[Client] Parse CRLF- and CR-delimited SSE events (#401)

HttpTransport looked for an event boundary with strpos($buffer, "\n\n") and
split event lines on "\n". Servers that terminate SSE lines with CRLF therefore
never produced a recognised event: the response accumulated in the buffer, no
message was ever dispatched, and connect() failed with "Initialization failed:
Request timed out".

This affects every MCP server built on the Python SDK. FastMCP's
streamable_http_app() returns sse_starlette's EventSourceResponse without a sep
argument, and its DEFAULT_SEPARATOR is "\r\n". The bug is invisible from within
this repository because the PHP server implementation emits LF, so client and
server agree in the test suite while disagreeing with most servers in the wild.

The SSE specification requires a parser to accept CRLF, LF or CR, so the
previous behaviour was non-compliant rather than merely unlucky.

Event extraction now accepts all three terminators, choosing whichever appears
first so a buffer split mid-sequence is handled correctly, and line splitting
uses the same set. A stream that ends without a trailing blank line now
dispatches its final event instead of discarding it.

Adds the first tests for src/Client/Transport/, covering LF, CRLF and CR
framing, an id field, a comment preamble, and a missing trailing blank line.
Without the fix four of the six fail with the timeout above.

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/Client/Transport/HttpTransport.php

@@ -230,16 +230,19 @@ class HttpTransport extends BaseTransport
             }
         }
 
-        while (false !== ($pos = strpos($this->sseBuffer, "\n\n"))) {
-            $event = substr($this->sseBuffer, 0, $pos);
-            $this->sseBuffer = substr($this->sseBuffer, $pos + 2);
-
+        while (null !== ($event = $this->extractSSEEvent())) {
             if (!empty(trim($event))) {
                 $this->processSSEEvent($event);
             }
         }
 
-        if ($this->activeStream->eof() && empty($this->sseBuffer)) {
+        if ($this->activeStream->eof()) {
+            // The stream ended without a trailing blank line: dispatch what is left.
+            if (!empty(trim($this->sseBuffer))) {
+                $this->processSSEEvent($this->sseBuffer);
+            }
+
+            $this->sseBuffer = '';
             $this->activeStream = null;
         }
     }
@@ -273,6 +276,37 @@ class HttpTransport extends BaseTransport
         }
     }
 
+    /**
+     * Take the next complete event off the buffer, or null if none is complete yet.
+     *
+     * Per the SSE specification, lines are terminated by CRLF, LF or CR, so an
+     * event is delimited by any pair of those. Servers built on sse-starlette
+     * (the MCP Python SDK) use CRLF.
+     */
+    private function extractSSEEvent(): ?string
+    {
+        $position = null;
+        $length = 0;
+
+        foreach (["\r\n\r\n", "\n\n", "\r\r"] as $delimiter) {
+            $found = strpos($this->sseBuffer, $delimiter);
+
+            if (false !== $found && (null === $position || $found < $position)) {
+                $position = $found;
+                $length = \strlen($delimiter);
+            }
+        }
+
+        if (null === $position) {
+            return null;
+        }
+
+        $event = substr($this->sseBuffer, 0, $position);
+        $this->sseBuffer = substr($this->sseBuffer, $position + $length);
+
+        return $event;
+    }
+
     /**
      * Parse a single SSE event and handle the message.
      */
@@ -280,7 +314,7 @@ class HttpTransport extends BaseTransport
     {
         $data = '';
 
-        foreach (explode("\n", $event) as $line) {
+        foreach (preg_split("/\r\n|\r|\n/", $event) ?: [] as $line) {
             if (str_starts_with($line, 'data:')) {
                 $data .= trim(substr($line, 5));
             }

tests/Unit/Client/Transport/HttpTransportTest.php

@@ -11,14 +11,19 @@
 
 namespace Mcp\Tests\Unit\Client\Transport;
 
+use Mcp\Client;
 use Mcp\Client\State\ClientState;
 use Mcp\Client\Transport\HttpTransport;
 use Mcp\Exception\InvalidArgumentException;
 use Mcp\Schema\JsonRpc\Error;
 use Nyholm\Psr7\Factory\Psr17Factory;
+use Nyholm\Psr7\Response;
+use PHPUnit\Framework\Attributes\DataProvider;
 use PHPUnit\Framework\Attributes\TestDox;
 use PHPUnit\Framework\TestCase;
 use Psr\Http\Client\ClientInterface;
+use Psr\Http\Message\RequestInterface;
+use Psr\Http\Message\ResponseInterface;
 use Psr\Http\Message\StreamInterface;
 
 final class HttpTransportTest extends TestCase
@@ -30,6 +35,65 @@ final class HttpTransportTest extends TestCase
         $this->factory = new Psr17Factory();
     }
 
+    /**
+     * @return iterable<string, array{string}>
+     */
+    public static function frameProvider(): iterable
+    {
+        yield 'LF line endings' => ["event: message\ndata: %s\n\n"];
+        // sse-starlette (and therefore every MCP Python SDK server) defaults to CRLF.
+        yield 'CRLF line endings' => ["event: message\r\ndata: %s\r\n\r\n"];
+        yield 'CR line endings' => ["event: message\rdata: %s\r\r"];
+        yield 'with an id field' => ["id: 1\r\nevent: message\r\ndata: %s\r\n\r\n"];
+        yield 'no trailing blank line' => ["event: message\ndata: %s\n"];
+        yield 'preceded by a comment' => [": ping\n\nevent: message\ndata: %s\n\n"];
+    }
+
+    #[DataProvider('frameProvider')]
+    #[TestDox('initialization succeeds for an SSE response framed as: $_dataName')]
+    public function testInitializeParsesSseFraming(string $frame): void
+    {
+        $httpClient = new class($frame) implements ClientInterface {
+            public function __construct(private readonly string $frame)
+            {
+            }
+
+            public function sendRequest(RequestInterface $request): ResponseInterface
+            {
+                $decoded = json_decode((string) $request->getBody(), true);
+
+                if ('initialize' !== ($decoded['method'] ?? null)) {
+                    return new Response(202);
+                }
+
+                $payload = json_encode([
+                    'jsonrpc' => '2.0',
+                    'id' => $decoded['id'],
+                    'result' => [
+                        'protocolVersion' => '2025-11-25',
+                        'capabilities' => ['tools' => ['listChanged' => false]],
+                        'serverInfo' => ['name' => 'test-server', 'version' => '1.0.0'],
+                    ],
+                ]);
+
+                return new Response(200, [
+                    'Content-Type' => 'text/event-stream',
+                    'Mcp-Session-Id' => 'abc123',
+                ], \sprintf($this->frame, $payload));
+            }
+        };
+
+        $client = Client::builder()
+            ->setClientInfo('test-client', '1.0.0')
+            ->setInitTimeout(1)
+            ->build();
+
+        $client->connect(new HttpTransport('http://localhost/mcp', [], $httpClient, $this->factory, $this->factory));
+
+        $this->assertTrue($client->isConnected());
+        $this->assertSame('test-server', $client->getServerInfo()?->name);
+    }
+
     #[TestDox('SSE stream is aborted before the buffer can exceed the configured cap')]
     public function testSseBufferIsBoundedByConfiguredCap(): void
     {