diff --git a/responses_api_agents/opencode_agent/app.py b/responses_api_agents/opencode_agent/app.py
index 87603386eaa..366a02202a4 100644
--- a/responses_api_agents/opencode_agent/app.py
+++ b/responses_api_agents/opencode_agent/app.py
@@ -582,7 +582,9 @@ def _build_opencode_config(self, rollout_id: Optional[str] = None) -> dict[str,
                 model,
                 {
                     "name": self.config.model,
-                    "interleaved": {"field": "reasoning"},
+                    # Gym model servers emit and accept `reasoning_content`; OpenCode replays assistant
+                    # history under this field, and Gym rejects an unknown `reasoning` key with a 422.
+                    "interleaved": {"field": "reasoning_content"},
                     "limit": {"context": self.config.context_window, "output": self.config.max_output_tokens},
                 },
             )
diff --git a/responses_api_agents/opencode_agent/tests/test_app.py b/responses_api_agents/opencode_agent/tests/test_app.py
index 9d0a7bb1721..1dcf0de12f7 100644
--- a/responses_api_agents/opencode_agent/tests/test_app.py
+++ b/responses_api_agents/opencode_agent/tests/test_app.py
@@ -22,6 +22,7 @@
 
 from nemo_gym.config_types import ModelServerRef, ResourcesServerRef
 from nemo_gym.openai_utils import (
+    NeMoGymChatCompletionCreateParamsNonStreaming,
     NeMoGymEasyInputMessage,
     NeMoGymFunctionCallOutput,
     NeMoGymResponseCreateParamsNonStreaming,
@@ -391,6 +392,30 @@ def test_model_server_builds_local_provider(self) -> None:
         assert provider["options"]["baseURL"] == "http://model/v1"
         assert provider["models"]["Qwen3.6-35B-A3B"]["limit"]["output"] == 131072
 
+    def test_model_server_replayed_reasoning_is_accepted_by_gym_chat_completions(self) -> None:
+        agent = _make_agent(model="m", model_server=ModelServerRef(type="responses_api_models", name="policy_model"))
+        with patch.object(agent, "_resolve_model_base_url", return_value="http://model/v1"):
+            field = agent._build_opencode_config()["provider"]["nemo"]["models"]["m"]["interleaved"]["field"]
+
+        # OpenCode replays an assistant turn after a tool call with the interleaved field set.
+        replayed = {
+            "role": "assistant",
+            "content": "",
+            field: "thinking",
+            "tool_calls": [{"id": "c1", "type": "function", "function": {"name": "task", "arguments": "{}"}}],
+        }
+        body = NeMoGymChatCompletionCreateParamsNonStreaming.model_validate(
+            {
+                "model": "m",
+                "messages": [
+                    {"role": "user", "content": "hi"},
+                    replayed,
+                    {"role": "tool", "tool_call_id": "c1", "content": "done"},
+                ],
+            }
+        )
+        assert body.messages[1]["reasoning_content"] == "thinking"
+
 
 class TestRolloutObservability:
     def test_routes_model_server_without_mutating_config(self, tmp_path: Path) -> None: