[Server] Send a stdio call's notifications before its response

CHANGELOG.md

@@ -20,6 +20,7 @@ All notable changes to `mcp/sdk` will be documented in this file.
 * 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 `HttpTransport` omitting the `MCP-Protocol-Version` header on requests after the `initialize` handshake, including the closing `DELETE`.
 * Fix memory growing with every `tools/call` in long-lived processes like FrankenPHP workers or stdio servers.
+* Fix the stdio transport answering a request before the notifications its handler sent, so a client could miss them.
 * 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 parallel elicitations on one session getting each other's answers: requests to the client get UUID ids instead of a session counter, and a request or notification a handler sends goes out on the stream of its own call.

src/Server/Transport/StdioTransport.php

@@ -425,6 +425,9 @@ private function handleFiberTermination(\Fiber $fiber): void
     {
         $finalResult = $fiber->getReturn();
 
+        // A client may stop reading once it has its answer, so what the call sent goes out first.
+        $this->flushOutgoingMessages();
+
         if (null !== $finalResult) {
             try {
                 $encoded = json_encode($finalResult, \JSON_THROW_ON_ERROR);

tests/Unit/Server/Transport/StdioSuspendedFibersTest.php

@@ -14,7 +14,9 @@
 use Mcp\Event\ServerRequestEvent;
 use Mcp\Schema\Elicitation\ElicitationSchema;
 use Mcp\Schema\Elicitation\StringSchemaDefinition;
+use Mcp\Schema\Enum\LoggingLevel;
 use Mcp\Schema\Enum\ProtocolVersion;
+use Mcp\Schema\Notification\LoggingMessageNotification;
 use Mcp\Server;
 use Mcp\Server\RequestContext;
 use Mcp\Server\Transport\StdioTransport;
@@ -64,6 +66,15 @@ public function testConcurrentElicitationsResumeWithTheirOwnAnswer(): void
         $this->assertSame('second', $this->answer($lines, 3)['result']['content'][0]['text'] ?? null);
     }
 
+    #[TestDox('notifications a call sent reach the client before its answer')]
+    public function testNotificationsPrecedeTheAnswer(): void
+    {
+        $lines = $this->exchange([self::call(2, 'announce')]);
+
+        $this->assertSame(['notifications/message', null], array_map(static fn (array $line): ?string => $line['method'] ?? null, \array_slice($lines, 1)));
+        $this->assertSame('announced', $lines[2]['result']['content'][0]['text'] ?? null);
+    }
+
     /**
      * @param list<array<string, mixed>> $lines
      *
@@ -149,6 +160,11 @@ public function dispatch(object $event): object
 
                 return $result->content['name'] ?? 'none';
             }, name: 'ask', description: 'Asks for a name')
+            ->addTool(static function (RequestContext $context): string {
+                $context->getClientGateway()->notify(new LoggingMessageNotification(LoggingLevel::Info, 'starting'));
+
+                return 'announced';
+            }, name: 'announce', description: 'Notifies, then answers')
             ->setEventDispatcher($dispatcher)
             ->build();