Refactor model alias handling and add copilot-fast support

[sbatten]

Remove the ModelAliasRegistry to simplify the codebase and enhance clarity. Introduce support for the copilot-fast endpoint, including logic for its creation and management. Update related components to accommodate these changes.

[Copilot]

Pull request overview

This PR removes the ModelAliasRegistry indirection and introduces a dedicated internal copilot-fast endpoint that selects a preferred “fast” model with fallback behavior, updating endpoint selection logic accordingly.

Changes:

• Removed ModelAliasRegistry and its alias-based model resolution/registration flow.
• Added CopilotFastChatEndpoint to resolve copilot-fast via a primary/fallback model and to force low-cost reasoning settings for Responses API requests.
• Updated ProductionEndpointProvider and LanguageModelAccess to route copilot-fast without alias expansion.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/platform/endpoint/node/modelMetadataFetcher.ts	Removes alias resolution when mapping ChatEndpointFamily → model metadata.
src/platform/endpoint/node/copilotChatEndpoint.ts	Adds CopilotFastChatEndpoint with primary/fallback selection and request override behavior.
src/platform/endpoint/common/modelAliasRegistry.ts	Deletes the alias registry implementation and the copilot-fast alias registration.
src/extension/prompt/vscode-node/endpointProviderImpl.ts	Special-cases copilot-fast to create/cache a dedicated fast endpoint instance.
src/extension/conversation/vscode-node/languageModelAccess.ts	Removes alias model injection and stops resolving endpoints via alias mapping.

Comments suppressed due to low confidence (3)

src/platform/endpoint/node/modelMetadataFetcher.ts:166

• getChatModelFromFamily still accepts ChatEndpointFamily (which includes 'copilot-fast'), but with the alias resolution removed this method will now try to look up 'copilot-fast' in _familyMap and throw. Either re-introduce explicit handling for 'copilot-fast' here, or change the typing/contract so callers can’t pass a value that will never resolve via _familyMap.

	public async getChatModelFromFamily(family: ChatEndpointFamily): Promise<IChatModelInformation> {
		await this._taskSingler.getOrCreate(ModelMetadataFetcher.ALL_MODEL_KEY, this._fetchModels.bind(this));
		let resolvedModel: IModelAPIResponse | undefined;

		if (family === 'copilot-base') {
			resolvedModel = this._copilotBaseModel;
		} else {
			resolvedModel = this._familyMap.get(family)?.[0];
		}

src/platform/endpoint/node/copilotChatEndpoint.ts:85

• CopilotFastChatEndpoint.create is calling modelFetcher.getChatModelFromFamily(...) with model capability families like 'gpt-5.4-nano'/'gpt-4o-mini' and forcing the types via as ChatEndpointFamily. This bypasses type-safety and makes it unclear whether getChatModelFromFamily is meant for internal endpoint families (copilot-*) or for CAPI model families. Consider introducing a separate fetcher method that takes the raw CAPI family string (or widening getChatModelFromFamily to accept string) and use that here instead of casting.

	static async create(modelFetcher: IModelMetadataFetcher, instantiationService: IInstantiationService): Promise<IChatEndpoint> {
		let modelMetadata: IChatModelInformation;
		try {
			modelMetadata = await modelFetcher.getChatModelFromFamily(CopilotFastChatEndpoint.primaryFamily as ChatEndpointFamily);
		} catch {
			modelMetadata = await modelFetcher.getChatModelFromFamily(CopilotFastChatEndpoint.fallbackFamily as ChatEndpointFamily);
		}

src/platform/endpoint/node/copilotChatEndpoint.ts:85

• The try { ... } catch { ... } fallback in CopilotFastChatEndpoint.create will fall back on any error (including network/JSON/parsing errors), which can mask real outages and also drops the original error context if the fallback fails too. Prefer catching only the “model not available” case (if distinguishable), and/or keep the original error and rethrow an aggregated error if the fallback resolution also fails.

		try {
			modelMetadata = await modelFetcher.getChatModelFromFamily(CopilotFastChatEndpoint.primaryFamily as ChatEndpointFamily);
		} catch {
			modelMetadata = await modelFetcher.getChatModelFromFamily(CopilotFastChatEndpoint.fallbackFamily as ChatEndpointFamily);
		}