diff --git a/extensions/copilot/src/extension/chatSessions/copilotcli/node/copilotcliSession.ts b/extensions/copilot/src/extension/chatSessions/copilotcli/node/copilotcliSession.ts
index 31ac8bbaa3f89..4129b67b595e8 100644
--- a/extensions/copilot/src/extension/chatSessions/copilotcli/node/copilotcliSession.ts
+++ b/extensions/copilot/src/extension/chatSessions/copilotcli/node/copilotcliSession.ts
@@ -941,6 +941,18 @@ export class CopilotCLISession extends DisposableStore implements ICopilotCLISes
 					this._stream?.progress(l10n.t('Compacting conversation...'));
 					await this._sdkSession.initializeAndValidateTools();
 					this._sdkSession.currentMode = 'interactive';
+					// Mirror the Copilot CLI SDK's own `messages.length < 2` guard to
+					// avoid its "Nothing to compact." throw, while distinguishing
+					// empty sessions from already-compacted sessions in the UI.
+					const messages = await this._sdkSession.getChatMessages();
+					if (messages.length === 0) {
+						this._stream?.markdown(l10n.t('Nothing to compact.'));
+						break;
+					}
+					if (messages.length < 2) {
+						this._stream?.markdown(l10n.t('Conversation already compacted.'));
+						break;
+					}
 					const result = await this._sdkSession.compactHistory();
 					if (result.success) {
 						this._stream?.markdown(l10n.t('Compacted conversation.'));
diff --git a/extensions/copilot/src/extension/chatSessions/copilotcli/node/test/copilotcliSession.spec.ts b/extensions/copilot/src/extension/chatSessions/copilotcli/node/test/copilotcliSession.spec.ts
index 364be208601d4..36d5d54ce72ce 100644
--- a/extensions/copilot/src/extension/chatSessions/copilotcli/node/test/copilotcliSession.spec.ts
+++ b/extensions/copilot/src/extension/chatSessions/copilotcli/node/test/copilotcliSession.spec.ts
@@ -124,6 +124,9 @@ class MockSdkSession {
 
 	async compactHistory() { return { success: true }; }
 
+	public chatMessages: Awaited<ReturnType<Session['getChatMessages']>> = [{ role: 'user', content: 'hi' }, { role: 'assistant', content: 'hello' }];
+	async getChatMessages() { return this.chatMessages; }
+
 	async abort() { }
 
 	isAbortable(): boolean { return true; }
@@ -730,6 +733,36 @@ describe('CopilotCLISession', () => {
 			expect(sdkSession.currentMode).toBe('interactive');
 			expect(stream.output.join('\n')).toContain('Compacted conversation.');
 		});
+
+		it('reports already-compacted when no new messages since last compaction (issue #311422)', async () => {
+			const session = await createSession();
+			const stream = new MockChatResponseStream();
+			session.attachStream(stream);
+			// Simulate post-compaction state: only the single summary message remains.
+			sdkSession.chatMessages = [{ role: 'system', content: 'summary' }];
+			let compactCalled = false;
+			sdkSession.compactHistory = async () => { compactCalled = true; return { success: true }; };
+
+			await session.handleRequest({ id: '', toolInvocationToken: undefined as never }, { command: 'compact', prompt: '' }, [], undefined, authInfo, CancellationToken.None);
+
+			expect(compactCalled).toBe(false);
+			expect(stream.output.join('\n')).toContain('Conversation already compacted.');
+		});
+
+		it('reports nothing-to-compact on an empty session', async () => {
+			const session = await createSession();
+			const stream = new MockChatResponseStream();
+			session.attachStream(stream);
+			// Simulate a brand-new session with no conversation yet.
+			sdkSession.chatMessages = [];
+			let compactCalled = false;
+			sdkSession.compactHistory = async () => { compactCalled = true; return { success: true }; };
+
+			await session.handleRequest({ id: '', toolInvocationToken: undefined as never }, { command: 'compact', prompt: '' }, [], undefined, authInfo, CancellationToken.None);
+
+			expect(compactCalled).toBe(false);
+			expect(stream.output.join('\n')).toContain('Nothing to compact.');
+		});
 	});
 
 	describe('steering (sending messages to a busy session)', () => {