fix(openrouter): strip Responses reasoning IDs (#38383)

Closes #37777

---

OpenRouter can return OpenAI Responses reasoning item IDs such as `rs_*`
in assistant reasoning details. Those IDs are not reliably resolvable on
a later OpenRouter turn, so replaying them can make otherwise-valid
multi-turn conversations fail with a provider 404.

This keeps the useful reasoning payload while removing only the
ephemeral Responses item IDs before serializing `reasoning_details` back
into request history. Non-Responses IDs and reasoning text are left
intact.

libs/partners/openrouter/langchain_openrouter/chat_models.py

@@ -1125,6 +1125,19 @@ def _merge_reasoning_run(run: list[dict[str, Any]]) -> dict[str, Any]:
     return merged_entry
 
 
+def _strip_ephemeral_reasoning_ids(value: Any) -> Any:
+    """Remove OpenAI Responses reasoning item IDs from outbound payloads."""
+    if isinstance(value, list):
+        return [_strip_ephemeral_reasoning_ids(item) for item in value]
+    if isinstance(value, dict):
+        return {
+            key: _strip_ephemeral_reasoning_ids(item)
+            for key, item in value.items()
+            if not (key == "id" and isinstance(item, str) and item.startswith("rs_"))
+        }
+    return value
+
+
 def _merge_reasoning_details(
     details: list[dict[str, Any]],
 ) -> list[dict[str, Any]]:
@@ -1244,8 +1257,8 @@ def _convert_message_to_dict(message: BaseMessage) -> dict[str, Any]:  # noqa: C
         if "reasoning_content" in message.additional_kwargs:
             message_dict["reasoning"] = message.additional_kwargs["reasoning_content"]
         if "reasoning_details" in message.additional_kwargs:
-            message_dict["reasoning_details"] = _merge_reasoning_details(
-                message.additional_kwargs["reasoning_details"]
+            message_dict["reasoning_details"] = _strip_ephemeral_reasoning_ids(
+                _merge_reasoning_details(message.additional_kwargs["reasoning_details"])
             )
     elif isinstance(message, SystemMessage):
         message_dict = {"role": "system", "content": message.content}

libs/partners/openrouter/tests/unit_tests/test_chat_models.py

@@ -1295,6 +1295,44 @@ class TestMessageConversion:
         result = _convert_message_to_dict(msg)
         assert result["reasoning_details"] == details
 
+    def test_ai_message_reasoning_details_strips_responses_ids(self) -> None:
+        """OpenAI Responses `rs_*` item IDs are stripped before replay."""
+        response_id = "rs_053a05e24b0da75e0169fa358ea9fc81908b18aff8157798c1"
+        details = [
+            {
+                "type": "reasoning.text",
+                "id": response_id,
+                "text": "step-by-step",
+                "index": 0,
+            }
+        ]
+        msg = AIMessage(
+            content="Answer",
+            additional_kwargs={"reasoning_details": details},
+        )
+        result = _convert_message_to_dict(msg)
+        assert result["reasoning_details"] == [
+            {"type": "reasoning.text", "text": "step-by-step", "index": 0}
+        ]
+        assert response_id.startswith("rs_")
+        assert details[0]["id"] == response_id
+
+    def test_ai_message_reasoning_details_preserves_non_responses_ids(self) -> None:
+        """Non-Responses IDs are preserved in reasoning details."""
+        details = [
+            {
+                "type": "reasoning.text",
+                "id": "reasoning_abc123",
+                "text": "step-by-step",
+            }
+        ]
+        msg = AIMessage(
+            content="Answer",
+            additional_kwargs={"reasoning_details": details},
+        )
+        result = _convert_message_to_dict(msg)
+        assert result["reasoning_details"] == details
+
     def test_ai_message_unindexed_reasoning_details_not_merged(self) -> None:
         """Entries without an `index` are passed through unchanged."""
         details = [