[Feature] Execution Subagent Custom Model

[vikramnitin9]

Tweaks to enable the use of a custom model with the execution subagent, in preparation for the Insiders flight this week.

1. Added an option to use a custom copilot proxy endpoint, with an experiment-based flag.
2. Updated the execution subagent prompt based on our experience training the custom model.
3. Small gating change in some prompts to fix a bug where the string execution_subagent could potentially appear in the system prompt even when disabled.

Tagging @bhavyaus and @roblourens for review.

[Copilot]

Pull request overview

Enables running the Execution Subagent against a custom/agentic-proxy model (via experiment-based config), and updates system prompts to better align with the execution subagent’s tool-calling constraints.

Changes:

• Added an experiment-based flag to route the execution subagent through the agentic proxy (with a default proxy model).
• Updated execution subagent prompt instructions for stricter terminal tool usage.
• Adjusted prompt gating so execution_subagent isn’t mentioned unless the tool is actually enabled.

Show a summary per file

File	Description
extensions/copilot/src/platform/endpoint/node/proxyAgenticExecutionEndpoint.ts	Adds a proxy-backed endpoint implementation for execution-subagent model routing.
extensions/copilot/src/platform/configuration/common/configurationService.ts	Introduces ExecutionSubagentUseAgenticProxy experiment-based config key and clarifies model behavior.
extensions/copilot/src/extension/prompts/node/agent/executionSubagentPrompt.tsx	Tightens execution subagent terminal tool rules and provides richer examples.
extensions/copilot/src/extension/prompts/node/agent/defaultAgentInstructions.tsx	Prevents referencing the execution subagent tool name when it’s disabled.
extensions/copilot/src/extension/prompts/node/agent/anthropicPrompts.tsx	Mirrors the same gating fix for Anthropic prompts and minor import reordering.
extensions/copilot/src/extension/prompt/node/executionSubagentToolCallingLoop.ts	Adds agentic-proxy endpoint selection and model fallback logic for the execution subagent.
extensions/copilot/package.nls.json	Adds localized description for the new setting and updates model-setting description.
extensions/copilot/package.json	Contributes the new github.copilot.chat.executionSubagent.useAgenticProxy setting.

Copilot's findings

• Files reviewed: 8/8 changed files
• Comments generated: 2

[Copilot]

Copilot's findings

• Files reviewed: 8/8 changed files
• Comments generated: 3

[Copilot]

ExecutionSubagentModel is read as a plain string setting but is cast to ChatEndpointFamily | undefined. Since ChatEndpointFamily is currently only 'copilot-base' | 'copilot-fast', this cast is unsound and hides invalid values at compile time. Consider treating the config value as string | undefined and explicitly narrowing/validating before calling endpointProvider.getChatEndpoint(...) (e.g., only call it when the value is one of the supported families), while still allowing arbitrary proxy model names when useAgenticProxy is enabled.

[Copilot]

When useAgenticProxy is enabled, getEndpoint() creates a new ProxyAgenticExecutionEndpoint instance on every call. This method is invoked from buildPrompt, getAvailableTools, and fetch, so the subagent may construct multiple endpoint instances per invocation/turn. Consider memoizing the created endpoint on the loop instance (or routing through IEndpointProvider caching) to avoid repeated instantiation and potential extra per-endpoint setup work.

See below for a potential fix:

	private endpointPromise: ReturnType<IEndpointProvider['getChatEndpoint']> | undefined;

	/**
	 * Get the endpoint to use for the execution subagent
	 */
	private async getEndpoint() {
		if (!this.endpointPromise) {
			this.endpointPromise = (async () => {
				const modelName = this._configurationService.getExperimentBasedConfig(ConfigKey.Advanced.ExecutionSubagentModel, this._experimentationService) as ChatEndpointFamily | undefined;
				const useAgenticProxy = this._configurationService.getExperimentBasedConfig(ConfigKey.Advanced.ExecutionSubagentUseAgenticProxy, this._experimentationService);

				if (useAgenticProxy) {
					// Use agentic proxy with ExecutionSubagentModel or default to DEFAULT_AGENTIC_PROXY_MODEL
					const agenticProxyModel = modelName || ExecutionSubagentToolCallingLoop.DEFAULT_AGENTIC_PROXY_MODEL;
					return this.instantiationService.createInstance(ProxyAgenticExecutionEndpoint, agenticProxyModel);
				}

				if (modelName) {
					try {
						// Try to get the specified model
						const endpoint = await this.endpointProvider.getChatEndpoint(modelName);
						if (endpoint.supportsToolCalls) {
							return endpoint;
						}

						this._logService.warn(`Configured execution subagent model ${modelName} does not support tool calls, falling back to main agent endpoint`);
						return await this.endpointProvider.getChatEndpoint(this.options.request);
					} catch (error) {
						// Model not available, fallback to main agent endpoint
						this._logService.warn(`Failed to get model ${modelName}, falling back to main agent endpoint: ${error}`);
						return await this.endpointProvider.getChatEndpoint(this.options.request);
					}
				}

				// No model name specified, use main agent endpoint
				return await this.endpointProvider.getChatEndpoint(this.options.request);
			})();
		}

		return await this.endpointPromise;

[bhavyaus]

nit: extra line