diff --git a/litellm/proxy/litellm_pre_call_utils.py b/litellm/proxy/litellm_pre_call_utils.py
index 5b3c0e885aa..22546a92af3 100644
--- a/litellm/proxy/litellm_pre_call_utils.py
+++ b/litellm/proxy/litellm_pre_call_utils.py
@@ -660,13 +660,17 @@ class LiteLLMProxyRequestSetup:
         if chain_id:
             metadata_from_headers["trace_id"] = chain_id
             metadata_from_headers["session_id"] = chain_id
-            # Prefer body trace_id over header so all three fields stay consistent.
-            _body_metadata = data.get(_metadata_variable_name)
-            _body_trace_id = (
-                _body_metadata.get("trace_id")
-                if isinstance(_body_metadata, dict)
-                else None
-            )
+            # Only prefer body trace_id on litellm_metadata routes (/v1/messages, threads, etc.)
+            # On /chat/completions, metadata["trace_id"] belongs to observability tools
+            # (e.g. LangFuse) and should not override the header chain ID.
+            _body_trace_id: Optional[str] = None
+            if _metadata_variable_name == "litellm_metadata":
+                _body_metadata = data.get(_metadata_variable_name)
+                _body_trace_id = (
+                    _body_metadata.get("trace_id")
+                    if isinstance(_body_metadata, dict)
+                    else None
+                )
             effective_id = _body_trace_id or chain_id
             data["litellm_session_id"] = effective_id
             data["litellm_trace_id"] = effective_id
@@ -675,7 +679,8 @@ class LiteLLMProxyRequestSetup:
             )
 
         if isinstance(data[_metadata_variable_name], dict):
-            # Body values take priority — only inject from headers if the key isn't already set.
+            # Only inject header-derived values for keys NOT already set by the user
+            # in the request body. This ensures body values take priority over headers.
             for key, value in metadata_from_headers.items():
                 if key not in data[_metadata_variable_name]:
                     data[_metadata_variable_name][key] = value
diff --git a/tests/test_litellm/proxy/test_litellm_pre_call_utils.py b/tests/test_litellm/proxy/test_litellm_pre_call_utils.py
index eda5611283f..55f0ae7c9cf 100644
--- a/tests/test_litellm/proxy/test_litellm_pre_call_utils.py
+++ b/tests/test_litellm/proxy/test_litellm_pre_call_utils.py
@@ -1975,6 +1975,28 @@ class TestAddLitellmMetadataFromRequestHeaders:
         assert result["litellm_session_id"] == "from-body"
         assert result["litellm_trace_id"] == "from-body"
 
+    def test_should_not_bleed_metadata_trace_id_into_chain_ids_on_chat_completions_route(self):
+        """On /chat/completions, metadata["trace_id"] (e.g. LangFuse) must not override the header chain ID."""
+        data = {
+            "model": "gpt-4o",
+            "messages": [{"role": "user", "content": "hi"}],
+            "metadata": {"trace_id": "langfuse-id"},
+        }
+        headers = {"x-litellm-trace-id": "chain-id", "content-type": "application/json"}
+
+        result = LiteLLMProxyRequestSetup.add_litellm_metadata_from_request_headers(
+            headers=headers,
+            data=data,
+            _metadata_variable_name="metadata",
+        )
+
+        # Header chain ID must win for top-level fields on this route.
+        assert result["litellm_session_id"] == "chain-id"
+        assert result["litellm_trace_id"] == "chain-id"
+        # The observability trace_id in the metadata dict must be untouched.
+        assert result["metadata"]["trace_id"] == "langfuse-id"
+
+
     @pytest.mark.asyncio
     async def test_should_preserve_body_trace_id_in_full_pipeline_on_messages_route(self):
         """Body trace_id must survive the full add_litellm_data_to_request pipeline."""