diff --git a/src/GrokProvider.php b/src/GrokProvider.php
index 300fc42..fb07c51 100644
--- a/src/GrokProvider.php
+++ b/src/GrokProvider.php
@@ -17,9 +17,11 @@
 use Generator;
 use PapiAI\Core\Contracts\NamedToolSelectableInterface;
 use PapiAI\Core\Contracts\ProviderInterface;
+use PapiAI\Core\Effort;
 use PapiAI\Core\Exception\AuthenticationException;
 use PapiAI\Core\Exception\ProviderException;
 use PapiAI\Core\Exception\RateLimitException;
+use PapiAI\Core\Exception\UnknownEffortException;
 use PapiAI\Core\Message;
 use PapiAI\Core\Response;
 use PapiAI\Core\Role;
@@ -44,9 +46,8 @@
  *   - grok-2 (multimodal)
  *
  * @see https://docs.x.ai/docs *
- * The neutral `effort` option is accepted and ignored here. xAI does expose a top-level reasoning_effort parameter, but papi does not map it yet, so the option is accepted and ignored for now. Note Grok 4.5 cannot disable reasoning at all. Ignoring it
- * degrades nothing the caller was promised, which is why it is silent where an unhonourable
- * `toolChoice` throws.
+ * The neutral effort option maps to xAI's top-level reasoning_effort. Note Grok 4.5 reasons
+ * always and cannot be switched off, so "none" narrows to the shallowest level it accepts.
  */
 class GrokProvider implements ProviderInterface, NamedToolSelectableInterface
 {
@@ -71,6 +72,7 @@ public function __construct(
         private readonly string $apiKey,
         private readonly string $defaultModel = self::MODEL_GROK_4_5,
         private readonly int $defaultMaxTokens = 4096,
+        private readonly ?Effort $defaultEffort = null,
     ) {
     }
 
@@ -214,9 +216,52 @@ private function buildPayload(array $messages, array $options): array
             }
         }
 
+        // Reasoning effort. xAI takes a flat level, but the accepted set varies: Grok 4.5 cannot
+        // switch reasoning off at all, so "none" narrows to the shallowest level it does accept.
+        $effort = $this->effortFor($options);
+
+        if ($effort !== null) {
+            $model = (string) ($options['model'] ?? $this->defaultModel);
+            $payload['reasoning_effort'] = $effort->nearestOf($this->levelsFor($model))->value;
+        }
+
         return $payload;
     }
 
+    /**
+     * The effort this request asks for: the per-call option, else the provider default.
+     *
+     * @param array<string, mixed> $options The caller's request options
+     *
+     * @throws UnknownEffortException When the level is not one core defines
+     */
+    private function effortFor(array $options): ?Effort
+    {
+        if (!isset($options['effort'])) {
+            return $this->defaultEffort;
+        }
+
+        $level = (string) $options['effort'];
+
+        return Effort::tryFrom($level) ?? throw new UnknownEffortException($level);
+    }
+
+    /**
+     * The levels this model accepts.
+     *
+     * Grok 4.5 reasons always and has no "none"; 4.3 and earlier can be switched off.
+     *
+     * @return non-empty-list<Effort>
+     */
+    private function levelsFor(string $model): array
+    {
+        if (str_contains($model, '4.5')) {
+            return [Effort::Low, Effort::Medium, Effort::High];
+        }
+
+        return [Effort::None, Effort::Low, Effort::Medium, Effort::High];
+    }
+
     /**
      * Convert a Message to OpenAI-compatible API format.
      */
diff --git a/tests/Unit/GrokEffortTest.php b/tests/Unit/GrokEffortTest.php
new file mode 100644
index 0000000..8627c9d
--- /dev/null
+++ b/tests/Unit/GrokEffortTest.php
@@ -0,0 +1,88 @@
+<?php
+
+/*
+ * This file is part of PapiAI,
+ * A simple but powerful PHP library for building AI agents.
+ *
+ * (c) Marcello Duarte <marcello.duarte@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+declare(strict_types=1);
+
+use PapiAI\Core\Effort;
+use PapiAI\Core\Message;
+use PapiAI\Grok\GrokProvider;
+
+/**
+ * Captures the request payload so effort mapping can be asserted without HTTP.
+ */
+class TestableGrokEffortProvider extends GrokProvider
+{
+    public array $lastPayload = [];
+
+    protected function request(array $payload): array
+    {
+        $this->lastPayload = $payload;
+
+        return ['choices' => [['message' => ['role' => 'assistant', 'content' => 'ok'], 'finish_reason' => 'stop']]];
+    }
+}
+
+describe('GrokProvider reasoning effort', function () {
+    beforeEach(function () {
+        $this->provider = new TestableGrokEffortProvider('test-api-key');
+        $this->chat = fn (array $options) => $this->provider->chat([Message::user('hi')], $options);
+    });
+
+    it('maps the middle levels straight through', function () {
+        foreach (['low', 'medium', 'high'] as $level) {
+            ($this->chat)(['effort' => $level, 'model' => 'grok-4.3']);
+
+            expect($this->provider->lastPayload['reasoning_effort'])->toBe($level);
+        }
+    });
+
+    it('lets 4.3 switch reasoning off', function () {
+        ($this->chat)(['effort' => 'none', 'model' => 'grok-4.3']);
+
+        expect($this->provider->lastPayload['reasoning_effort'])->toBe('none');
+    });
+
+    it('will not claim 4.5 can switch reasoning off, because it cannot', function () {
+        ($this->chat)(['effort' => 'none', 'model' => 'grok-4.5']);
+
+        expect($this->provider->lastPayload['reasoning_effort'])->toBe('low');
+    });
+
+    it('narrows the levels above what xAI offers', function () {
+        foreach (['extra-high', 'maximum'] as $level) {
+            ($this->chat)(['effort' => $level, 'model' => 'grok-4.5']);
+
+            expect($this->provider->lastPayload['reasoning_effort'])->toBe('high');
+        }
+    });
+
+    it('sends nothing when the caller does not ask', function () {
+        ($this->chat)([]);
+
+        expect($this->provider->lastPayload)->not->toHaveKey('reasoning_effort');
+    });
+
+    it('rejects a level it does not recognise', function () {
+        expect(fn () => ($this->chat)(['effort' => 'enormous']))
+            ->toThrow(InvalidArgumentException::class, 'enormous');
+    });
+
+    it('accepts a provider-level default the call can override', function () {
+        $provider = new TestableGrokEffortProvider('k', 'grok-4.3', 4096, Effort::High);
+
+        $provider->chat([Message::user('hi')], []);
+        expect($provider->lastPayload['reasoning_effort'])->toBe('high');
+
+        $provider->chat([Message::user('hi')], ['effort' => 'low']);
+        expect($provider->lastPayload['reasoning_effort'])->toBe('low');
+    });
+});