diff --git a/extensions/copilot/src/extension/chatSessions/copilotcli/vscode-node/test/diagnosticsChanged.spec.ts b/extensions/copilot/src/extension/chatSessions/copilotcli/vscode-node/test/diagnosticsChanged.spec.ts
index 65d45a1cbbe44..c6218254492f2 100644
--- a/extensions/copilot/src/extension/chatSessions/copilotcli/vscode-node/test/diagnosticsChanged.spec.ts
+++ b/extensions/copilot/src/extension/chatSessions/copilotcli/vscode-node/test/diagnosticsChanged.spec.ts
@@ -197,6 +197,25 @@ describe('diagnosticsChanged push notification', () => {
 		expect(notifiedDiag.range.end.character).toBe(20);
 	});
 
+	it('should serialize a null diagnostic code without crashing', async () => {
+		registerDiagnosticsChangedNotification(logger, httpServer as unknown as InProcHttpServer);
+
+		const uri = createMockUri('/test/file.ts');
+		const diag = createMockDiagnostic('Null code message', 0, 0, 0, 0, 10, 'test-source');
+		(diag as { code?: unknown }).code = null;
+
+		mockGetDiagnostics.mockReturnValue([diag]);
+
+		registeredCallback!({ uris: [uri] });
+		await vi.advanceTimersByTimeAsync(250);
+
+		const params = httpServer.broadcastNotification.mock.calls[0][1] as unknown as DiagnosticNotificationParams;
+		const notifiedDiag = params.uris[0].diagnostics[0];
+
+		expect(notifiedDiag.message).toBe('Null code message');
+		expect(notifiedDiag.code).toBeNull();
+	});
+
 	it('should handle multiple URIs in a single change event', async () => {
 		registerDiagnosticsChangedNotification(logger, httpServer as unknown as InProcHttpServer);
 
diff --git a/extensions/copilot/src/extension/chatSessions/copilotcli/vscode-node/test/getDiagnostics.spec.ts b/extensions/copilot/src/extension/chatSessions/copilotcli/vscode-node/test/getDiagnostics.spec.ts
index 038a2cf7016c4..40cfdbcf3b26d 100644
--- a/extensions/copilot/src/extension/chatSessions/copilotcli/vscode-node/test/getDiagnostics.spec.ts
+++ b/extensions/copilot/src/extension/chatSessions/copilotcli/vscode-node/test/getDiagnostics.spec.ts
@@ -176,4 +176,23 @@ describe('getDiagnostics tool', () => {
 
 		expect(result[0].diagnostics[0].code).toBe(2304);
 	});
+
+	it('should handle a null code in diagnostics without crashing', async () => {
+		const mockDiag = {
+			message: 'Null code error',
+			severity: 0,
+			range: {
+				start: { line: 0, character: 0 },
+				end: { line: 0, character: 5 },
+			},
+			source: 'ts',
+			code: null,
+		};
+		mockGetDiagnostics.mockReturnValue([mockDiag]);
+
+		const handler = server.getToolHandler('get_diagnostics')!;
+		const result = parseToolResult<DiagnosticsFileResult[]>(await handler({ uri: 'file:///test.ts' }));
+
+		expect(result[0].diagnostics[0].code).toBeNull();
+	});
 });
diff --git a/extensions/copilot/src/extension/chatSessions/copilotcli/vscode-node/tools/getDiagnostics.ts b/extensions/copilot/src/extension/chatSessions/copilotcli/vscode-node/tools/getDiagnostics.ts
index 13f09111cb4ae..e894080c60494 100644
--- a/extensions/copilot/src/extension/chatSessions/copilotcli/vscode-node/tools/getDiagnostics.ts
+++ b/extensions/copilot/src/extension/chatSessions/copilotcli/vscode-node/tools/getDiagnostics.ts
@@ -6,7 +6,7 @@
 import * as vscode from 'vscode';
 import type { McpServer } from '@modelcontextprotocol/sdk/server/mcp.js';
 import { z } from 'zod';
-import { makeTextResult } from './utils';
+import { makeTextResult, normalizeDiagnosticCode } from './utils';
 import { ILogger } from '../../../../../platform/log/common/logService';
 
 export function registerGetDiagnosticsTool(server: McpServer, logger: ILogger): void {
@@ -44,7 +44,7 @@ export function registerGetDiagnosticsTool(server: McpServer, logger: ILogger):
 						end: { line: d.range.end.line, character: d.range.end.character },
 					},
 					source: d.source,
-					code: typeof d.code === 'object' ? d.code.value : d.code,
+					code: normalizeDiagnosticCode(d.code),
 				})),
 			})).filter(item => item.diagnostics.length > 0);
 
diff --git a/extensions/copilot/src/extension/chatSessions/copilotcli/vscode-node/tools/push/diagnosticsChanged.ts b/extensions/copilot/src/extension/chatSessions/copilotcli/vscode-node/tools/push/diagnosticsChanged.ts
index d0bfa4d2b354e..3e34612e89d3b 100644
--- a/extensions/copilot/src/extension/chatSessions/copilotcli/vscode-node/tools/push/diagnosticsChanged.ts
+++ b/extensions/copilot/src/extension/chatSessions/copilotcli/vscode-node/tools/push/diagnosticsChanged.ts
@@ -7,6 +7,7 @@ import * as vscode from 'vscode';
 import { ILogger } from '../../../../../../platform/log/common/logService';
 import { Delayer } from '../../../../../../util/vs/base/common/async';
 import { InProcHttpServer } from '../../inProcHttpServer';
+import { normalizeDiagnosticCode } from '../utils';
 
 interface DiagnosticInfo {
 	uri: string;
@@ -49,7 +50,7 @@ function getDiagnosticsForUri(uri: vscode.Uri): DiagnosticInfo {
 			message: d.message,
 			severity: severityToString(d.severity),
 			source: d.source,
-			code: typeof d.code === 'object' ? d.code.value : d.code,
+			code: normalizeDiagnosticCode(d.code),
 		})),
 	};
 }
diff --git a/extensions/copilot/src/extension/chatSessions/copilotcli/vscode-node/tools/utils.ts b/extensions/copilot/src/extension/chatSessions/copilotcli/vscode-node/tools/utils.ts
index 1a2d4479e6553..836a54fd5208d 100644
--- a/extensions/copilot/src/extension/chatSessions/copilotcli/vscode-node/tools/utils.ts
+++ b/extensions/copilot/src/extension/chatSessions/copilotcli/vscode-node/tools/utils.ts
@@ -3,6 +3,24 @@
  *  Licensed under the MIT License. See License.txt in the project root for license information.
  *--------------------------------------------------------------------------------------------*/
 
+import type * as vscode from 'vscode';
+
+/**
+ * Normalizes a `vscode.Diagnostic.code` into a plain string or number for serialization.
+ *
+ * `Diagnostic.code` may be a string, a number, `undefined`, `null` (language servers and
+ * extensions are free to set it), or a `{ value, target }` object. Because
+ * `typeof null === 'object'`, callers must guard against `null` before reading `.value`;
+ * this helper centralizes that guard so every consumer of `vscode.languages.getDiagnostics`
+ * handles the value the same way.
+ */
+export function normalizeDiagnosticCode(code: vscode.Diagnostic['code']): string | number | undefined {
+	if (typeof code === 'object' && code !== null) {
+		return code.value;
+	}
+	return code;
+}
+
 export function makeTextResult(data: unknown): { content: [{ type: 'text'; text: string }] } {
 	return {
 		content: [