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

.changeset/stdio-max-buffer-size.md

@@ -0,0 +1,7 @@
+---
+'@modelcontextprotocol/core': patch
+'@modelcontextprotocol/client': patch
+'@modelcontextprotocol/server': patch
+---
+
+Add a configurable `maxBufferSize` (default 10 MB) to the stdio transports. When a single message would push the read buffer past the limit, the transport now emits an `onerror` and closes instead of growing the buffer unbounded. Configure via `new StdioClientTransport({ ..., maxBufferSize })` or `new StdioServerTransport(stdin, stdout, { maxBufferSize })`. The default is exported from `@modelcontextprotocol/core` as `STDIO_DEFAULT_MAX_BUFFER_SIZE`.

packages/client/src/client/stdio.ts

@@ -38,6 +38,14 @@ export type StdioServerParameters = {
      * If not specified, the current working directory will be inherited.
      */
     cwd?: string;
+
+    /**
+     * 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;
 };
 
 /**
@@ -92,7 +100,7 @@ export function getDefaultEnvironment(): Record<string, string> {
  */
 export class StdioClientTransport implements Transport {
     private _process?: ChildProcess;
-    private _readBuffer: ReadBuffer = new ReadBuffer();
+    private _readBuffer: ReadBuffer;
     private _serverParams: StdioServerParameters;
     private _stderrStream: PassThrough | null = null;
 
@@ -102,6 +110,7 @@ export class StdioClientTransport implements Transport {
 
     constructor(server: StdioServerParameters) {
         this._serverParams = server;
+        this._readBuffer = new ReadBuffer({ maxBufferSize: server.maxBufferSize });
         if (server.stderr === 'pipe' || server.stderr === 'overlapped') {
             this._stderrStream = new PassThrough();
         }
@@ -149,8 +158,13 @@ export class StdioClientTransport implements Transport {
             });
 
             this._process.stdout?.on('data', chunk => {
-                this._readBuffer.append(chunk);
-                this.processReadBuffer();
+                try {
+                    this._readBuffer.append(chunk);
+                    this.processReadBuffer();
+                } catch (error) {
+                    this.onerror?.(error as Error);
+                    this.close().catch(() => {});
+                }
             });
 
             this._process.stdout?.on('error', error => {

packages/client/test/client/stdio.test.ts

@@ -77,3 +77,44 @@ test('should return child process pid', async () => {
     await client.close();
     expect(client.pid).toBeNull();
 });
+
+test('should respect custom maxBufferSize option', async () => {
+    const client = new StdioClientTransport({
+        command: 'node',
+        args: ['-e', 'process.stdout.write(Buffer.alloc(200, 0x41))'],
+        maxBufferSize: 100
+    });
+
+    const errorReceived = new Promise<Error>(resolve => {
+        client.onerror = resolve;
+    });
+    const closed = new Promise<void>(resolve => {
+        client.onclose = () => resolve();
+    });
+
+    await client.start();
+
+    const error = await errorReceived;
+    expect(error.message).toMatch(/ReadBuffer exceeded maximum size/);
+    await closed;
+});
+
+test('should fire onerror and close when ReadBuffer overflows', async () => {
+    const client = new StdioClientTransport({
+        command: 'node',
+        args: ['-e', 'process.stdout.write(Buffer.alloc(11 * 1024 * 1024, 0x41))']
+    });
+
+    const errorReceived = new Promise<Error>(resolve => {
+        client.onerror = resolve;
+    });
+    const closed = new Promise<void>(resolve => {
+        client.onclose = () => resolve();
+    });
+
+    await client.start();
+
+    const error = await errorReceived;
+    expect(error.message).toMatch(/ReadBuffer exceeded maximum size/);
+    await closed;
+});

packages/core/src/exports/public/index.ts

@@ -53,7 +53,7 @@ export type {
 export { DEFAULT_REQUEST_TIMEOUT_MSEC } from '../../shared/protocol.js';
 
 // stdio message framing utilities (for custom transport authors)
-export { deserializeMessage, ReadBuffer, serializeMessage } from '../../shared/stdio.js';
+export { deserializeMessage, ReadBuffer, serializeMessage, STDIO_DEFAULT_MAX_BUFFER_SIZE } from '../../shared/stdio.js';
 
 // Transport types (NOT normalizeHeaders)
 export type { FetchLike, Transport, TransportSendOptions } from '../../shared/transport.js';

packages/core/src/shared/stdio.ts

@@ -1,13 +1,25 @@
 import type { JSONRPCMessage } from '../types/index.js';
 import { JSONRPCMessageSchema } from '../types/index.js';
 
+export const STDIO_DEFAULT_MAX_BUFFER_SIZE = 10 * 1024 * 1024;
+
 /**
  * Buffers a continuous stdio stream into discrete JSON-RPC messages.
  */
 export class ReadBuffer {
     private _buffer?: Buffer;
+    private _maxBufferSize: number;
+
+    constructor(options?: { maxBufferSize?: number }) {
+        this._maxBufferSize = options?.maxBufferSize ?? STDIO_DEFAULT_MAX_BUFFER_SIZE;
+    }
 
     append(chunk: Buffer): void {
+        const newSize = (this._buffer?.length ?? 0) + chunk.length;
+        if (newSize > this._maxBufferSize) {
+            this.clear();
+            throw new Error(`ReadBuffer exceeded maximum size of ${this._maxBufferSize} bytes`);
+        }
         this._buffer = this._buffer ? Buffer.concat([this._buffer, chunk]) : chunk;
     }
 

packages/core/test/shared/stdio.test.ts

@@ -1,4 +1,4 @@
-import { ReadBuffer } from '../../src/shared/stdio.js';
+import { ReadBuffer, STDIO_DEFAULT_MAX_BUFFER_SIZE } from '../../src/shared/stdio.js';
 import type { JSONRPCMessage } from '../../src/types/index.js';
 
 const testMessage: JSONRPCMessage = {
@@ -113,3 +113,46 @@ describe('non-JSON line filtering', () => {
         expect(() => readBuffer.readMessage()).toThrow();
     });
 });
+
+describe('buffer size limit', () => {
+    test('should throw when buffer exceeds default max size', () => {
+        const readBuffer = new ReadBuffer();
+        const chunkSize = 1024 * 1024; // 1 MB
+        const chunk = Buffer.alloc(chunkSize);
+        const chunksToFill = Math.floor(STDIO_DEFAULT_MAX_BUFFER_SIZE / chunkSize);
+        for (let i = 0; i < chunksToFill; i++) {
+            readBuffer.append(chunk);
+        }
+        expect(() => readBuffer.append(chunk)).toThrow(/ReadBuffer exceeded maximum size/);
+    });
+
+    test('should throw when buffer exceeds custom max size', () => {
+        const readBuffer = new ReadBuffer({ maxBufferSize: 100 });
+        readBuffer.append(Buffer.alloc(50));
+        expect(() => readBuffer.append(Buffer.alloc(51))).toThrow(/ReadBuffer exceeded maximum size/);
+    });
+
+    test('should clear buffer before throwing on overflow', () => {
+        const readBuffer = new ReadBuffer({ maxBufferSize: 100 });
+        readBuffer.append(Buffer.alloc(50));
+        expect(() => readBuffer.append(Buffer.alloc(51))).toThrow();
+
+        // Buffer should be cleared — can append again
+        readBuffer.append(Buffer.alloc(50));
+        // And read messages normally
+        expect(readBuffer.readMessage()).toBeNull();
+    });
+
+    test('should allow appending up to exactly the max size', () => {
+        const readBuffer = new ReadBuffer({ maxBufferSize: 100 });
+        // Should not throw — exactly at limit
+        expect(() => readBuffer.append(Buffer.alloc(100))).not.toThrow();
+    });
+
+    test('should work with no options (backwards compatible)', () => {
+        const readBuffer = new ReadBuffer();
+        // Small append should always work
+        readBuffer.append(Buffer.from(JSON.stringify({ jsonrpc: '2.0', method: 'ping' }) + '\n'));
+        expect(readBuffer.readMessage()).not.toBeNull();
+    });
+});

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);
+});