From a93f3a73103acc0fbecf82adfaffb0fd667dbc59 Mon Sep 17 00:00:00 2001
From: Christopher Hertel <mail@christopher-hertel.de>
Date: Sat, 10 Oct 2026 03:55:28 +0200
Subject: [PATCH] [Client] Mark the client disconnected when the stdio server
 exits

---
 CHANGELOG.md                                  |  1 +
 src/Client/Transport/StdioTransport.php       |  4 ++
 tests/Integration/DeadServerTest.php          | 46 +++++++++++++++++++
 tests/Integration/Fixture/dead_server.php     | 38 +++++++++++++++
 .../Client/Transport/StdioTransportTest.php   | 23 ++++++++++
 5 files changed, 112 insertions(+)
 create mode 100644 tests/Integration/DeadServerTest.php
 create mode 100644 tests/Integration/Fixture/dead_server.php

diff --git a/CHANGELOG.md b/CHANGELOG.md
index e34852a9..a2123096 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -45,6 +45,7 @@ All notable changes to `mcp/sdk` will be documented in this file.
 * On a `2026-07-28` connection, `Client::setLoggingLevel()` stamps the level on every following request, `Client::ping()` sends `server/discover` and `Client::sendRootsListChanged()` sends nothing.
 * Fail a client request at once when the HTTP server refuses it with an error status or the stdio server process exits, instead of waiting out the timeout.
 * [BC Break] Bump `MessageInterface::PROTOCOL_VERSION` to `2026-07-28`. Use `ProtocolVersion::latestHandshake()` where a handshake revision is needed, e.g. in an `initialize` answer.
+* Fix `Client::isConnected()` staying true after the stdio server process exited, which left later calls failing against the dead process.
 
 0.8.0
 -----
diff --git a/src/Client/Transport/StdioTransport.php b/src/Client/Transport/StdioTransport.php
index 2108e64e..38ddf350 100644
--- a/src/Client/Transport/StdioTransport.php
+++ b/src/Client/Transport/StdioTransport.php
@@ -118,6 +118,8 @@ public function send(string $data): void
         }
 
         if (false === @fwrite($this->stdin, $data."\n")) {
+            $this->state?->setInitialized(false);
+
             throw new ConnectionException('Could not write to the server process; it is no longer running.');
         }
 
@@ -246,6 +248,8 @@ private function processInput(): void
 
         // Only on an empty read, so an answer arriving with the end of output is taken first.
         if (('' === $data || false === $data) && \is_resource($this->stdout) && feof($this->stdout)) {
+            // The server is gone: the client has to connect again before its next call.
+            $this->state?->setInitialized(false);
             $this->failPending('The server process closed its output; it is no longer running.', [self::CONNECTION_LOST => true]);
         }
     }
diff --git a/tests/Integration/DeadServerTest.php b/tests/Integration/DeadServerTest.php
new file mode 100644
index 00000000..f7d9e563
--- /dev/null
+++ b/tests/Integration/DeadServerTest.php
@@ -0,0 +1,46 @@
+<?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\Integration;
+
+use Mcp\Exception\ConnectionException;
+use Mcp\Exception\ExceptionInterface;
+use Mcp\Schema\Enum\ProtocolVersion;
+use PHPUnit\Framework\Attributes\DataProvider;
+use PHPUnit\Framework\Attributes\TestDox;
+
+/**
+ * What a server process that dies leaves behind on the client.
+ *
+ * @see Fixture/dead_server.php for the server under test
+ */
+final class DeadServerTest extends IntegrationTestCase
+{
+    #[DataProvider('provideEras')]
+    #[TestDox('a server exiting mid-call leaves the client disconnected on $_dataName')]
+    public function testExitedServerDisconnectsTheClient(ProtocolVersion $version): void
+    {
+        $client = $this->connect('dead_server', $this->clientBuilder()->setProtocolVersion($version));
+
+        try {
+            $client->callTool('exit');
+            $this->fail('A call the server dies in must fail.');
+        } catch (ExceptionInterface) {
+        }
+
+        $this->assertFalse($client->isConnected());
+
+        $this->expectException(ConnectionException::class);
+        $this->expectExceptionMessage('Client is not connected.');
+
+        $client->callTool('fast');
+    }
+}
diff --git a/tests/Integration/Fixture/dead_server.php b/tests/Integration/Fixture/dead_server.php
new file mode 100644
index 00000000..bdf0496c
--- /dev/null
+++ b/tests/Integration/Fixture/dead_server.php
@@ -0,0 +1,38 @@
+<?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.
+ */
+
+/*
+ * Server for {@see \Mcp\Tests\Integration\DeadServerTest}.
+ *
+ * `exit` ends the server process in the middle of the call.
+ */
+
+use Mcp\Server;
+use Mcp\Server\Transport\StdioTransport;
+
+require_once dirname(__DIR__, 3).'/vendor/autoload.php';
+
+Server::builder()
+    ->setServerInfo('integration-server', '1.0.0')
+    ->addTool(
+        static function (): string {
+            exit(1);
+        },
+        name: 'exit',
+        description: 'Ends the server process without answering.',
+    )
+    ->addTool(
+        static fn (): string => 'quick',
+        name: 'fast',
+        description: 'Returns at once.',
+    )
+    ->build()
+    ->run(new StdioTransport());
diff --git a/tests/Unit/Client/Transport/StdioTransportTest.php b/tests/Unit/Client/Transport/StdioTransportTest.php
index 5b85a05e..18f08963 100644
--- a/tests/Unit/Client/Transport/StdioTransportTest.php
+++ b/tests/Unit/Client/Transport/StdioTransportTest.php
@@ -86,6 +86,7 @@ public function testClosedOutputFailsPendingRequests(): void
     {
         $transport = new StdioTransport(command: 'true');
         $state = new ClientState();
+        $state->setInitialized(true);
         $transport->setState($state);
         $state->addPendingRequest(1, 120);
 
@@ -97,6 +98,28 @@ public function testClosedOutputFailsPendingRequests(): void
         $this->assertInstanceOf(Error::class, $response);
         $this->assertStringContainsString('no longer running', $response->message);
         $this->assertSame([TransportInterface::CONNECTION_LOST => true], $response->data);
+        $this->assertFalse($state->isInitialized());
+    }
+
+    #[TestDox('a server that can no longer be written to leaves the client uninitialized')]
+    public function testFailedWriteUninitializesTheClient(): void
+    {
+        $transport = new StdioTransport(command: 'true');
+        $state = new ClientState();
+        $state->setInitialized(true);
+        $transport->setState($state);
+
+        $stdin = fopen('php://temp', 'r');
+        $this->assertNotFalse($stdin);
+        (new \ReflectionProperty($transport, 'stdin'))->setValue($transport, $stdin);
+
+        try {
+            $transport->send('{"jsonrpc":"2.0","id":1,"method":"ping"}');
+            $this->fail('Writing to a server that is gone must fail.');
+        } catch (ConnectionException) {
+        }
+
+        $this->assertFalse($state->isInitialized());
     }
 
     /**