diff --git a/src/Client/Exception/HttpTransportException.php b/src/Client/Exception/HttpTransportException.php
new file mode 100644
index 00000000..fbc6c5eb
--- /dev/null
+++ b/src/Client/Exception/HttpTransportException.php
@@ -0,0 +1,33 @@
+<?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\Client\Exception;
+
+/**
+ * Thrown when the server answers a request with a non-success HTTP status
+ * code. Carries the status code and a snippet of the response body so callers
+ * can surface the server-side failure instead of waiting on a timeout.
+ */
+class HttpTransportException extends \Mcp\Exception\Exception
+{
+    private readonly int $statusCode;
+
+    public function __construct(string $message, int $statusCode, ?\Throwable $previous = null)
+    {
+        parent::__construct($message, $statusCode, $previous);
+        $this->statusCode = $statusCode;
+    }
+
+    public function getStatusCode(): int
+    {
+        return $this->statusCode;
+    }
+}
diff --git a/src/Client/Exception/SessionExpiredException.php b/src/Client/Exception/SessionExpiredException.php
new file mode 100644
index 00000000..e3182788
--- /dev/null
+++ b/src/Client/Exception/SessionExpiredException.php
@@ -0,0 +1,21 @@
+<?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\Client\Exception;
+
+/**
+ * Thrown when the server reports that the current session no longer exists
+ * (HTTP 404 on a request that carried a session id). The transport clears the
+ * local session id so the application can re-initialize and start a new one.
+ */
+class SessionExpiredException extends \Mcp\Exception\Exception
+{
+}
diff --git a/src/Client/Transport/HttpTransport.php b/src/Client/Transport/HttpTransport.php
index ddb662f7..840bdac4 100644
--- a/src/Client/Transport/HttpTransport.php
+++ b/src/Client/Transport/HttpTransport.php
@@ -13,6 +13,8 @@
 
 use Http\Discovery\Psr17FactoryDiscovery;
 use Http\Discovery\Psr18ClientDiscovery;
+use Mcp\Client\Exception\HttpTransportException;
+use Mcp\Client\Exception\SessionExpiredException;
 use Mcp\Exception\ConnectionException;
 use Mcp\Exception\InvalidArgumentException;
 use Mcp\Schema\JsonRpc\Error;
@@ -124,6 +126,15 @@ public function send(string $data): void
             $request = $request->withHeader('Mcp-Session-Id', $this->sessionId);
         }
 
+        // Spec: clients MUST echo the negotiated protocol version on every
+        // request after the initialize handshake. The handshake itself runs
+        // before a version is negotiated, so the header is omitted for that
+        // first request and the server falls back to its default version.
+        $protocolVersion = $this->state?->getProtocolVersion();
+        if (null !== $protocolVersion) {
+            $request = $request->withHeader('MCP-Protocol-Version', $protocolVersion->value);
+        }
+
         foreach ($this->headers as $name => $value) {
             $request = $request->withHeader($name, $value);
         }
@@ -137,6 +148,29 @@ public function send(string $data): void
             throw new ConnectionException('HTTP request failed: '.$e->getMessage(), 0, $e);
         }
 
+        $statusCode = $response->getStatusCode();
+
+        // A 404 on a request carrying a session id means the server has dropped
+        // the session: the client must re-initialize. Clear the local session id
+        // and surface the failure instead of parsing the error body as a normal
+        // message.
+        if (404 === $statusCode && null !== $this->sessionId) {
+            $this->logger->warning('Server no longer knows the current session (HTTP 404); clearing the session id so the client re-initializes.', ['session_id' => $this->sessionId]);
+            $this->sessionId = null;
+
+            throw new SessionExpiredException('The MCP session no longer exists (HTTP 404); re-initialize the client to start a new session.');
+        }
+
+        // Any other non-success status is a transport-level failure. Reading the
+        // body here also surfaces plain-text error pages instead of silently
+        // dropping them and leaving the caller waiting on a timeout.
+        if ($statusCode < 200 || $statusCode >= 300) {
+            $body = $response->getBody()->getContents();
+            $snippet = '' === trim($body) ? 'empty body' : substr(trim($body), 0, 500);
+
+            throw new HttpTransportException(\sprintf('MCP server returned HTTP %d: %s', $statusCode, $snippet), $statusCode);
+        }
+
         if ($response->hasHeader('Mcp-Session-Id')) {
             $this->sessionId = $response->getHeaderLine('Mcp-Session-Id');
             $this->logger->debug('Received session ID', ['session_id' => $this->sessionId]);
diff --git a/tests/Unit/Client/Transport/HttpTransportTest.php b/tests/Unit/Client/Transport/HttpTransportTest.php
index 6c6119e1..b70f9799 100644
--- a/tests/Unit/Client/Transport/HttpTransportTest.php
+++ b/tests/Unit/Client/Transport/HttpTransportTest.php
@@ -12,9 +12,12 @@
 namespace Mcp\Tests\Unit\Client\Transport;
 
 use Mcp\Client;
+use Mcp\Client\Exception\HttpTransportException;
+use Mcp\Client\Exception\SessionExpiredException;
 use Mcp\Client\State\ClientState;
 use Mcp\Client\Transport\HttpTransport;
 use Mcp\Exception\InvalidArgumentException;
+use Mcp\Schema\Enum\ProtocolVersion;
 use Mcp\Schema\JsonRpc\Error;
 use Nyholm\Psr7\Factory\Psr17Factory;
 use Nyholm\Psr7\Response;
@@ -153,6 +156,137 @@ public function testRejectsNonPositiveCap(): void
         $this->createTransport(maxSseBufferBytes: 0);
     }
 
+    #[TestDox('HTTP 404 with a session id clears the session and throws a session-expired error')]
+    public function test404WithSessionClearsSessionAndThrowsSessionExpiredException(): void
+    {
+        $httpClient = new class implements ClientInterface {
+            public function sendRequest(RequestInterface $request): ResponseInterface
+            {
+                return new Response(404, ['Content-Type' => 'application/json'], '{"jsonrpc":"2.0","id":1,"error":{"code":-32000,"message":"Session not found"}}');
+            }
+        };
+
+        $transport = new HttpTransport('https://example.test/mcp', [], $httpClient, $this->factory, $this->factory);
+        $this->setSessionId($transport, 'abc-123');
+
+        try {
+            $transport->send('{"jsonrpc":"2.0","method":"ping","id":1}');
+            $this->fail('Expected SessionExpiredException to be thrown.');
+        } catch (SessionExpiredException $e) {
+            $this->assertStringContainsString('404', $e->getMessage());
+        }
+
+        $this->assertNull($this->readPrivate($transport, 'sessionId'), 'The session id must be cleared so the client can re-initialize.');
+    }
+
+    #[TestDox('HTTP 404 without a session id is a plain transport error')]
+    public function test404WithoutSessionThrowsHttpTransportException(): void
+    {
+        $httpClient = new class implements ClientInterface {
+            public function sendRequest(RequestInterface $request): ResponseInterface
+            {
+                return new Response(404, ['Content-Type' => 'application/json'], '{"jsonrpc":"2.0","id":1,"error":{"code":-32000,"message":"Not found"}}');
+            }
+        };
+
+        $transport = new HttpTransport('https://example.test/mcp', [], $httpClient, $this->factory, $this->factory);
+
+        try {
+            $transport->send('{"jsonrpc":"2.0","method":"ping","id":1}');
+            $this->fail('Expected HttpTransportException to be thrown.');
+        } catch (HttpTransportException $e) {
+            $this->assertSame(404, $e->getStatusCode());
+        }
+    }
+
+    #[TestDox('a non-success status throws a transport error carrying the status and a body snippet')]
+    public function testNonSuccessStatusThrowsHttpTransportException(): void
+    {
+        $httpClient = new class implements ClientInterface {
+            public function sendRequest(RequestInterface $request): ResponseInterface
+            {
+                return new Response(500, ['Content-Type' => 'text/plain'], 'Internal Server Error');
+            }
+        };
+
+        $transport = new HttpTransport('https://example.test/mcp', [], $httpClient, $this->factory, $this->factory);
+
+        try {
+            $transport->send('{"jsonrpc":"2.0","method":"ping","id":1}');
+            $this->fail('Expected HttpTransportException to be thrown.');
+        } catch (HttpTransportException $e) {
+            $this->assertSame(500, $e->getStatusCode());
+            $this->assertStringContainsString('500', $e->getMessage());
+            $this->assertStringContainsString('Internal Server Error', $e->getMessage());
+        }
+    }
+
+    #[TestDox('a 200 application/json response is dispatched normally')]
+    public function test200JsonResponseIsHandledNormally(): void
+    {
+        $httpClient = new class implements ClientInterface {
+            public function sendRequest(RequestInterface $request): ResponseInterface
+            {
+                return new Response(200, ['Content-Type' => 'application/json'], '{"jsonrpc":"2.0","id":1,"result":{"status":"ok"}}');
+            }
+        };
+
+        $transport = new HttpTransport('https://example.test/mcp', [], $httpClient, $this->factory, $this->factory);
+        $messages = [];
+        $transport->onMessage(static function (string $message) use (&$messages): void {
+            $messages[] = $message;
+        });
+
+        $transport->send('{"jsonrpc":"2.0","method":"ping","id":1}');
+
+        $this->assertSame(['{"jsonrpc":"2.0","id":1,"result":{"status":"ok"}}'], $messages);
+    }
+
+    #[TestDox('the MCP-Protocol-Version header is sent once a protocol version is negotiated')]
+    public function testSendsProtocolVersionHeaderWhenNegotiated(): void
+    {
+        $httpClient = new class implements ClientInterface {
+            public ?string $protocolVersionHeader = null;
+
+            public function sendRequest(RequestInterface $request): ResponseInterface
+            {
+                $this->protocolVersionHeader = $request->getHeaderLine('MCP-Protocol-Version');
+
+                return new Response(200, ['Content-Type' => 'application/json']);
+            }
+        };
+
+        $transport = new HttpTransport('https://example.test/mcp', [], $httpClient, $this->factory, $this->factory);
+        $state = new ClientState();
+        $state->setProtocolVersion(ProtocolVersion::V2025_11_25);
+        $transport->setState($state);
+
+        $transport->send('{"jsonrpc":"2.0","method":"ping","id":1}');
+
+        $this->assertSame('2025-11-25', $httpClient->protocolVersionHeader);
+    }
+
+    #[TestDox('the MCP-Protocol-Version header is omitted before a protocol version is negotiated')]
+    public function testOmitsProtocolVersionHeaderBeforeNegotiation(): void
+    {
+        $httpClient = new class implements ClientInterface {
+            public ?string $protocolVersionHeader = 'unset';
+
+            public function sendRequest(RequestInterface $request): ResponseInterface
+            {
+                $this->protocolVersionHeader = $request->getHeaderLine('MCP-Protocol-Version');
+
+                return new Response(200, ['Content-Type' => 'application/json']);
+            }
+        };
+
+        $transport = new HttpTransport('https://example.test/mcp', [], $httpClient, $this->factory, $this->factory);
+
+        $transport->send('{"jsonrpc":"2.0","method":"ping","id":1}');
+
+        $this->assertSame('', $httpClient->protocolVersionHeader);
+    }
+
     private function createTransport(int $maxSseBufferBytes = 8 * 1024 * 1024): HttpTransport
     {
         return new HttpTransport(
@@ -169,6 +303,11 @@ private function setActiveStream(HttpTransport $transport, StreamInterface $stre
         (new \ReflectionProperty($transport, 'activeStream'))->setValue($transport, $stream);
     }
 
+    private function setSessionId(HttpTransport $transport, ?string $sessionId): void
+    {
+        (new \ReflectionProperty($transport, 'sessionId'))->setValue($transport, $sessionId);
+    }
+
     private function invokeProcessSseStream(HttpTransport $transport): void
     {
         (new \ReflectionMethod($transport, 'processSSEStream'))->invoke($transport);