internal/config/config.go

@@ -36,6 +36,10 @@ type ModelUpstream struct {
 	Address string
 	// Path is the upstream path for this model (e.g., "/v1/chat/completions").
 	Path string
+	// UpstreamModel is the model name to send to the upstream server.
+	// If empty, the client-provided model name (Name) is used as-is.
+	// Use this when the upstream expects a different model name than clients send.
+	UpstreamModel string
 	// AuthRequired indicates whether this model requires JWT authentication.
 	AuthRequired bool
 }

internal/config/loader.go

@@ -37,10 +37,11 @@ type rawRoute struct {
 
 // rawModel represents a single model entry in the YAML configuration.
 type rawModel struct {
-	Name         string `yaml:"name"`
-	Address      string `yaml:"address"`
-	Path         string `yaml:"path"`
-	AuthRequired *bool  `yaml:"authRequired"`
+	Name          string `yaml:"name"`
+	Address       string `yaml:"address"`
+	Path          string `yaml:"path"`
+	UpstreamModel string `yaml:"upstreamModel"`
+	AuthRequired  *bool  `yaml:"authRequired"`
 }
 
 // rawAdapter represents a service adapter in the YAML configuration.
@@ -132,10 +133,11 @@ func LoadRoutesAndModelsFromFile(path string) (map[string]*Route, map[string]*Mo
 			authRequired = *rawModel.AuthRequired
 		}
 		models[rawModel.Name] = &ModelUpstream{
-			Name:         rawModel.Name,
-			Address:      rawModel.Address,
-			Path:         rawModel.Path,
-			AuthRequired: authRequired,
+			Name:          rawModel.Name,
+			Address:       rawModel.Address,
+			Path:          rawModel.Path,
+			UpstreamModel: rawModel.UpstreamModel,
+			AuthRequired:  authRequired,
 		}
 	}
 

internal/proxy/bodydispatch_test.go

@@ -381,3 +381,66 @@ func TestBodySizeCappedDispatch(t *testing.T) {
 		t.Errorf("expected 200 for reasonable body, got %d", resp.StatusCode)
 	}
 }
+
+// TestUpstreamModelRewrite verifies that the model field is rewritten when upstreamModel is set.
+func TestUpstreamModelRewrite(t *testing.T) {
+	var receivedModel string
+
+	upstreamServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		body, _ := io.ReadAll(r.Body)
+		var payload map[string]interface{}
+		json.Unmarshal(body, &payload)
+		receivedModel = payload["model"].(string)
+		w.Header().Set("Content-Type", "application/json")
+		w.WriteHeader(http.StatusOK)
+		fmt.Fprint(w, `{}`)
+	}))
+	defer upstreamServer.Close()
+
+	upstreamAddr := strings.TrimPrefix(upstreamServer.URL, "http://")
+
+	cfg := &config.Config{
+		Models: map[string]*config.ModelUpstream{
+			// Client sends "ornith:35b", upstream expects "qwen2.5:72b-instruct"
+			"ornith:35b": {
+				Name:          "ornith:35b",
+				Address:       upstreamAddr,
+				UpstreamModel: "qwen2.5:72b-instruct",
+			},
+			// No rewrite - upstream model same as client model
+			"reasoning": {
+				Name:    "reasoning",
+				Address: upstreamAddr,
+			},
+		},
+		Routes: make(map[string]*config.Route),
+	}
+
+	handler := New(cfg)
+	defer handler.Close()
+
+	server := httptest.NewServer(handler)
+	defer server.Close()
+
+	// Test 1: Model should be rewritten
+	requestBody := `{"model":"ornith:35b","messages":[{"role":"user","content":"hi"}]}`
+	resp, err := http.Post(server.URL+"/v1/chat/completions", "application/json", strings.NewReader(requestBody))
+	if err != nil {
+		t.Fatalf("request failed: %v", err)
+	}
+	resp.Body.Close()
+
+	if receivedModel != "qwen2.5:72b-instruct" {
+		t.Errorf("expected upstream to receive model 'qwen2.5:72b-instruct', got '%s'", receivedModel)
+	}
+
+	// Test 2: No rewrite when upstreamModel is empty
+	receivedModel = ""
+	requestBody = `{"model":"reasoning","messages":[{"role":"user","content":"hi"}]}`
+	resp, _ = http.Post(server.URL+"/v1/chat/completions", "application/json", strings.NewReader(requestBody))
+	resp.Body.Close()
+
+	if receivedModel != "reasoning" {
+		t.Errorf("expected upstream to receive model 'reasoning', got '%s'", receivedModel)
+	}
+}

internal/proxy/router.go

@@ -122,6 +122,20 @@ func (h *Handler) routeByModel(r *http.Request, path string) (*Route, error) {
 		}
 	}
 
+	// If upstream expects a different model name, rewrite the body
+	if modelUpstream.UpstreamModel != "" && modelUpstream.UpstreamModel != modelName {
+		payload["model"] = modelUpstream.UpstreamModel
+		newBody, err := json.Marshal(payload)
+		if err != nil {
+			return nil, &modelValidationError{
+				Kind:    "invalid_request",
+				Message: fmt.Sprintf("failed to rewrite model name: %v", err),
+			}
+		}
+		r.Body = io.NopCloser(bytes.NewReader(newBody))
+		r.ContentLength = int64(len(newBody))
+	}
+
 	// Determine the upstream path based on the request path
 	upstreamPath := path
 	if path == "/v1/rerank" {