feat(express): add jsonOptions to createMcpExpressApp()

.changeset/express-json-options.md

@@ -0,0 +1,5 @@
+---
+'@modelcontextprotocol/express': patch
+---
+
+Add `jsonOptions` option to `createMcpExpressApp()` to allow configuring `express.json()` body parser options (e.g. request body size limit).

packages/middleware/express/src/express.examples.ts

@@ -39,3 +39,13 @@ function createMcpExpressApp_allowedHosts() {
     //#endregion createMcpExpressApp_allowedHosts
     return app;
 }
+
+/**
+ * Example: Custom JSON body parser options.
+ */
+function createMcpExpressApp_jsonOptions() {
+    //#region createMcpExpressApp_jsonOptions
+    const app = createMcpExpressApp({ jsonOptions: { limit: '5mb' } });
+    //#endregion createMcpExpressApp_jsonOptions
+    return app;
+}

packages/middleware/express/src/express.ts

@@ -22,6 +22,13 @@ export interface CreateMcpExpressAppOptions {
      * to restrict which hostnames are allowed.
      */
     allowedHosts?: string[];
+
+    /**
+     * Options passed directly to `express.json()` body parser.
+     * Use this to configure the request body size limit, e.g. `{ limit: '5mb' }`.
+     * @see https://expressjs.com/en/api.html#express.json
+     */
+    jsonOptions?: Parameters<typeof express.json>[0];
 }
 
 /**
@@ -49,12 +56,17 @@ export interface CreateMcpExpressAppOptions {
  * ```ts source="./express.examples.ts#createMcpExpressApp_allowedHosts"
  * const app = createMcpExpressApp({ host: '0.0.0.0', allowedHosts: ['myapp.local', 'localhost'] });
  * ```
+ *
+ * @example Custom JSON body parser options
+ * ```ts source="./express.examples.ts#createMcpExpressApp_jsonOptions"
+ * const app = createMcpExpressApp({ jsonOptions: { limit: '5mb' } });
+ * ```
  */
 export function createMcpExpressApp(options: CreateMcpExpressAppOptions = {}): Express {
-    const { host = '127.0.0.1', allowedHosts } = options;
+    const { host = '127.0.0.1', allowedHosts, jsonOptions } = options;
 
     const app = express();
-    app.use(express.json());
+    app.use(express.json(jsonOptions));
 
     // If allowedHosts is explicitly provided, use that for validation
     if (allowedHosts) {

packages/middleware/express/test/express.test.ts

@@ -167,6 +167,18 @@ describe('@modelcontextprotocol/express', () => {
             warn.mockRestore();
         });
 
+        test('should pass jsonOptions to express.json()', () => {
+            const app = createMcpExpressApp({ jsonOptions: { limit: '5mb' } });
+            expect(app).toBeDefined();
+            expect(typeof app.use).toBe('function');
+        });
+
+        test('should use default json options when jsonOptions not provided', () => {
+            const app = createMcpExpressApp();
+            expect(app).toBeDefined();
+            expect(typeof app.use).toBe('function');
+        });
+
         test('should not apply host validation for non-localhost hosts without allowedHosts', () => {
             const warn = vi.spyOn(console, 'warn').mockImplementation(() => {});