diff --git a/.github/workflows/pipeline.yaml b/.github/workflows/pipeline.yaml
index b1f1e27f..957d6f91 100644
--- a/.github/workflows/pipeline.yaml
+++ b/.github/workflows/pipeline.yaml
@@ -72,7 +72,8 @@ jobs:
       - name: Tests
         run: vendor/bin/phpunit --testsuite=inspector
 
-  conformance:
+  conformance-server:
+    name: conformance / server
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
@@ -122,6 +123,47 @@ jobs:
         if: always()
         run: docker compose -f tests/Conformance/Fixtures/docker-compose.yml down
 
+  conformance-client:
+    name: conformance / client
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v6
+
+      - name: Setup PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: '8.4'
+          coverage: "none"
+
+      - name: Setup Node
+        uses: actions/setup-node@v6
+        with:
+          node-version: '22'
+
+      - name: Install Composer
+        uses: "ramsey/composer-install@v4"
+
+      - name: Create log directory
+        run: mkdir -p tests/Conformance/logs
+
+      - name: Run client conformance tests
+        working-directory: ./tests/Conformance
+        run: npx @modelcontextprotocol/conformance client --command "php ${{ github.workspace }}/tests/Conformance/client.php" --suite all --expected-failures conformance-baseline.yml
+
+      - name: Show logs on failure
+        if: failure()
+        run: |
+          echo "=== Client Conformance Log ==="
+          cat tests/Conformance/logs/client-conformance.log 2>/dev/null || echo "No client conformance log found"
+          echo ""
+          echo "=== Test Results ==="
+          find tests/Conformance/results -name "checks.json" 2>/dev/null | head -3 | while read f; do
+            echo "--- $f ---"
+            cat "$f"
+            echo ""
+          done || echo "No results found"
+
   qa:
     runs-on: ubuntu-latest
     steps:
diff --git a/Makefile b/Makefile
index 5358de25..49f0d9ec 100644
--- a/Makefile
+++ b/Makefile
@@ -1,4 +1,4 @@
-.PHONY: deps-stable deps-low cs phpstan tests unit-tests inspector-tests coverage ci ci-stable ci-lowest conformance-tests docs
+.PHONY: deps-stable deps-low cs phpstan tests unit-tests inspector-tests coverage ci ci-stable ci-lowest conformance-tests conformance-server conformance-client docs
 
 deps-stable:
 	composer update --prefer-stable
@@ -21,13 +21,18 @@ unit-tests:
 inspector-tests:
 	vendor/bin/phpunit --testsuite=inspector
 
-conformance-tests:
+conformance-tests: conformance-server conformance-client
+
+conformance-server:
 	docker compose -f tests/Conformance/Fixtures/docker-compose.yml up -d
 	@echo "Waiting for server to start..."
 	@sleep 5
 	cd tests/Conformance && npx @modelcontextprotocol/conformance server --url http://localhost:8000/ || true
 	docker compose -f tests/Conformance/Fixtures/docker-compose.yml down
 
+conformance-client:
+	cd tests/Conformance && npx @modelcontextprotocol/conformance client --command "php $(CURDIR)/tests/Conformance/client.php" --suite all --expected-failures conformance-baseline.yml || true
+
 coverage:
 	XDEBUG_MODE=coverage vendor/bin/phpunit --testsuite=unit --coverage-html=coverage
 
diff --git a/src/Schema/ClientCapabilities.php b/src/Schema/ClientCapabilities.php
index f6f277a0..bfe4e6ed 100644
--- a/src/Schema/ClientCapabilities.php
+++ b/src/Schema/ClientCapabilities.php
@@ -78,9 +78,9 @@ public static function fromArray(array $data): self
      *     sampling?: object,
      *     elicitation?: object,
      *     experimental?: object,
-     * }
+     * }|\stdClass
      */
-    public function jsonSerialize(): array
+    public function jsonSerialize(): array|object
     {
         $data = [];
         if ($this->roots || $this->rootsListChanged) {
@@ -102,6 +102,6 @@ public function jsonSerialize(): array
             $data['experimental'] = (object) $this->experimental;
         }
 
-        return $data;
+        return $data ?: new \stdClass();
     }
 }
diff --git a/src/Schema/Request/CallToolRequest.php b/src/Schema/Request/CallToolRequest.php
index 49bcd0d8..0674066c 100644
--- a/src/Schema/Request/CallToolRequest.php
+++ b/src/Schema/Request/CallToolRequest.php
@@ -65,7 +65,7 @@ protected function getParams(): array
     {
         return [
             'name' => $this->name,
-            'arguments' => $this->arguments,
+            'arguments' => $this->arguments ?: new \stdClass(),
         ];
     }
 }
diff --git a/tests/Conformance/client.php b/tests/Conformance/client.php
new file mode 100644
index 00000000..5c2ee2c7
--- /dev/null
+++ b/tests/Conformance/client.php
@@ -0,0 +1,108 @@
+<?php
+
+/*
+ * This file is part of the official PHP MCP SDK.
+ *
+ * A collaboration between Symfony and the PHP Foundation.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+require_once dirname(__DIR__, 2).'/vendor/autoload.php';
+
+use Mcp\Client;
+use Mcp\Client\Handler\Request\RequestHandlerInterface;
+use Mcp\Client\Transport\HttpTransport;
+use Mcp\Schema\ClientCapabilities;
+use Mcp\Schema\Enum\ElicitAction;
+use Mcp\Schema\JsonRpc\Request;
+use Mcp\Schema\JsonRpc\Response;
+use Mcp\Schema\Request\ElicitRequest;
+use Mcp\Schema\Result\ElicitResult;
+use Mcp\Tests\Conformance\FileLogger;
+
+$url = $argv[1] ?? null;
+$scenario = getenv('MCP_CONFORMANCE_SCENARIO') ?: null;
+
+if (!$url || !$scenario) {
+    fwrite(\STDERR, "Usage: MCP_CONFORMANCE_SCENARIO=<scenario> php client.php <server-url>\n");
+    exit(1);
+}
+
+@mkdir(__DIR__.'/logs', 0777, true);
+$logger = new FileLogger(__DIR__.'/logs/client-conformance.log', true);
+$logger->info(sprintf('Starting client conformance test: scenario=%s, url=%s', $scenario, $url));
+
+$builder = Client::builder()
+    ->setClientInfo('mcp-conformance-test-client', '1.0.0')
+    ->setInitTimeout(30)
+    ->setRequestTimeout(60)
+    ->setLogger($logger);
+
+if ('elicitation-sep1034-client-defaults' === $scenario) {
+    $builder->setCapabilities(new ClientCapabilities(elicitation: true));
+    $builder->addRequestHandler(new class($logger) implements RequestHandlerInterface {
+        public function __construct(private readonly Psr\Log\LoggerInterface $logger)
+        {
+        }
+
+        public function supports(Request $request): bool
+        {
+            return $request instanceof ElicitRequest;
+        }
+
+        public function handle(Request $request): Response
+        {
+            $this->logger->info('Received elicitation request, accepting with empty content');
+
+            return new Response($request->getId(), new ElicitResult(ElicitAction::Accept, []));
+        }
+    });
+}
+
+$client = $builder->build();
+$transport = new HttpTransport($url, logger: $logger);
+
+try {
+    $client->connect($transport);
+    $logger->info('Connected to server');
+
+    $toolsResult = $client->listTools();
+    $logger->info(sprintf('Listed %d tools', count($toolsResult->tools)));
+
+    switch ($scenario) {
+        case 'initialize':
+            break;
+
+        case 'tools_call':
+            $toolName = $toolsResult->tools[0]->name ?? 'test-tool';
+            $client->callTool($toolName, []);
+            $logger->info(sprintf('Called tool: %s', $toolName));
+            break;
+
+        case 'elicitation-sep1034-client-defaults':
+            $toolName = $toolsResult->tools[0]->name ?? 'test_client_elicitation_defaults';
+            $client->callTool($toolName, []);
+            $logger->info(sprintf('Called tool: %s', $toolName));
+            break;
+
+        default:
+            $logger->warning(sprintf('Unknown scenario: %s', $scenario));
+            break;
+    }
+
+    $client->disconnect();
+    $logger->info('Disconnected');
+    exit(0);
+} catch (Throwable $e) {
+    $logger->error(sprintf('Error: %s', $e->getMessage()));
+    fwrite(\STDERR, sprintf("Error: %s\n%s\n", $e->getMessage(), $e->getTraceAsString()));
+
+    try {
+        $client->disconnect();
+    } catch (Throwable $ignored) {
+    }
+
+    exit(1);
+}
diff --git a/tests/Conformance/conformance-baseline.yml b/tests/Conformance/conformance-baseline.yml
index de676e85..61f9783f 100644
--- a/tests/Conformance/conformance-baseline.yml
+++ b/tests/Conformance/conformance-baseline.yml
@@ -1,3 +1,28 @@
 server:
   - dns-rebinding-protection
 
+client:
+  - elicitation-sep1034-client-defaults
+  - sse-retry
+  - auth/metadata-default
+  - auth/metadata-var1
+  - auth/metadata-var2
+  - auth/metadata-var3
+  - auth/basic-cimd
+  - auth/scope-from-www-authenticate
+  - auth/scope-from-scopes-supported
+  - auth/scope-omitted-when-undefined
+  - auth/scope-step-up
+  - auth/scope-retry-limit
+  - auth/token-endpoint-auth-basic
+  - auth/token-endpoint-auth-post
+  - auth/token-endpoint-auth-none
+  - auth/pre-registration
+  - auth/2025-03-26-oauth-metadata-backcompat
+  - auth/2025-03-26-oauth-endpoint-fallback
+  - auth/offline-access-scope
+  - auth/offline-access-not-supported
+  - auth/client-credentials-jwt
+  - auth/client-credentials-basic
+  - auth/cross-app-access-complete-flow
+
diff --git a/tests/Unit/Server/Handler/Request/InitializeHandlerTest.php b/tests/Unit/Server/Handler/Request/InitializeHandlerTest.php
index bdd79d42..c56eac4d 100644
--- a/tests/Unit/Server/Handler/Request/InitializeHandlerTest.php
+++ b/tests/Unit/Server/Handler/Request/InitializeHandlerTest.php
@@ -41,10 +41,10 @@ public function testHandleUsesConfigurationProtocolVersion(): void
         $session = $this->createMock(SessionInterface::class);
         $session->expects($this->exactly(2))
             ->method('set')
-            ->willReturnCallback(function (string $key, array $value): void {
+            ->willReturnCallback(function (string $key, mixed $value): void {
                 match ($key) {
                     'client_info' => $this->assertSame(['name' => 'client-app', 'version' => '1.0.0'], $value),
-                    'client_capabilities' => $this->assertSame([], $value),
+                    'client_capabilities' => $this->assertEquals(new \stdClass(), $value),
                     default => $this->fail("Unexpected session key: {$key}"),
                 };
             });