diff --git a/CHANGELOG.md b/CHANGELOG.md
index eb0b7062..ddca338c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -67,6 +67,7 @@ All notable changes to `mcp/sdk` will be documented in this file.
   * [BC Break] Keep the JSON types of `tools/call` arguments, so validation sees what the client sent: `{}` and `{"0":"x"}` are objects, `[]` and `["x"]` are arrays. Nested objects in `CallToolRequest::$arguments` and in the arguments of `ToolHandlerInterface::execute()` are `\stdClass`, as they are for parameters without a type or typed `mixed`, `object` or `\stdClass`; parameters typed `array` still receive arrays at every level. `array<string, T>` parameters generate `{"type": "object"}`, and `array<int, T>` parameters accept a JSON array or object.
   * [BC Break] Reject `tools/call` whose `arguments` is not a JSON object, including `[]`, with `-32602`, and one whose message has a member name starting with a NUL byte with `-32600`.
   * [BC Break] Answer params that do not fit a known method (e.g. `tools/call` without `name`) with `-32602` instead of `-32600`, as JSON-RPC requires. `Request::fromArray()` throws the new `InvalidParamsException`, a subclass of `InvalidArgumentException`, and `InvalidInputMessageException::isInvalidParams()` tells the protocols which code to send.
+  * Fix a handshake-era request for a method the SDK does not know being answered with `-32600`: it gets `-32601`, as JSON-RPC requires and as the `2026-07-28` era already answers.
   * [BC Break] Answer an argument that cannot be cast to its parameter type with `-32602` instead of `-32603`, for tools, prompts and resources. The message names the parameter. A `string` parameter refuses a list or an object instead of receiving `"Array"`, and an int-backed enum refuses a value with a fraction instead of truncating it. A missing required argument of a prompt, or of a tool that gets past schema validation, gets `-32602` too.
   * [BC Break] Modern tools with an `outputSchema` send and validate a null result as structured content, with JSON text `null` alongside it. A tool whose schema does not allow null, including a `void` tool, returns a tool error instead of the text `(null)`. `ToolReference::extractStructuredContent()` returns `StructuredContent::Absent` when there is nothing to send.
   * [BC Break] `ReferenceHandler` resolves a string handler that is not an invokable class from the container by service id. The container goes first, so a function handler no longer runs if the container has an entry of that name.
diff --git a/src/Server/Protocol.php b/src/Server/Protocol.php
index 90c40e81..5dcbbc12 100644
--- a/src/Server/Protocol.php
+++ b/src/Server/Protocol.php
@@ -12,6 +12,7 @@
 namespace Mcp\Server;
 
 use Mcp\Schema\Exception\InvalidInputMessageException;
+use Mcp\Schema\Exception\UnknownMethodException;
 use Mcp\Schema\JsonRpc\Error;
 use Mcp\Schema\JsonRpc\MessageInterface;
 use Mcp\Schema\JsonRpc\Notification;
@@ -376,9 +377,11 @@ private function handleInvalidMessage(TransportInterface $transport, InvalidInpu
     {
         $this->logger->warning('Failed to create message.', ['exception' => $exception]);
 
-        $error = $exception->isInvalidParams()
-            ? Error::forInvalidParams($exception->getMessage(), $exception->getRequestId())
-            : Error::forInvalidRequest($exception->getMessage(), $exception->getRequestId());
+        $error = match (true) {
+            $exception instanceof UnknownMethodException => Error::forMethodNotFound($exception->getMessage(), $exception->getRequestId()),
+            $exception->isInvalidParams() => Error::forInvalidParams($exception->getMessage(), $exception->getRequestId()),
+            default => Error::forInvalidRequest($exception->getMessage(), $exception->getRequestId()),
+        };
         $this->sendResponse($transport, $error, $session, $context);
     }
 
diff --git a/tests/Unit/Server/ProtocolTest.php b/tests/Unit/Server/ProtocolTest.php
index 292d9acc..aba2e47d 100644
--- a/tests/Unit/Server/ProtocolTest.php
+++ b/tests/Unit/Server/ProtocolTest.php
@@ -959,6 +959,35 @@ public function testRequestWithoutHandlerReturnsMethodNotFoundError(): void
         $this->assertStringContainsString('No handler found', $message['error']['message']);
     }
 
+    #[TestDox('Request for a method the SDK does not know returns method not found error')]
+    public function testUnknownMethodReturnsMethodNotFoundError(): void
+    {
+        $session = $this->createMock(SessionInterface::class);
+
+        $this->sessionManager->method('createWithId')->willReturn($session);
+        $this->sessionManager->method('exists')->willReturn(true);
+
+        $transport = new RecordingTransport();
+
+        $protocol = new Protocol(
+            requestHandlers: [],
+            notificationHandlers: [],
+            messageFactory: MessageFactory::make(),
+            sessionManager: $this->sessionManager,
+        );
+
+        $protocol->processInput(
+            $transport,
+            '{"jsonrpc": "2.0", "id": 7, "method": "nonexistent/method", "params": {}}',
+            Uuid::v4()
+        );
+
+        $this->assertCount(1, $transport->sent);
+        $message = json_decode($transport->sent[0]['message'], true);
+        $this->assertSame(Error::METHOD_NOT_FOUND, $message['error']['code']);
+        $this->assertSame(7, $message['id']);
+    }
+
     #[TestDox('Handler throwing InvalidArgumentException returns invalid params error')]
     public function testHandlerInvalidArgumentReturnsInvalidParamsError(): void
     {
diff --git a/tests/Unit/Server/Transport/StdioDualEraTest.php b/tests/Unit/Server/Transport/StdioDualEraTest.php
index 61907ca4..02325e1d 100644
--- a/tests/Unit/Server/Transport/StdioDualEraTest.php
+++ b/tests/Unit/Server/Transport/StdioDualEraTest.php
@@ -426,7 +426,7 @@ public function testCustomDiscoverHandlerIsNotReachedOverTheHandshake(): void
             ['jsonrpc' => '2.0', 'id' => 2, 'method' => 'server/discover'],
         ]);
 
-        $this->assertSame(Error::INVALID_REQUEST, $answers[2]['error']['code']);
+        $this->assertSame(Error::METHOD_NOT_FOUND, $answers[2]['error']['code']);
         $this->assertArrayNotHasKey('result', $answers[2]);
     }