Merge commit from fork

processInput() appended every 8 KiB stdout chunk to $inputBuffer and only
drained it on a "\n". A spawned server that streams stdout without ever
emitting a newline grew the buffer without bound, exhausting client
memory (GHSA-6vrr-g4hw-f3hj).

Cap the buffer (default 4 MiB, configurable via the constructor). When
appending the next chunk would exceed the cap before a newline is seen,
abort the read and resolve the in-flight request with an error
immediately, so the caller fails fast instead of spinning until its
request timeout.

Based on localden's suggested fix in the advisory: makes the cap
configurable, but resolves the pending request with an error (keeping
runRequest's Response|Error contract) instead of throwing a
TransportException out of the read loop, and leaves connection teardown
to the caller. Adds regression tests.

Co-authored-by: localden <53200638+localden@users.noreply.github.com>

src/Client/Transport/StdioTransport.php

@@ -12,6 +12,7 @@
 namespace Mcp\Client\Transport;
 
 use Mcp\Exception\ConnectionException;
+use Mcp\Exception\InvalidArgumentException;
 use Mcp\Schema\JsonRpc\Error;
 use Mcp\Schema\JsonRpc\Response;
 use Psr\Log\LoggerInterface;
@@ -45,6 +46,11 @@ class StdioTransport extends BaseTransport
 
     private string $inputBuffer = '';
 
+    /**
+     * Default cap on the bytes buffered while waiting for a complete line.
+     */
+    public const DEFAULT_MAX_BUFFER_SIZE = 4 * 1024 * 1024;
+
     /** @var McpFiber|null */
     private ?\Fiber $activeFiber = null;
 
@@ -56,6 +62,12 @@ class StdioTransport extends BaseTransport
      * @param array<int, string>         $args          Command arguments
      * @param string|null                $cwd           Working directory
      * @param array<string, string>|null $env           Environment variables
+     * @param int                        $maxBufferSize Maximum bytes buffered while waiting for a complete line. The
+     *                                                  buffer is only drained on a "\n"; a spawned server that streams
+     *                                                  stdout without ever emitting a newline would otherwise grow it
+     *                                                  without bound and exhaust client memory. Reaching the cap aborts
+     *                                                  the read instead. Raise it for servers that emit single frames
+     *                                                  larger than the default.
      */
     public function __construct(
         private readonly string $command,
@@ -63,8 +75,13 @@ class StdioTransport extends BaseTransport
         private readonly ?string $cwd = null,
         private readonly ?array $env = null,
         ?LoggerInterface $logger = null,
+        private readonly int $maxBufferSize = self::DEFAULT_MAX_BUFFER_SIZE,
     ) {
         parent::__construct($logger);
+
+        if ($maxBufferSize < 1) {
+            throw new InvalidArgumentException(\sprintf('The maximum buffer size must be a positive number of bytes, got %d.', $maxBufferSize));
+        }
     }
 
     public function connect(): void
@@ -223,6 +240,12 @@ class StdioTransport extends BaseTransport
 
         $data = fread($this->stdout, 8192);
         if (false !== $data && '' !== $data) {
+            if (\strlen($this->inputBuffer) + \strlen($data) > $this->maxBufferSize) {
+                $this->abortInput(\sprintf('buffered %d bytes without a newline, exceeding the %d byte limit', \strlen($this->inputBuffer) + \strlen($data), $this->maxBufferSize));
+
+                return;
+            }
+
             $this->inputBuffer .= $data;
         }
 
@@ -237,6 +260,33 @@ class StdioTransport extends BaseTransport
         }
     }
 
+    /**
+     * Discard the input buffer and fail any in-flight request.
+     *
+     * The waiting fiber is resolved with an error immediately so the caller
+     * fails fast, rather than spinning until the request timeout elapses.
+     */
+    private function abortInput(string $reason): void
+    {
+        $bufferedBytes = \strlen($this->inputBuffer);
+        $this->inputBuffer = '';
+
+        $this->logger->warning('Aborting stdio input: '.$reason, [
+            'buffered_bytes' => $bufferedBytes,
+            'max_buffer_size' => $this->maxBufferSize,
+        ]);
+
+        if (null === $this->state) {
+            return;
+        }
+
+        foreach ($this->state->getPendingRequests() as $pending) {
+            $requestId = $pending['request_id'];
+            $error = Error::forInternalError('stdio input aborted: '.$reason, $requestId);
+            $this->state->storeResponse($requestId, $error->jsonSerialize());
+        }
+    }
+
     private function processFiber(): void
     {
         if (null === $this->activeFiber || !$this->activeFiber->isSuspended()) {

tests/Unit/Client/Transport/StdioTransportTest.php

@@ -0,0 +1,107 @@
+<?php
+
+/*
+ * This file is part of the official PHP MCP SDK.
+ *
+ * A collaboration between Symfony and the PHP Foundation.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Mcp\Tests\Unit\Client\Transport;
+
+use Mcp\Client\State\ClientState;
+use Mcp\Client\Transport\StdioTransport;
+use Mcp\Exception\InvalidArgumentException;
+use Mcp\Schema\JsonRpc\Error;
+use PHPUnit\Framework\Attributes\TestDox;
+use PHPUnit\Framework\TestCase;
+
+final class StdioTransportTest extends TestCase
+{
+    #[TestDox('input buffer is bounded: an over-length frame without a newline is aborted')]
+    public function testInputBufferIsBounded(): void
+    {
+        $transport = new StdioTransport(command: 'true', maxBufferSize: 64);
+        $state = new ClientState();
+        $state->addPendingRequest(1, 30);
+        $transport->setState($state);
+
+        // A server that floods stdout without ever emitting a newline.
+        $this->setStdout($transport, $this->stream(str_repeat('a', 8192)));
+
+        $this->invokeProcessInput($transport);
+
+        self::assertSame('', $this->readPrivate($transport, 'inputBuffer'), 'buffer must be cleared on abort');
+    }
+
+    #[TestDox('aborting the input fails the in-flight request immediately')]
+    public function testAbortFailsPendingRequestFast(): void
+    {
+        $transport = new StdioTransport(command: 'true', maxBufferSize: 64);
+        $state = new ClientState();
+        $state->addPendingRequest(1, 30);
+        $transport->setState($state);
+
+        $this->setStdout($transport, $this->stream(str_repeat('a', 8192)));
+
+        $this->invokeProcessInput($transport);
+
+        $response = $state->consumeResponse(1);
+        self::assertInstanceOf(Error::class, $response);
+        self::assertSame(Error::INTERNAL_ERROR, $response->code);
+        self::assertSame(1, $response->id);
+    }
+
+    #[TestDox('newline-delimited frames within the cap are parsed and dispatched')]
+    public function testWellFormedFramesStillParse(): void
+    {
+        $transport = new StdioTransport(command: 'true');
+        $messages = [];
+        $transport->onMessage(static function (string $message) use (&$messages): void {
+            $messages[] = $message;
+        });
+
+        $this->setStdout($transport, $this->stream('{"a":1}'."\n".'{"b":2}'."\n"));
+
+        $this->invokeProcessInput($transport);
+
+        self::assertSame(['{"a":1}', '{"b":2}'], $messages);
+    }
+
+    #[TestDox('the buffer cap must be a positive number of bytes')]
+    public function testRejectsNonPositiveCap(): void
+    {
+        $this->expectException(InvalidArgumentException::class);
+
+        new StdioTransport(command: 'true', maxBufferSize: 0);
+    }
+
+    /**
+     * @return resource
+     */
+    private function stream(string $contents)
+    {
+        $stream = fopen('php://temp', 'r+');
+        fwrite($stream, $contents);
+        rewind($stream);
+
+        return $stream;
+    }
+
+    private function setStdout(StdioTransport $transport, mixed $stream): void
+    {
+        (new \ReflectionProperty($transport, 'stdout'))->setValue($transport, $stream);
+    }
+
+    private function invokeProcessInput(StdioTransport $transport): void
+    {
+        (new \ReflectionMethod($transport, 'processInput'))->invoke($transport);
+    }
+
+    private function readPrivate(StdioTransport $transport, string $property): mixed
+    {
+        return (new \ReflectionProperty($transport, $property))->getValue($transport);
+    }
+}