[Client] Expose the capabilities the server declared

CHANGELOG.md

@@ -22,6 +22,8 @@ All notable changes to `mcp/sdk` will be documented in this file.
 * [BC Break] `AbstractSchemaDefinition` declares an abstract `getDefault()`, which a custom schema definition has to implement.
 * Fix `Client::getPrompt()` without arguments sending `"arguments": []`, which servers validating the spec's object type (e.g. the TypeScript SDK) reject: empty arguments are now omitted.
 * Add `Tool::$execution` (`ToolExecution` with a `TaskSupport` enum) and `ServerCapabilities::$tasks`, which were dropped when parsing a 2025-11-25 server's `tools/list` and `initialize` results.
+* Add `Client::getServerCapabilities()`, returning what the server declared in `initialize` or, from `2026-07-28` on, in `server/discover`.
+* [BC Break] `ClientStateInterface` declares `setServerCapabilities()` and `getServerCapabilities()`, which a custom implementation has to add.
 
 0.8.0
 -----

src/Client.php

@@ -48,6 +48,7 @@
 use Mcp\Schema\Result\ListResourceTemplatesResult;
 use Mcp\Schema\Result\ListToolsResult;
 use Mcp\Schema\Result\ReadResourceResult;
+use Mcp\Schema\ServerCapabilities;
 use Psr\Log\LoggerInterface;
 use Psr\Log\NullLogger;
 
@@ -147,6 +148,18 @@ public function getInstructions(): ?string
         return $this->protocol->getState()->getInstructions();
     }
 
+    /**
+     * Capabilities the server declared: during the handshake, or in its
+     * `server/discover` result from 2026-07-28 on.
+     *
+     * Null until a server declared them, e.g. before connecting or when a
+     * 2026-07-28 server does not answer `server/discover`.
+     */
+    public function getServerCapabilities(): ?ServerCapabilities
+    {
+        return $this->protocol->getState()->getServerCapabilities();
+    }
+
     /**
      * Protocol revision negotiated during the handshake.
      *

src/Client/Protocol.php

@@ -38,6 +38,7 @@
 use Mcp\Schema\Request\DiscoverRequest;
 use Mcp\Schema\Request\InitializeRequest;
 use Mcp\Schema\Result\InitializeResult;
+use Mcp\Schema\ServerCapabilities;
 use Mcp\Server\Stateless\RequestMeta;
 use Psr\Log\LoggerInterface;
 use Psr\Log\NullLogger;
@@ -228,6 +229,7 @@ public function initialize(Configuration $config): Response|Error
             $this->state->setProtocolVersion($negotiated);
             $this->state->setServerInfo($initResult->serverInfo);
             $this->state->setInstructions($initResult->instructions);
+            $this->state->setServerCapabilities($initResult->capabilities);
             $this->state->setInitialized(true);
 
             $this->sendNotification(new InitializedNotification());
@@ -246,9 +248,10 @@ public function initialize(Configuration $config): Response|Error
      *
      * There is nothing to negotiate: the revision travels on every request, so
      * the connection is usable the moment the transport is. `server/discover`
-     * is only asked because the facade exposes `getServerInfo()`, and a server
-     * that will not answer it still serves every other method — so a failure
-     * here is logged and the connection proceeds.
+     * is only asked because the facade exposes `getServerInfo()` and
+     * `getServerCapabilities()`, and a server that will not answer it still
+     * serves every other method — so a failure here is logged and the
+     * connection proceeds.
      *
      * @return Response<array<string, mixed>>
      */
@@ -300,6 +303,10 @@ private function readDiscovery(array $result): void
             $this->state->setInstructions($result['instructions']);
         }
 
+        if (\is_array($result['capabilities'] ?? null)) {
+            $this->state->setServerCapabilities(ServerCapabilities::fromArray($result['capabilities']));
+        }
+
         $this->reconcileVersion($result['supportedVersions'] ?? null);
 
         $this->logger->info('Discovery complete', [

src/Client/State/ClientState.php

@@ -15,6 +15,7 @@
 use Mcp\Schema\Implementation;
 use Mcp\Schema\JsonRpc\Error;
 use Mcp\Schema\JsonRpc\Response;
+use Mcp\Schema\ServerCapabilities;
 
 /**
  * In-memory client state implementation.
@@ -32,6 +33,7 @@ class ClientState implements ClientStateInterface
     private ?ProtocolVersion $protocolVersion = null;
     private ?Implementation $serverInfo = null;
     private ?string $instructions = null;
+    private ?ServerCapabilities $serverCapabilities = null;
 
     /** @var array<int|string, array{request_id: int|string, timestamp: int, timeout: int}> */
     private array $pendingRequests = [];
@@ -128,6 +130,16 @@ public function getInstructions(): ?string
         return $this->instructions;
     }
 
+    public function setServerCapabilities(ServerCapabilities $capabilities): void
+    {
+        $this->serverCapabilities = $capabilities;
+    }
+
+    public function getServerCapabilities(): ?ServerCapabilities
+    {
+        return $this->serverCapabilities;
+    }
+
     public function storeProgress(string $token, float $progress, ?float $total, ?string $message): void
     {
         $this->progressUpdates[] = [

src/Client/State/ClientStateInterface.php

@@ -15,6 +15,7 @@
 use Mcp\Schema\Implementation;
 use Mcp\Schema\JsonRpc\Error;
 use Mcp\Schema\JsonRpc\Response;
+use Mcp\Schema\ServerCapabilities;
 
 /**
  * Interface for client state management.
@@ -109,6 +110,16 @@ public function setInstructions(?string $instructions): void;
      */
     public function getInstructions(): ?string;
 
+    /**
+     * Set the capabilities the server declared.
+     */
+    public function setServerCapabilities(ServerCapabilities $capabilities): void;
+
+    /**
+     * Get the capabilities the server declared, null until it declared them.
+     */
+    public function getServerCapabilities(): ?ServerCapabilities;
+
     /**
      * Store progress data received from a notification.
      *

tests/Integration/HandshakeTest.php

@@ -82,6 +82,26 @@ public function testServerInfoIsExchanged(): void
         $this->assertTrue($client->isConnected());
     }
 
+    #[TestDox('the handshake carries the server capabilities to the client')]
+    public function testServerCapabilitiesAreExchanged(): void
+    {
+        $capabilities = $this->connect('handshake')->getServerCapabilities();
+
+        // The fixture registers no tools, resources or prompts.
+        $this->assertNotNull($capabilities);
+        $this->assertTrue($capabilities->logging);
+        $this->assertTrue($capabilities->completions);
+        $this->assertFalse($capabilities->tools);
+        $this->assertFalse($capabilities->prompts);
+        $this->assertFalse($capabilities->resources);
+    }
+
+    #[TestDox('the server capabilities are unset before the handshake')]
+    public function testServerCapabilitiesAreNullBeforeConnecting(): void
+    {
+        $this->assertNull($this->clientBuilder()->build()->getServerCapabilities());
+    }
+
     #[TestDox('the negotiated revision is unset before the handshake')]
     public function testProtocolVersionIsNullBeforeConnecting(): void
     {

tests/Integration/StatelessClientTest.php

@@ -75,6 +75,7 @@ public function testConnectWithoutHandshake(): void
         $this->assertTrue($client->isConnected());
         $this->assertSame(ProtocolVersion::V2026_07_28, $client->getProtocolVersion());
         $this->assertSame('Stateless Lifecycle Demo', $client->getServerInfo()?->name);
+        $this->assertTrue($client->getServerCapabilities()?->tools);
 
         $client->disconnect();
     }

tests/Unit/Client/ProtocolTest.php

@@ -93,6 +93,7 @@ public function testDiscoveryFailureIsNotFatal(): void
         $protocol->initialize($config);
 
         $this->assertTrue($protocol->getState()->isInitialized());
+        $this->assertNull($protocol->getState()->getServerCapabilities());
     }
 
     #[TestDox('refuses to continue when discovery shows the server has no modern revision')]