feat: add Implement agent using CustomAgentProvider API

package.json

@@ -23,7 +23,7 @@
 	"icon": "assets/copilot.png",
 	"pricing": "Trial",
 	"engines": {
-		"vscode": "^1.109.0-20260121",
+		"vscode": "^1.109.0-20260124",
 		"npm": ">=9.0.0",
 		"node": ">=22.14.0"
 	},
@@ -3579,6 +3579,15 @@
 						"tags": [
 							"experimental"
 						]
+					},
+					"github.copilot.chat.implementAgent.model": {
+						"type": "string",
+						"default": "",
+						"scope": "resource",
+						"markdownDescription": "%github.copilot.config.implementAgent.model%",
+						"tags": [
+							"experimental"
+						]
 					}
 				}
 			},

package.nls.json

@@ -302,6 +302,7 @@
 	"github.copilot.config.organizationInstructions.enabled": "When enabled, Copilot will load custom instructions defined by your GitHub Organization.",
 	"github.copilot.config.planAgent.additionalTools": "Additional tools to enable for the Plan agent, on top of built-in tools. Use fully-qualified tool names (e.g., `github/issue_read`, `mcp_server/tool_name`).",
 	"github.copilot.config.planAgent.model": "Override the language model used by the Plan agent. Leave empty to use the default model.",
+	"github.copilot.config.implementAgent.model": "Override the language model used by the Implement agent. Leave empty to use the default model.",
 	"copilot.toolSet.editing.description": "Edit files in your workspace",
 	"copilot.toolSet.read.description": "Read files in your workspace",
 	"copilot.toolSet.search.description": "Search files in your workspace",

src/extension/agents/vscode-node/implementAgentProvider.ts

@@ -0,0 +1,166 @@
+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *  Licensed under the MIT License. See License.txt in the project root for license information.
+ *--------------------------------------------------------------------------------------------*/
+
+import * as vscode from 'vscode';
+import { ConfigKey, IConfigurationService } from '../../../platform/configuration/common/configurationService';
+import { AGENT_FILE_EXTENSION } from '../../../platform/customInstructions/common/promptTypes';
+import { IVSCodeExtensionContext } from '../../../platform/extContext/common/extensionContext';
+import { IFileSystemService } from '../../../platform/filesystem/common/fileSystemService';
+import { ILogService } from '../../../platform/log/common/logService';
+import { Disposable } from '../../../util/vs/base/common/lifecycle';
+
+/**
+ * Complete Implement agent configuration
+ */
+interface ImplementAgentConfig {
+	name: string;
+	description: string;
+	model?: string;
+	body: string;
+}
+
+/**
+ * Base Implement agent configuration - embedded from Implement.agent.md
+ * This avoids runtime file loading and YAML parsing dependencies.
+ */
+const BASE_IMPLEMENT_AGENT_CONFIG: ImplementAgentConfig = {
+	name: 'Implement',
+	description: 'Executes an existing plan',
+	body: `You are an IMPLEMENTATION AGENT.
+
+You receive a plan that has already been created by the user or a planning agent. Your role is to carry out that plan by executing its steps in order.
+
+Focus on implementation, not planning or redesigning. Follow the plan as written and aim to achieve its intended outcome.
+
+If the plan is unclear or cannot be followed as written, pause and ask for clarification
+
+You are successful when the plan's stated outcome is achieved, with **no unrequested changes**.
+
+## Guidelines
+- Follow the plan's steps in sequence
+- Complete each step before moving to the next
+- Avoid introducing new scope or features
+- Limit changes to what is necessary to implement the plan
+
+<stopping_rules>
+Stop implementation and hand back to **Plan** if:
+- Required information or files are missing
+- A step cannot be completed as described
+- You need to make a significant decision not covered by the plan
+</stopping_rules>
+
+<implement_style_guide>
+- Be concise and practical
+- Explain actions briefly when helpful
+- Summarize changes at the end if appropriate
+
+Your tone should reflect: **"I am implementing the plan."**
+</implement_style_guide>`
+};
+
+/**
+ * Builds .agent.md content from a configuration object using string formatting.
+ */
+export function buildImplementAgentMarkdown(config: ImplementAgentConfig): string {
+	const lines: string[] = ['---'];
+
+	// Simple scalar fields
+	lines.push(`name: ${config.name}`);
+	lines.push(`description: ${config.description}`);
+
+	// Model (optional)
+	if (config.model) {
+		lines.push(`model: ${config.model}`);
+	}
+
+	lines.push('---');
+	lines.push(config.body);
+
+	return lines.join('\n');
+}
+
+/**
+ * Provides the Implement agent dynamically with settings-based customization.
+ *
+ * This provider uses an embedded configuration and generates .agent.md content
+ * with settings-based customization (model override).
+ */
+export class ImplementAgentProvider extends Disposable implements vscode.ChatCustomAgentProvider {
+	readonly label = vscode.l10n.t('Implement Agent');
+
+	private static readonly CACHE_DIR = 'implement-agent';
+	private static readonly AGENT_FILENAME = `Implement${AGENT_FILE_EXTENSION}`;
+
+	private readonly _onDidChangeCustomAgents = this._register(new vscode.EventEmitter<void>());
+	readonly onDidChangeCustomAgents = this._onDidChangeCustomAgents.event;
+
+	constructor(
+		@IConfigurationService private readonly configurationService: IConfigurationService,
+		@IVSCodeExtensionContext private readonly extensionContext: IVSCodeExtensionContext,
+		@IFileSystemService private readonly fileSystemService: IFileSystemService,
+		@ILogService private readonly logService: ILogService,
+	) {
+		super();
+
+		// Listen for settings changes to refresh agents
+		this._register(this.configurationService.onDidChangeConfiguration(e => {
+			if (e.affectsConfiguration(ConfigKey.ImplementAgentModel.fullyQualifiedId)) {
+				this.logService.trace('[ImplementAgentProvider] Settings changed, refreshing agent');
+				this._onDidChangeCustomAgents.fire();
+			}
+		}));
+	}
+
+	async provideCustomAgents(
+		_context: unknown,
+		_token: vscode.CancellationToken
+	): Promise<vscode.ChatResource[]> {
+		// Build config with settings-based customization
+		const config = this.buildCustomizedConfig();
+
+		// Generate .agent.md content
+		const content = buildImplementAgentMarkdown(config);
+
+		// Write to cache file and return URI
+		const fileUri = await this.writeCacheFile(content);
+		return [{ uri: fileUri }];
+	}
+
+	private async writeCacheFile(content: string): Promise<vscode.Uri> {
+		const cacheDir = vscode.Uri.joinPath(
+			this.extensionContext.globalStorageUri,
+			ImplementAgentProvider.CACHE_DIR
+		);
+
+		// Ensure cache directory exists
+		try {
+			await this.fileSystemService.stat(cacheDir);
+		} catch {
+			await this.fileSystemService.createDirectory(cacheDir);
+		}
+
+		const fileUri = vscode.Uri.joinPath(cacheDir, ImplementAgentProvider.AGENT_FILENAME);
+		await this.fileSystemService.writeFile(fileUri, new TextEncoder().encode(content));
+		this.logService.trace(`[ImplementAgentProvider] Wrote agent file: ${fileUri.toString()}`);
+		return fileUri;
+	}
+
+	private buildCustomizedConfig(): ImplementAgentConfig {
+		const modelOverride = this.configurationService.getConfig(ConfigKey.ImplementAgentModel);
+
+		// Start with base config
+		const config: ImplementAgentConfig = {
+			...BASE_IMPLEMENT_AGENT_CONFIG,
+		};
+
+		// Apply model override
+		if (modelOverride) {
+			config.model = modelOverride;
+			this.logService.trace(`[ImplementAgentProvider] Applied model override: ${modelOverride}`);
+		}
+
+		return config;
+	}
+}

src/extension/agents/vscode-node/planAgentProvider.ts

@@ -58,7 +58,7 @@ const BASE_PLAN_AGENT_CONFIG: PlanAgentConfig = {
 	handoffs: [
 		{
 			label: 'Start Implementation',
-			agent: 'agent',
+			agent: 'Implement',
 			prompt: 'Start implementation',
 			send: true
 		},

src/extension/agents/vscode-node/promptFileContrib.ts

@@ -11,6 +11,7 @@ import { IInstantiationService } from '../../../util/vs/platform/instantiation/c
 import { IExtensionContribution } from '../../common/contributions';
 import { GitHubOrgCustomAgentProvider } from './githubOrgCustomAgentProvider';
 import { GitHubOrgInstructionsProvider } from './githubOrgInstructionsProvider';
+import { ImplementAgentProvider } from './implementAgentProvider';
 import { PlanAgentProvider } from './planAgentProvider';
 
 export class PromptFileContribution extends Disposable implements IExtensionContribution {
@@ -33,6 +34,10 @@ export class PromptFileContribution extends Disposable implements IExtensionCont
 			// Register Plan agent provider for dynamic settings-based customization
 			const planProvider = instantiationService.createInstance(PlanAgentProvider);
 			this._register(vscode.chat.registerCustomAgentProvider(planProvider));
+
+			// Register Implement agent provider for dynamic settings-based customization
+			const implementProvider = instantiationService.createInstance(ImplementAgentProvider);
+			this._register(vscode.chat.registerCustomAgentProvider(implementProvider));
 		}
 
 		// Register instructions provider