[Server] Fix stdio answering a request on an idle connection only after ~50ms

CHANGELOG.md

@@ -45,6 +45,7 @@ All notable changes to `mcp/sdk` will be documented in this file.
 * On a `2026-07-28` connection, `Client::setLoggingLevel()` stamps the level on every following request, `Client::ping()` sends `server/discover` and `Client::sendRootsListChanged()` sends nothing.
 * Fail a client request at once when the HTTP server refuses it with an error status or the stdio server process exits, instead of waiting out the timeout.
 * [BC Break] Bump `MessageInterface::PROTOCOL_VERSION` to `2026-07-28`. Use `ProtocolVersion::latestHandshake()` where a handshake revision is needed, e.g. in an `initialize` answer.
+* Fix the server's `StdioTransport` answering a request on an idle connection only after ~50ms: it now waits on the input with `stream_select()` instead of sleeping.
 
 0.8.0
 -----

src/Server/Transport/StdioTransport.php

@@ -149,7 +149,7 @@ protected function processInput(): void
     {
         $line = fgets($this->input, $this->maxLineBytes);
         if (false === $line) {
-            usleep(50000); // 50ms
+            $this->waitForInput();
 
             return;
         }
@@ -183,6 +183,21 @@ protected function processInput(): void
         }
     }
 
+    /**
+     * Idles until input arrives, for at most 50ms, so the run loop still gets
+     * to suspended fibers, open streams and queued messages in between.
+     */
+    private function waitForInput(): void
+    {
+        $read = [$this->input];
+        $write = $except = null;
+
+        // Streams that cannot be selected, like php://memory, fall back to sleeping.
+        if (false === @stream_select($read, $write, $except, 0, 50000)) {
+            usleep(50000);
+        }
+    }
+
     /**
      * Hands one message to the era it belongs to, settling the connection's
      * era on the first request.

tests/Unit/Server/Transport/StdioTransportTest.php

@@ -13,6 +13,7 @@
 
 use Mcp\Exception\InvalidArgumentException;
 use Mcp\Server\Transport\StdioTransport;
+use Mcp\Server\Transport\TransportInterface;
 use PHPUnit\Framework\Attributes\TestDox;
 use PHPUnit\Framework\TestCase;
 
@@ -51,6 +52,39 @@ public function testNormalLineIsDispatched(): void
         $this->assertSame(['{"jsonrpc":"2.0","id":1}'], $messages);
     }
 
+    #[TestDox('a line arriving on an idle input is read as soon as it arrives')]
+    public function testIdleInputWakesOnArrival(): void
+    {
+        // The child sends a request, waits for the answer, idles for 10ms, then sends another and exits.
+        $process = proc_open(
+            [\PHP_BINARY, '-r', 'echo "{\"first\":1}\n"; fgets(STDIN); usleep(10000); echo "{\"second\":1}\n";'],
+            [0 => ['pipe', 'r'], 1 => ['pipe', 'w']],
+            $pipes,
+        );
+        $this->assertIsResource($process);
+
+        $answeredAt = null;
+        $arrivedAt = null;
+        $transport = new StdioTransport(input: $pipes[1], output: $pipes[0]);
+        $transport->onMessage(static function (TransportInterface $transport) use (&$answeredAt, &$arrivedAt): void {
+            if (null === $answeredAt) {
+                $transport->send('{"answer":1}', []);
+                $answeredAt = hrtime(true);
+
+                return;
+            }
+
+            $arrivedAt = hrtime(true);
+        });
+
+        $transport->listen();
+        proc_close($process);
+
+        $this->assertNotNull($answeredAt);
+        $this->assertNotNull($arrivedAt);
+        $this->assertLessThan(45, ($arrivedAt - $answeredAt) / 1e6, 'the idle wait must end when input arrives');
+    }
+
     #[TestDox('the line byte cap must be a positive number of bytes')]
     public function testRejectsNonPositiveCap(): void
     {