fix: update response model field to match routing decision

The response JSON model field now correctly reflects the semantic router's
decision instead of using the model name from the vLLM endpoint.

Changes:
- Parse response JSON and update model field to ctx.RequestModel
- Re-marshal modified response for cache and client
- Only modify non-streaming responses
- Fallback to original response on marshal errors

This ensures API consumers can determine which model was selected by
examining the standard model field, rather than requiring custom headers
or log inspection.

Fixes #430

Co-Authored-By: Claude <[email protected]>
Signed-off-by: Yossi Ovadia <[email protected]>

Author: yossiovadia

src/semantic-router/pkg/extproc/response_handler.go

@@ -217,6 +217,25 @@ func (r *OpenAIRouter) handleResponseBody(v *ext_proc.ProcessingRequest_Response
 		observability.Errorf("Error parsing tokens from response: %v", err)
 		metrics.RecordRequestError(ctx.RequestModel, "parse_error")
 	}
+
+	// Update the model field in the response to match the routing decision
+	// This ensures the API response reflects which model was selected by the semantic router
+	modelFieldUpdated := false
+	if ctx.RequestModel != "" && parsed.Model != ctx.RequestModel {
+		observability.Infof("Updating response model field from '%s' to '%s' (routing decision)", parsed.Model, ctx.RequestModel)
+		parsed.Model = ctx.RequestModel
+
+		// Re-marshal the response with the updated model field
+		modifiedBody, err := json.Marshal(parsed)
+		if err != nil {
+			observability.Errorf("Error re-marshaling response with updated model field: %v", err)
+			// Fall back to original response body on error
+		} else {
+			responseBody = modifiedBody
+			modelFieldUpdated = true
+		}
+	}
+
 	promptTokens := int(parsed.Usage.PromptTokens)
 	completionTokens := int(parsed.Usage.CompletionTokens)
 
@@ -281,7 +300,7 @@ func (r *OpenAIRouter) handleResponseBody(v *ext_proc.ProcessingRequest_Response
 		}
 	}
 
-	// Allow the response to continue without modification
+	// Return the response (with modified model field if updated)
 	response := &ext_proc.ProcessingResponse{
 		Response: &ext_proc.ProcessingResponse_ResponseBody{
 			ResponseBody: &ext_proc.BodyResponse{
@@ -292,5 +311,14 @@ func (r *OpenAIRouter) handleResponseBody(v *ext_proc.ProcessingRequest_Response
 		},
 	}
 
+	// If we updated the model field, include the modified body in the response
+	if modelFieldUpdated {
+		response.GetResponseBody().Response.BodyMutation = &ext_proc.BodyMutation{
+			Mutation: &ext_proc.BodyMutation_Body{
+				Body: responseBody,
+			},
+		}
+	}
+
 	return response, nil
 }