[Client] Fix connecting to a server with an empty serverInfo.version

CHANGELOG.md

@@ -13,6 +13,7 @@ All notable changes to `mcp/sdk` will be documented in this file.
 * [BC Break] Reject a `Tool` input schema whose `properties` is not an object or whose `required` is neither a list nor `null`, instead of silently replacing the member. Reject a `completion/complete` whose `argument` is missing `name` or `value`, instead of completing against an empty prefix.
 * Add `HttpTransport::getSessionId()` to read the server-minted `Mcp-Session-Id`: a request-scoped caller can persist it and pass it back through the constructor's `$headers` on a later transport. Always `null` on `2026-07-28`, which removed protocol-level sessions.
 * Fix OIDC discovery rejecting issuers with a trailing slash (e.g. Authentik, Auth0).
+* Fix `Client::connect()` rejecting an empty `serverInfo.version`, as the Python SDK sends, and leaving the transport open when initialization fails on a malformed result: the transport is now closed and the failure wrapped in a `ConnectionException`.
 * Fix stateless SSE streams holding back frames until close when PHP output buffering is enabled.
 * Reject a recognized `Mcp-Param-*` header whose mirrored argument is absent from the body with `-32020`, instead of accepting the request (SEP-2243).
 * Fix `RequestEvent`, `ResponseEvent` and `ErrorEvent` not being dispatched for `2026-07-28` requests.

src/Client.php

@@ -17,6 +17,7 @@
 use Mcp\Client\Protocol;
 use Mcp\Client\Transport\TransportInterface;
 use Mcp\Exception\ConnectionException;
+use Mcp\Exception\ExceptionInterface;
 use Mcp\Exception\InvalidArgumentException;
 use Mcp\Exception\RequestCancelledException;
 use Mcp\Exception\RequestException;
@@ -103,13 +104,19 @@ public function connect(TransportInterface $transport): void
                 $this->logger->info('Client connected and initialized', ['attempt' => $attempt]);
 
                 return;
-            } catch (ConnectionException $e) {
+            } catch (ExceptionInterface $e) {
                 // initialize() flags the session before sending the initialized
                 // notification, so a failure in between leaves the flag set.
                 $this->protocol->getState()->setInitialized(false);
 
                 $transport->close();
 
+                // Anything else, like a malformed initialize result, is not
+                // transient and would fail every retry the same way.
+                if (!$e instanceof ConnectionException) {
+                    throw new ConnectionException('Initialization failed: '.$e->getMessage(), 0, $e);
+                }
+
                 if ($attempt === $maxAttempts) {
                     throw $e;
                 }

src/Schema/Implementation.php

@@ -51,7 +51,8 @@ public static function fromArray(array $data): self
         if (!isset($data['name']) || !\is_string($data['name']) || '' === $data['name']) {
             throw new InvalidArgumentException('Invalid or missing "name" in Implementation data.');
         }
-        if (!isset($data['version']) || !\is_string($data['version']) || '' === $data['version']) {
+        // Only typed as a string, and the Python SDK sends "" when none is configured.
+        if (!isset($data['version']) || !\is_string($data['version'])) {
             throw new InvalidArgumentException('Invalid or missing "version" in Implementation data.');
         }
 

tests/Unit/ClientTest.php

@@ -157,6 +157,25 @@ public function testFailureAfterHandshakeResultDoesNotLeaveClientConnected(): vo
         $this->assertFalse($client->isConnected());
     }
 
+    #[TestDox('a malformed handshake result fails with a ConnectionException and closes the transport')]
+    public function testMalformedHandshakeResultClosesTheTransport(): void
+    {
+        $transport = new FakeTransport([FakeTransport::MALFORMED]);
+
+        $client = Client::builder()->setMaxRetries(2)->build();
+
+        try {
+            $client->connect($transport);
+            $this->fail(\sprintf('Expected a "%s" to be thrown.', ConnectionException::class));
+        } catch (ConnectionException $e) {
+            $this->assertInstanceOf(InvalidArgumentException::class, $e->getPrevious());
+        }
+
+        $this->assertSame(1, $transport->connectCalls, 'a malformed result is not retried');
+        $this->assertSame(1, $transport->closeCalls);
+        $this->assertFalse($client->isConnected());
+    }
+
     #[TestDox('a timed out attempt does not leave state behind that fails the retry')]
     public function testTimedOutAttemptDoesNotPoisonTheRetry(): void
     {
@@ -197,6 +216,9 @@ final class FakeTransport extends BaseTransport
     /** The initialize request is answered, but the connection breaks right after. */
     public const BREAK_AFTER_ACCEPT = 'break_after_accept';
 
+    /** The initialize request is answered with a result lacking `serverInfo.version`. */
+    public const MALFORMED = 'malformed';
+
     public int $connectCalls = 0;
     public int $closeCalls = 0;
 
@@ -206,7 +228,7 @@ final class FakeTransport extends BaseTransport
     private array $outbox = [];
 
     /**
-     * @param list<self::ACCEPT|self::ACCEPT_MODERN|self::REJECT|self::IGNORE|self::BREAK_AFTER_ACCEPT> $attempts How each successive connect() call behaves
+     * @param list<self::ACCEPT|self::ACCEPT_MODERN|self::REJECT|self::IGNORE|self::BREAK_AFTER_ACCEPT|self::MALFORMED> $attempts How each successive connect() call behaves
      */
     public function __construct(private array $attempts = [self::ACCEPT])
     {
@@ -256,7 +278,9 @@ public function send(string $data): void
             : ['result' => [
                 'protocolVersion' => ProtocolVersion::V2025_11_25->value,
                 'capabilities' => [],
-                'serverInfo' => ['name' => 'Test Server', 'version' => '1.0.0'],
+                'serverInfo' => self::MALFORMED === $this->outcome
+                    ? ['name' => 'Test Server']
+                    : ['name' => 'Test Server', 'version' => '1.0.0'],
             ]];
 
         $this->outbox[] = json_encode(['jsonrpc' => '2.0', 'id' => $message['id']] + $answer, \JSON_THROW_ON_ERROR);

tests/Unit/Schema/ImplementationTest.php

@@ -120,12 +120,15 @@ public function testFromArrayThrowsOnMissingVersion(): void
         Implementation::fromArray(['name' => 'my-client']);
     }
 
-    public function testFromArrayThrowsOnEmptyVersion(): void
+    /**
+     * The spec types the version as a plain string, and the Python SDK sends an
+     * empty one when the server has none configured.
+     */
+    public function testFromArrayAcceptsEmptyVersion(): void
     {
-        $this->expectException(InvalidArgumentException::class);
-        $this->expectExceptionMessage('Invalid or missing "version" in Implementation data.');
+        $implementation = Implementation::fromArray(['name' => 'my-server', 'version' => '']);
 
-        Implementation::fromArray(['name' => 'my-client', 'version' => '']);
+        $this->assertSame('', $implementation->version);
     }
 
     public function testFromArrayThrowsOnNonStringVersion(): void