fix(codex): drop extra_headers for chatgpt.com backend

tests/agent/transports/test_codex_transport.py

@@ -155,6 +155,46 @@ class TestCodexBuildKwargs:
         )
         assert "max_output_tokens" not in kw
 
+    def test_codex_backend_does_not_set_extra_headers(self, transport):
+        messages = [{"role": "user", "content": "Hi"}]
+
+        kw = transport.build_kwargs(
+            model="gpt-5.4",
+            messages=messages,
+            tools=[],
+            session_id="conv-codex-1",
+            is_codex_backend=True,
+        )
+
+        assert "extra_headers" not in kw
+
+    def test_codex_backend_strips_caller_extra_headers(self, transport):
+        messages = [{"role": "user", "content": "Hi"}]
+
+        kw = transport.build_kwargs(
+            model="gpt-5.4",
+            messages=messages,
+            tools=[],
+            session_id="conv-codex-1",
+            is_codex_backend=True,
+            request_overrides={"extra_headers": {"x-test": "1"}},
+        )
+
+        assert "extra_headers" not in kw
+
+    def test_non_codex_responses_preserves_caller_extra_headers(self, transport):
+        messages = [{"role": "user", "content": "Hi"}]
+
+        kw = transport.build_kwargs(
+            model="gpt-5.4",
+            messages=messages,
+            tools=[],
+            is_codex_backend=False,
+            request_overrides={"extra_headers": {"x-test": "1"}},
+        )
+
+        assert kw["extra_headers"] == {"x-test": "1"}
+
     def test_xai_headers(self, transport):
         messages = [{"role": "user", "content": "Hi"}]
         kw = transport.build_kwargs(