fix: allow controlling the error code for POST error responses

- Updated the Protocol class to set a 404 status code for invalid or expired sessions and improved logging for response sending and error handling.
- Adjusted response status code handling to utilize the context's status code, ensuring more accurate HTTP responses.

Author: CodeWithKyrian

src/Protocol.php

@@ -121,7 +121,16 @@ public function processMessage(Request|Notification|BatchRequest $message, strin
 
         if ($session === null) {
             $error = Error::forInvalidRequest('Invalid or expired session. Please re-initialize the session.', $message->id);
-            $this->transport->sendMessage($error, $sessionId, $context);
+            $context['status_code'] = 404;
+
+            $this->transport->sendMessage($error, $sessionId, $context)
+                ->then(function () use ($sessionId, $error, $context) {
+                    $this->logger->debug('Response sent.', ['sessionId' => $sessionId, 'payload' => $error, 'context' => $context]);
+                })
+                ->catch(function (Throwable $e) use ($sessionId, $error, $context) {
+                    $this->logger->error('Failed to send response.', ['sessionId' => $sessionId, 'error' => $e->getMessage()]);
+                });
+
             return;
         }
 

src/Transports/StreamableHttpServerTransport.php

@@ -68,12 +68,16 @@ class StreamableHttpServerTransport implements ServerTransportInterface, LoggerA
 
     private ?ThroughStream $getStream = null;
 
+    /**
+     * @param bool $enableJsonResponse If true, the server will return JSON responses instead of starting an SSE stream. 
+     * This can be useful for simple request/response scenarios without streaming.
+     */
     public function __construct(
         private readonly string $host = '127.0.0.1',
         private readonly int $port = 8080,
         private string $mcpPath = '/mcp',
         private ?array $sslContext = null,
-        private readonly bool $preferDirectJsonResponse = true,
+        private readonly bool $enableJsonResponse = true,
         ?IdGeneratorInterface $idGenerator = null,
         ?EventStoreInterface $eventStore = null
     ) {
@@ -295,11 +299,7 @@ private function handlePostRequest(ServerRequestInterface $request): PromiseInte
             $deferred->resolve(new HttpResponse(202));
             $context['type'] = 'post_202_sent';
         } else {
-            $clientPrefersSse = str_contains($acceptHeader, 'text/event-stream');
-            $clientAcceptsJson = str_contains($acceptHeader, 'application/json');
-            $useSse = $clientPrefersSse && !($this->preferDirectJsonResponse && $clientAcceptsJson);
-
-            if ($useSse) {
+            if (!$this->enableJsonResponse) {
                 $streamId = $this->idGenerator->generateId();
                 $sseStream = new ThroughStream();
                 $this->activeSseStreams[$streamId] = [
@@ -480,7 +480,8 @@ public function sendMessage(Message $message, string $sessionId, array $context
                     $headers['Mcp-Session-Id'] = $sessionId;
                 }
 
-                $deferred->resolve(new HttpResponse(200, $headers, $responseBody));
+                $statusCode = $context['status_code'] ?? 200;
+                $deferred->resolve(new HttpResponse($statusCode, $headers, $responseBody . "\n"));
                 return resolve(null);
 
             default: