feat(express): add limit option for JSON body parser

.changeset/express-json-limit.md

@@ -0,0 +1,5 @@
+---
+'@modelcontextprotocol/express': minor
+---
+
+Add `limit` option to `createMcpExpressApp()` to allow overriding the default 100kb JSON body parser size limit.

packages/middleware/express/src/express.ts

@@ -22,6 +22,15 @@ export interface CreateMcpExpressAppOptions {
      * to restrict which hostnames are allowed.
      */
     allowedHosts?: string[];
+
+    /**
+     * Controls the maximum request body size for the JSON body parser.
+     * Passed directly to `express.json({ limit })`.
+     * Defaults to Express's built-in default of '100kb' when not specified.
+     *
+     * @example '10mb'
+     */
+    limit?: string | number;
 }
 
 /**
@@ -48,10 +57,10 @@ export interface CreateMcpExpressAppOptions {
  * ```
  */
 export function createMcpExpressApp(options: CreateMcpExpressAppOptions = {}): Express {
-    const { host = '127.0.0.1', allowedHosts } = options;
+    const { host = '127.0.0.1', allowedHosts, limit } = options;
 
     const app = express();
-    app.use(express.json());
+    app.use(express.json(limit !== undefined ? { limit } : undefined));
 
     // If allowedHosts is explicitly provided, use that for validation
     if (allowedHosts) {

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

@@ -167,6 +167,16 @@ describe('@modelcontextprotocol/express', () => {
             warn.mockRestore();
         });
 
+        test('should accept limit option for JSON body parser', () => {
+            const app = createMcpExpressApp({ limit: '10mb' });
+            expect(app).toBeDefined();
+        });
+
+        test('should accept numeric limit option for JSON body parser', () => {
+            const app = createMcpExpressApp({ limit: 1048576 });
+            expect(app).toBeDefined();
+        });
+
         test('should not apply host validation for non-localhost hosts without allowedHosts', () => {
             const warn = vi.spyOn(console, 'warn').mockImplementation(() => {});