diff --git a/src/Mcp/EventListener/CoerceJsonEncodedArgumentsListener.php b/src/Mcp/EventListener/CoerceJsonEncodedArgumentsListener.php
new file mode 100644
index 0000000..a756dd8
--- /dev/null
+++ b/src/Mcp/EventListener/CoerceJsonEncodedArgumentsListener.php
@@ -0,0 +1,99 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Mcp\EventListener;
+
+use App\Mcp\Schema\CoercingSchemaGenerator;
+use Mcp\Capability\RegistryInterface;
+use Mcp\Event\RequestEvent;
+use Mcp\Schema\Request\CallToolRequest;
+use Symfony\Component\DependencyInjection\Attribute\Autowire;
+use Symfony\Component\EventDispatcher\Attribute\AsEventListener;
+use Throwable;
+
+use function is_array;
+use function is_string;
+
+/**
+ * Decodes JSON-encoded structured arguments before tool calls are validated.
+ *
+ * Some MCP clients/proxies serialize array and object arguments as JSON strings
+ * (e.g. `tagIds` arrives as the string `"[3]"` instead of the array `[3]`). The
+ * SDK validates arguments against the JSON Schema BEFORE casting, so an `array`
+ * schema rejects the string with a 422, and ReferenceHandler::castToArray does
+ * not decode JSON strings either.
+ *
+ * This listener runs on the SDK RequestEvent (dispatched before any handler) and,
+ * driven by the tool's input schema, decodes string arguments whose target type
+ * is `array` or `object`. Scalar stringification is handled separately by
+ * {@see CoercingSchemaGenerator}.
+ */
+#[AsEventListener(event: RequestEvent::class)]
+final class CoerceJsonEncodedArgumentsListener
+{
+    public function __construct(
+        #[Autowire(service: 'mcp.registry')]
+        private readonly RegistryInterface $registry,
+    ) {}
+
+    public function __invoke(RequestEvent $event): void
+    {
+        $request = $event->getRequest();
+        if (!$request instanceof CallToolRequest) {
+            return;
+        }
+
+        $arguments = $request->arguments;
+        if ([] === $arguments) {
+            return;
+        }
+
+        $properties = $this->toolProperties($request->name);
+        if (null === $properties) {
+            return;
+        }
+
+        $changed = false;
+        foreach ($arguments as $name => $value) {
+            if (!is_string($value) || !is_array($properties[$name] ?? null)) {
+                continue;
+            }
+
+            $types = (array) ($properties[$name]['type'] ?? []);
+            if ([] === array_intersect(['array', 'object'], $types)) {
+                continue;
+            }
+
+            $decoded = json_decode($value, true);
+            if (is_array($decoded)) {
+                $arguments[$name] = $decoded;
+                $changed          = true;
+            }
+        }
+
+        if ($changed) {
+            $event->setRequest(
+                new CallToolRequest($request->name, $arguments)
+                    ->withId($request->getId())
+                    ->withMeta($request->getMeta()),
+            );
+        }
+    }
+
+    /**
+     * @return null|array<string, mixed>
+     */
+    private function toolProperties(string $toolName): ?array
+    {
+        try {
+            $schema = $this->registry->getTool($toolName)->tool->inputSchema;
+        } catch (Throwable) {
+            return null;
+        }
+
+        $properties = $schema['properties'] ?? null;
+
+        return is_array($properties) ? $properties : null;
+    }
+}
diff --git a/tests/Unit/Mcp/CoerceJsonEncodedArgumentsListenerTest.php b/tests/Unit/Mcp/CoerceJsonEncodedArgumentsListenerTest.php
new file mode 100644
index 0000000..4b42db8
--- /dev/null
+++ b/tests/Unit/Mcp/CoerceJsonEncodedArgumentsListenerTest.php
@@ -0,0 +1,91 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Tests\Unit\Mcp;
+
+use App\Mcp\EventListener\CoerceJsonEncodedArgumentsListener;
+use Mcp\Capability\Registry\ToolReference;
+use Mcp\Capability\RegistryInterface;
+use Mcp\Event\RequestEvent;
+use Mcp\Schema\Request\CallToolRequest;
+use Mcp\Schema\Tool;
+use Mcp\Server\Session\SessionInterface;
+use PHPUnit\Framework\TestCase;
+
+/**
+ * @internal
+ */
+class CoerceJsonEncodedArgumentsListenerTest extends TestCase
+{
+    private const SCHEMA = [
+        'type'       => 'object',
+        'properties' => [
+            'id'              => ['type' => ['integer', 'string']],
+            'title'           => ['type' => 'string'],
+            'tagIds'          => ['type' => ['array', 'null'], 'items' => ['type' => ['integer', 'string']]],
+            'collaboratorIds' => ['type' => ['array', 'null'], 'items' => ['type' => ['integer', 'string']]],
+        ],
+    ];
+
+    public function testDecodesJsonStringArrayForArrayTypedParam(): void
+    {
+        $result = $this->handle(['tagIds' => '[3]', 'collaboratorIds' => '[5,6]']);
+
+        self::assertSame([3], $result->arguments['tagIds']);
+        self::assertSame([5, 6], $result->arguments['collaboratorIds']);
+    }
+
+    public function testLeavesRealArrayUntouched(): void
+    {
+        $result = $this->handle(['tagIds' => [3]]);
+
+        self::assertSame([3], $result->arguments['tagIds']);
+    }
+
+    public function testDoesNotTouchStringTypedParamEvenIfItLooksLikeJson(): void
+    {
+        $result = $this->handle(['title' => '[1,2]']);
+
+        // title is schema-typed string -> must stay the literal string.
+        self::assertSame('[1,2]', $result->arguments['title']);
+    }
+
+    public function testLeavesScalarTypedParamUntouched(): void
+    {
+        // id is integer/string typed -> not an array/object, handled by the schema
+        // relaxation + SDK cast, not by this listener.
+        $result = $this->handle(['id' => '463']);
+
+        self::assertSame('463', $result->arguments['id']);
+    }
+
+    public function testPreservesRequestId(): void
+    {
+        $result = $this->handle(['tagIds' => '[3]']);
+
+        self::assertSame(1, $result->getId());
+    }
+
+    /**
+     * @param array<string, mixed> $arguments
+     */
+    private function handle(array $arguments): CallToolRequest
+    {
+        $tool      = new Tool('update-task', self::SCHEMA, null, null);
+        $reference = new ToolReference($tool, static fn () => null);
+
+        $registry = $this->createMock(RegistryInterface::class);
+        $registry->method('getTool')->willReturn($reference);
+
+        $request = new CallToolRequest('update-task', $arguments)->withId(1);
+        $event   = new RequestEvent($request, $this->createMock(SessionInterface::class));
+
+        (new CoerceJsonEncodedArgumentsListener($registry))($event);
+
+        $result = $event->getRequest();
+        self::assertInstanceOf(CallToolRequest::class, $result);
+
+        return $result;
+    }
+}