Clarify usage of prompt file specifications in Copilot CLI (#4705)

* Copikot CLI: Clarify usage of prompt file specifications

* Update tests

Author: DonJayamanne

src/extension/chatSessions/vscode-node/copilotCLIChatSessions.ts

@@ -1536,10 +1536,9 @@ export class CopilotCLIChatSessionParticipant extends Disposable {
 
 	/**
 	 * Gets the agent to be used.
-	 * If creating a new session, then uses the agent configured in settings.
-	 * If opening an existing session, then uses the agent associated with that session.
-	 * If creating a new session with a prompt file that specifies an agent, then uses that agent.
+	 * If the request has a prompt file (modeInstructions2) that specifies an agent, uses that agent.
 	 * If the prompt file specifies tools, those tools override the agent's default tools.
+	 * Otherwise returns undefined (no agent).
 	 */
 	private async getAgent(sessionId: string | undefined, request: vscode.ChatRequest | undefined, token: vscode.CancellationToken): Promise<SweCustomAgent | undefined> {
 		// If we have a prompt file that specifies an agent or tools, use that.
@@ -1553,8 +1552,8 @@ export class CopilotCLIChatSessionParticipant extends Disposable {
 				return customAgent;
 			}
 		}
-		const sessionAgent = sessionId ? await this.chatSessionMetadataStore.getSessionAgent(sessionId) : undefined;
-		return sessionAgent ? await this.copilotCLIAgents.resolveAgent(sessionAgent) : undefined;
+		// If not found, don't use any agent, default to empty agent.
+		return undefined;
 	}
 
 	private async getPromptInfoFromRequest(request: vscode.ChatRequest, token: vscode.CancellationToken): Promise<ParsedPromptFile | undefined> {

src/extension/chatSessions/vscode-node/copilotCLIChatSessionsContribution.ts

@@ -15,8 +15,8 @@ import { IFileSystemService } from '../../../platform/filesystem/common/fileSyst
 import { IGitExtensionService } from '../../../platform/git/common/gitExtensionService';
 import { getGitHubRepoInfoFromContext, IGitService, RepoContext } from '../../../platform/git/common/gitService';
 import { toGitUri } from '../../../platform/git/common/utils';
-import { IOctoKitService } from '../../../platform/github/common/githubService';
 import { derivePullRequestState } from '../../../platform/github/common/githubAPI';
+import { IOctoKitService } from '../../../platform/github/common/githubService';
 import { ILogService } from '../../../platform/log/common/logService';
 import { IPromptsService, ParsedPromptFile } from '../../../platform/promptFiles/common/promptsService';
 import { ITelemetryService } from '../../../platform/telemetry/common/telemetry';
@@ -1660,10 +1660,9 @@ export class CopilotCLIChatSessionParticipant extends Disposable {
 
 	/**
 	 * Gets the agent to be used.
-	 * If creating a new session, then uses the agent configured in settings.
-	 * If opening an existing session, then uses the agent associated with that session.
-	 * If creating a new session with a prompt file that specifies an agent, then uses that agent.
+	 * If the request has a prompt file (modeInstructions2) that specifies an agent, uses that agent.
 	 * If the prompt file specifies tools, those tools override the agent's default tools.
+	 * Otherwise returns undefined (no agent).
 	 */
 	private async getAgent(sessionId: string | undefined, request: vscode.ChatRequest | undefined, token: vscode.CancellationToken): Promise<SweCustomAgent | undefined> {
 		// If we have a prompt file that specifies an agent or tools, use that.
@@ -1677,8 +1676,8 @@ export class CopilotCLIChatSessionParticipant extends Disposable {
 				return customAgent;
 			}
 		}
-		const sessionAgent = sessionId ? await this.chatSessionMetadataStore.getSessionAgent(sessionId) : undefined;
-		return sessionAgent ? await this.copilotCLIAgents.resolveAgent(sessionAgent) : undefined;
+		// If not found, don't use any agent, default to empty agent.
+		return undefined;
 	}
 
 	private async getPromptInfoFromRequest(request: vscode.ChatRequest, token: vscode.CancellationToken): Promise<ParsedPromptFile | undefined> {

src/extension/chatSessions/vscode-node/test/copilotCLIChatSessionParticipant.spec.ts

@@ -1961,6 +1961,23 @@ describe('CopilotCLIChatSessionParticipant.handleRequest', () => {
 			const { agent } = createSessionSpy.mock.calls[0][0];
 			expect(agent?.tools).toBeNull();
 		});
+
+		it('does not use session agent when no modeInstructions2 is provided', async () => {
+			const agentParticipant = makeParticipantWithAgents(new MockCopilotCLIAgentsWithCustomAgent(['tool-a']));
+			const createSessionSpy = vi.spyOn(sessionService, 'createSession');
+
+			const request = new TestChatRequest('Do something');
+			// No modeInstructions2 set — agent should be undefined regardless of session state
+			const context = createChatContext('temp-new', true);
+			const stream = new MockChatResponseStream();
+			const token = disposables.add(new CancellationTokenSource()).token;
+
+			await agentParticipant.createHandler()(request, context, stream, token);
+
+			expect(createSessionSpy).toHaveBeenCalled();
+			const { agent } = createSessionSpy.mock.calls[0][0];
+			expect(agent).toBeUndefined();
+		});
 	});
 
 	describe('PR detection with retry', () => {