Port of #2239 (v1.x) to v2 (#2275)

packages/server/src/server/stdio.ts

@@ -17,14 +17,25 @@ import { process } from '@modelcontextprotocol/server/_shims';
  * ```
  */
 export class StdioServerTransport implements Transport {
-    private _readBuffer: ReadBuffer = new ReadBuffer();
+    private _readBuffer: ReadBuffer;
     private _started = false;
     private _closed = false;
 
     constructor(
         private _stdin: Readable = process.stdin,
-        private _stdout: Writable = process.stdout
-    ) {}
+        private _stdout: Writable = process.stdout,
+        options?: {
+            /**
+             * Maximum size of the read buffer in bytes. If a single message exceeds
+             * this size the transport will emit an error and close.
+             *
+             * Defaults to 10 MB.
+             */
+            maxBufferSize?: number;
+        }
+    ) {
+        this._readBuffer = new ReadBuffer({ maxBufferSize: options?.maxBufferSize });
+    }
 
     onclose?: () => void;
     onerror?: (error: Error) => void;
@@ -32,8 +43,13 @@ export class StdioServerTransport implements Transport {
 
     // Arrow functions to bind `this` properly, while maintaining function identity.
     _ondata = (chunk: Buffer) => {
-        this._readBuffer.append(chunk);
-        this.processReadBuffer();
+        try {
+            this._readBuffer.append(chunk);
+            this.processReadBuffer();
+        } catch (error) {
+            this.onerror?.(error as Error);
+            this.close().catch(() => {});
+        }
     };
     _onerror = (error: Error) => {
         this.onerror?.(error);

packages/server/test/server/stdio.test.ts

@@ -179,3 +179,51 @@ test('should fire onerror before onclose on stdout error', async () => {
 
     expect(events).toEqual(['error', 'close']);
 });
+
+test('should respect custom maxBufferSize option', async () => {
+    const server = new StdioServerTransport(input, output, { maxBufferSize: 100 });
+
+    let receivedError: Error | undefined;
+    server.onerror = err => {
+        receivedError = err;
+    };
+    let closeCount = 0;
+    server.onclose = () => {
+        closeCount++;
+    };
+
+    await server.start();
+
+    // Push 101 bytes without a newline — exceeds the 100-byte limit
+    input.push(Buffer.alloc(101, 0x41));
+
+    await new Promise(resolve => setTimeout(resolve, 10));
+
+    expect(receivedError?.message).toMatch(/ReadBuffer exceeded maximum size/);
+    expect(closeCount).toBe(1);
+});
+
+test('should fire onerror and close when ReadBuffer overflows', async () => {
+    const server = new StdioServerTransport(input, output);
+
+    let receivedError: Error | undefined;
+    server.onerror = err => {
+        receivedError = err;
+    };
+    let closeCount = 0;
+    server.onclose = () => {
+        closeCount++;
+    };
+
+    await server.start();
+
+    // Push data exceeding the default 10 MB limit without a newline
+    const chunk = Buffer.alloc(11 * 1024 * 1024, 0x41);
+    input.push(chunk);
+
+    // Allow the close() promise to settle
+    await new Promise(resolve => setTimeout(resolve, 10));
+
+    expect(receivedError?.message).toMatch(/ReadBuffer exceeded maximum size/);
+    expect(closeCount).toBe(1);
+});