diff --git a/agent/transports/codex.py b/agent/transports/codex.py
index 2ebc396fbb1..f011034dae8 100644
--- a/agent/transports/codex.py
+++ b/agent/transports/codex.py
@@ -105,6 +105,7 @@ class ResponsesApiTransport(ProviderTransport):
 
         if reasoning_enabled and is_xai_responses:
             kwargs["include"] = ["reasoning.encrypted_content"]
+            kwargs["reasoning"] = {"effort": reasoning_effort}
         elif reasoning_enabled:
             if is_github_responses:
                 github_reasoning = params.get("github_reasoning_extra")
diff --git a/tests/agent/transports/test_codex_transport.py b/tests/agent/transports/test_codex_transport.py
index 26145660cca..7217f2e9e6a 100644
--- a/tests/agent/transports/test_codex_transport.py
+++ b/tests/agent/transports/test_codex_transport.py
@@ -149,6 +149,37 @@ class TestCodexBuildKwargs:
         # "minimal" should be clamped to "low"
         assert kw.get("reasoning", {}).get("effort") == "low"
 
+    def test_xai_reasoning_effort_passed(self, transport):
+        messages = [{"role": "user", "content": "Hi"}]
+        kw = transport.build_kwargs(
+            model="grok-4.3", messages=messages, tools=[],
+            is_xai_responses=True,
+            reasoning_config={"effort": "high"},
+        )
+        # xAI Responses must receive both encrypted reasoning content and the effort
+        assert kw.get("reasoning") == {"effort": "high"}
+        assert "reasoning.encrypted_content" in kw.get("include", [])
+
+    def test_xai_reasoning_disabled_no_reasoning_key(self, transport):
+        messages = [{"role": "user", "content": "Hi"}]
+        kw = transport.build_kwargs(
+            model="grok-4.3", messages=messages, tools=[],
+            is_xai_responses=True,
+            reasoning_config={"enabled": False},
+        )
+        # When reasoning is disabled, do not send the reasoning key at all
+        assert "reasoning" not in kw
+
+    def test_xai_minimal_effort_clamped(self, transport):
+        messages = [{"role": "user", "content": "Hi"}]
+        kw = transport.build_kwargs(
+            model="grok-4.3", messages=messages, tools=[],
+            is_xai_responses=True,
+            reasoning_config={"effort": "minimal"},
+        )
+        # "minimal" should be clamped to "low" for xAI as well
+        assert kw.get("reasoning", {}).get("effort") == "low"
+
 
 class TestCodexValidateResponse: