Fix optional argument handling in prompts for Zod V4 (#1199)

Co-authored-by: Matt <77928207+mattzcarey@users.noreply.github.com>

src/server/mcp.test.ts

@@ -3879,6 +3879,51 @@ describe.each(zodTestMatrix)('$zodVersionLabel', (entry: ZodMatrixEntry) => {
             expect(result.resources[0].description).toBe('Overridden description');
             expect(result.resources[0].mimeType).toBe('text/markdown');
         });
+
+        test('should support optional prompt arguments', async () => {
+            const mcpServer = new McpServer({
+                name: 'test server',
+                version: '1.0'
+            });
+
+            const client = new Client({
+                name: 'test client',
+                version: '1.0'
+            });
+
+            mcpServer.registerPrompt(
+                'test-prompt',
+                {
+                    argsSchema: {
+                        name: z.string().optional()
+                    }
+                },
+                () => ({
+                    messages: []
+                })
+            );
+
+            const [clientTransport, serverTransport] = InMemoryTransport.createLinkedPair();
+
+            await Promise.all([client.connect(clientTransport), mcpServer.server.connect(serverTransport)]);
+
+            const result = await client.request(
+                {
+                    method: 'prompts/list'
+                },
+                ListPromptsResultSchema
+            );
+
+            expect(result.prompts).toHaveLength(1);
+            expect(result.prompts[0].name).toBe('test-prompt');
+            expect(result.prompts[0].arguments).toEqual([
+                {
+                    name: 'name',
+                    description: undefined,
+                    required: false
+                }
+            ]);
+        });
     });
 
     describe('Tool title precedence', () => {

src/server/zod-compat.ts

@@ -31,7 +31,7 @@ export interface ZodV3Internal {
 export interface ZodV4Internal {
     _zod?: {
         def?: {
-            typeName?: string;
+            type?: string;
             value?: unknown;
             values?: unknown[];
             shape?: Record<string, AnySchema> | (() => Record<string, AnySchema>);
@@ -175,7 +175,7 @@ export function normalizeObjectSchema(schema: AnySchema | ZodRawShapeCompat | un
         // Check if it's a v4 object
         const v4Schema = schema as unknown as ZodV4Internal;
         const def = v4Schema._zod?.def;
-        if (def && (def.typeName === 'object' || def.shape !== undefined)) {
+        if (def && (def.type === 'object' || def.shape !== undefined)) {
             return schema as AnyObjectSchema;
         }
     } else {
@@ -238,7 +238,7 @@ export function getSchemaDescription(schema: AnySchema): string | undefined {
 export function isSchemaOptional(schema: AnySchema): boolean {
     if (isZ4Schema(schema)) {
         const v4Schema = schema as unknown as ZodV4Internal;
-        return v4Schema._zod?.def?.typeName === 'ZodOptional';
+        return v4Schema._zod?.def?.type === 'optional';
     }
     const v3Schema = schema as unknown as ZodV3Internal;
     // v3 has isOptional() method