was dispatched and its outcome is known, from any origin.	It also delivers rejected actions that never reduced — check rejectionReason. Implementing this and onDidApplyClientAction together sees every client action twice.	sessionInputNeeded, persistedTurnUsage, sessionFlags
onOutgoingTurn	Awaited after admission and provider lookup, before the turn is sent.	Rejected turns and noAgent failures never reach it. IOutgoingTurn carries the full Message; contributions can replace its text in order and add instructions, but cannot replace its already-committed attachments, model, agent, origin, or metadata. Prefer the message-text channel for context injection: changing instructions invalidates the provider prompt cache and increases user cost. Do not bypass the send path.

The call sites are in node/agentSideEffects.ts, node/agentHostTurnStarter.ts, and node/agentService.ts.

Ordering is behavior

order is load-bearing. The dispatcher sorts lower values first; registration order breaks ties only. Ordering is per hook, so the 100-series can be reused for hydration independently of outgoing turns. See node/agentHostChatContributionsService.ts.

Use explicit orders to preserve a required sequence:

• LocalCommandContribution is 50 and TurnAdmissionContribution is 100 on onIncomingRequest. That order is load-bearing: local commands were always intercepted before the read-only guard, so /rename still works in a read-only or archived chat.
• CheckpointAndChangesetContribution runs at 100 so it captures a checkpoint before later changeset-related work.
• QueueDrainContribution is 200 and SessionInputNeededContribution is 200.
• GitHubReferencesContribution and ChatSurfaceContribution are 300 where applicable.
• SessionTitleContribution is 400, MarkUnreadContribution and SideChatContribution are 500, ChatDraftContribution is 600, and SessionFlagsContribution is 700.

markUnread and sideChat both declare 500 and both implement onTurnEnd, so their relative sequence rests on registration order. Give a new contribution a distinct order rather than joining that tie.

A silent reordering regression already happened. The built-in-sequence regression tests in test/node/chatContributions.test.ts protect the original turn-end, outgoing-turn, and hydration sequences. Add or update those tests when ordering changes intentionally.

Depend on services, not a growing context

Never add callbacks or services to IAgentHostChatContributionContext. It is only contribution identity plus centrally evicted mementos. Coordinate through @Injectable services.

IAgentHostChatContributionHost is intentionally the narrow bridge to AgentSideEffects: it has only hostLaunchKind and sendTurnMessage. It grew to seven members before being cut back to two by promoting collaborators to real services:

• IAgentHostSessionTitleController
• IAgentHostProviderService
• IAgentHostTelemetryReporter
• IAgentHostTurnTracker
• IAgentHostToolCallTracker
• IAgentHostLocalCommands

IAgentHostWorktreeIsolation is injectable too. "It isn't injectable" was a wrong conclusion caused by service-registration order and later disproved. Trace registration from node/agentHostServices.ts through node/agentHostBootstrap.ts and createAgentServiceComposition in node/agentServiceComposition.ts. Register a real service before the instantiation path constructs contributions; do not smuggle it through the context or host bridge.

Adding a service is not free: roughly six hand-built test ServiceCollections construct AgentSideEffects or call registerBuiltInChatContributions, and each needs the new registration or fails with UNKNOWN service <id>. Two related traps, both hit for real:

• Only wrap a test registration in disposables.add(...) when the class extends Disposable; wrapping a plain object throws d.dispose is not a function at teardown.
• If the collaborator is stateless, prefer a plain function taking ServicesAccessor, invoked through IInstantiationService.invokeFunction, and skip DI entirely. node/agentHostTurnStarter.ts is the reference: making it a service bought nothing and cost a registration in every graph.

Mementos have a lifecycle

Create keys with createChatMementoKey or createSessionMementoKey in common/agentHostChatContributionsService.ts. Contexts are contribution-scoped, so identical debug names in two contributions do not collide.

• Chat mementos are evicted when their chat is disposed.
• Session mementos and all supplied chat mementos are evicted when their owning session is disposed.
• Extra key segments are part of the identity. Use them for bounded identities such as a queued message id.
• Call deleteMemento when an extra-segment value is no longer needed. Setting its observable to undefined leaves its map entry alive until chat or session disposal, so a long-lived chat otherwise accumulates one entry per segment value.
• onHydrateChat runs before its chat is registered, so a chat memento taken there would outlive a registration that then failed. No current contribution does this, but do not be the first without handling it.

queueDrain/queueDrainContribution.ts demonstrates the required delete on queued-message removal and consumption. Central eviction lives in node/agentHostChatContributionsService.ts and is called by node/agentService.ts.

Failure isolation, and the one exception

The dispatcher logs and isolates failures independently for every observer hook:

• A throwing onTurnEnd, onDidApplyClientAction, or onDidDispatchAction contribution does not stop later contributions.
• A failed onOutgoingTurn contribution adds no instructions and does not block the send or later contributions.
• A failed onHydrateTurns or onHydrateChat contribution deliberately passes the previous value to the next stage. Losing chat history, a title, or a draft is worse than losing one enrichment.

onIncomingRequest inverts this and fails closed. A contribution that throws there rejects the request with internalError at stage validation. That gate is the enforcement behind the UI hiding the composer, so failing open would let a buggy or remote client run work in a session that may no longer have its isolated worktree on disk. Losing one enrichment is survivable; letting a request past a guard is not.

It is also deliberately synchronous, for the same reason: a gate has to decide before the send path performs any await, so the state it reads cannot change between the decision and its effect. Every admission check the host makes today is already synchronous. An async version was tried first and broke three agentSideEffects tests by deferring the read-only guard one microtask past the assertions — which was the design saying the gate belongs before the first await.

Implement normal error handling where it improves the feature, but never depend on a thrown error to control lifecycle flow. The exact dispatch behavior is in node/agentHostChatContributionsService.ts.

Tests and extraction proof

Add focused dispatcher tests in test/node/chatContributions.test.ts for hook dispatch, ordering, memento eviction, and failure isolation. The same file owns the built-in-sequence regression tests.

When extracting existing behavior from AgentSideEffects, leave the existing test/node/agentSideEffects.test.ts suite unchanged. Its passing behavior is the proof that the extraction preserved semantics; test new dispatcher mechanics separately rather than rewriting the old assertions to fit the new structure.

One caveat that has bitten twice: some hand-built test graphs construct AgentSideEffects without calling registerBuiltInChatContributions, so they silently stop exercising behavior the moment it moves into a contribution. When an extraction breaks such a suite, add the missing registration so the graph mirrors production wiring — do not weaken the assertion. The failure means the test was verifying an incomplete graph, not that the extraction was wrong.