fix(relay): ignore rewrites after codec baseline failure

Signed-off-by: Alex Fournier <afournier@nvidia.com>

agent/relay_llm.py

@@ -855,13 +855,13 @@ def _provider_request(
     request: Any,
     *,
     relay_request_body: dict[str, Any],
-    codec_baseline_body: dict[str, Any],
+    codec_baseline_body: dict[str, Any] | None,
     metadata: dict[str, Any] | None,
 ) -> dict[str, Any]:
     content = getattr(request, "content", request)
     if not isinstance(content, dict):
         content = relay_request_body
-    if _json_equal(content, relay_request_body):
+    if codec_baseline_body is None or _json_equal(content, relay_request_body):
         final = dict(original)
     else:
         baseline = codec_baseline_body
@@ -1006,7 +1006,7 @@ def _codec_round_trip_request_body(
     *,
     relay_request_body: dict[str, Any],
     metadata: dict[str, Any] | None,
-) -> dict[str, Any]:
+) -> dict[str, Any] | None:
     """Return the codec-only request shape used to identify real rewrites."""
     codec = _codec(relay, metadata)
     if codec is None:
@@ -1018,8 +1018,16 @@ def _codec_round_trip_request_body(
         if isinstance(content, dict):
             return _provider_request_body(content, metadata)
     except Exception:
-        logger.debug("NeMo Relay request codec baseline failed", exc_info=True)
-    return _provider_request_body(relay_request_body, metadata)
+        logger.warning(
+            "NeMo Relay request codec baseline failed; ignoring request rewrites",
+            exc_info=True,
+        )
+        return None
+    logger.warning(
+        "NeMo Relay request codec returned an unsupported baseline; "
+        "ignoring request rewrites"
+    )
+    return None
 
 
 def _provider_request_body(

tests/agent/test_relay_llm.py

@@ -1349,6 +1349,72 @@ def test_request_rewrite_preserves_fields_dropped_by_codec(relay_turn, monkeypat
     ]
 
 
+def test_request_rewrite_is_ignored_when_codec_baseline_fails(
+    relay_turn, monkeypatch
+):
+    relay, _turn = relay_turn
+    captured_requests = []
+    original = {
+        "model": "test-model",
+        "messages": [],
+        "temperature": 0.0,
+        "extra_body": {"routing": {"provider": "nim"}},
+    }
+
+    async def lossy_execute(_name, request, callback, **_kwargs):
+        rewritten = {
+            key: value
+            for key, value in request.content.items()
+            if key != "extra_body"
+        }
+        rewritten["temperature"] = 0.25
+        return callback(relay.LLMRequest(request.headers, rewritten))
+
+    monkeypatch.setattr(relay.llm, "execute", lossy_execute)
+    monkeypatch.setattr(
+        relay_llm,
+        "_codec_round_trip_request_body",
+        lambda *_args, **_kwargs: None,
+    )
+
+    relay_llm.execute(
+        original,
+        lambda request: captured_requests.append(request) or {"content": "ok"},
+        session_id="session-1",
+        name="test-provider",
+        model_name="test-model",
+        metadata={
+            "api_mode": "chat_completions",
+            "api_request_id": "request-codec-failure",
+        },
+    )
+
+    assert captured_requests == [original]
+
+
+def test_codec_baseline_failure_is_explicit(relay_turn, monkeypatch, caplog):
+    relay, _turn = relay_turn
+    request_body = {"model": "test-model", "messages": []}
+    request = relay.LLMRequest({}, request_body)
+
+    class FailingCodec:
+        def decode(self, _request):
+            raise RuntimeError("simulated codec failure")
+
+    monkeypatch.setattr(relay_llm, "_codec", lambda *_args, **_kwargs: FailingCodec())
+
+    with caplog.at_level("WARNING", logger="agent.relay_llm"):
+        baseline = relay_llm._codec_round_trip_request_body(
+            relay,
+            request,
+            relay_request_body=request_body,
+            metadata={"api_mode": "chat_completions"},
+        )
+
+    assert baseline is None
+    assert "ignoring request rewrites" in caplog.text
+
+
 def test_request_rewrite_can_remove_codec_represented_field(relay_turn, monkeypatch):
     relay, _turn = relay_turn
     captured_requests = []