Add native Responses API upstream proxy

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

internal/gateway/gateway_test.go

@@ -19,15 +19,35 @@ import (
 type fakeUpstream struct {
 	lastModel   string
 	lastAuth    string
+	lastPath    string
 	lastRequest map[string]any
 }
 
 func (f *fakeUpstream) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	f.lastPath = r.URL.Path
 	if r.Method == http.MethodGet && r.URL.Path == "/models" {
 		w.Header().Set("Content-Type", "application/json")
 		_, _ = io.WriteString(w, `{"object":"list","data":[{"id":"ds-chat"},{"id":"ds-haiku"}]}`)
 		return
 	}
+	if r.Method == http.MethodPost && r.URL.Path == "/v1/responses" {
+		f.lastAuth = r.Header.Get("Authorization")
+		body, _ := io.ReadAll(r.Body)
+		_ = json.Unmarshal(body, &f.lastRequest)
+		f.lastModel, _ = f.lastRequest["model"].(string)
+		stream, _ := f.lastRequest["stream"].(bool)
+		if stream {
+			w.Header().Set("Content-Type", "text/event-stream")
+			_, _ = io.WriteString(w, "event: response.completed\ndata: {\"type\":\"response.completed\"}\n\n")
+			return
+		}
+		w.Header().Set("Content-Type", "application/json")
+		_ = json.NewEncoder(w).Encode(map[string]any{
+			"id": "resp_native", "object": "response", "status": "completed", "model": f.lastModel,
+			"output": []any{}, "usage": map[string]any{"input_tokens": 9, "output_tokens": 2, "total_tokens": 11},
+		})
+		return
+	}
 	if r.URL.Path != "/chat/completions" {
 		w.WriteHeader(http.StatusNotFound)
 		return
@@ -290,6 +310,74 @@ func TestResponsesStream(t *testing.T) {
 	}
 }
 
+func TestNativeResponsesProtocol(t *testing.T) {
+	for _, stream := range []bool{false, true} {
+		name := "non-streaming"
+		if stream {
+			name = "streaming"
+		}
+		t.Run(name, func(t *testing.T) {
+			env := newCodexTestEnv(t)
+			provider, err := env.store.AddProvider("native-responses", "", env.upstreamURL+"/v1", "up-key",
+				config.Models{Default: "native-model"}, config.TargetCodex, config.ProtocolOpenAIResponses)
+			if err != nil {
+				t.Fatalf("AddProvider: %v", err)
+			}
+			if _, err := env.store.ActivateCodex(provider.ID); err != nil {
+				t.Fatalf("ActivateCodex: %v", err)
+			}
+
+			body := map[string]any{
+				"model":     "gpt-5-codex",
+				"input":     "What model are you?",
+				"stream":    stream,
+				"reasoning": map[string]any{"effort": "high"},
+				"tools": []any{
+					map[string]any{"type": "web_search_preview"},
+					map[string]any{"type": "function", "name": "read_file", "parameters": map[string]any{"type": "object"}},
+				},
+			}
+			resp := env.post(t, "/v1/responses", body, env.store.Config().RelayToken)
+			defer resp.Body.Close()
+			if resp.StatusCode != http.StatusOK {
+				data, _ := io.ReadAll(resp.Body)
+				t.Fatalf("status = %d, body = %s", resp.StatusCode, data)
+			}
+			data, err := io.ReadAll(resp.Body)
+			if err != nil {
+				t.Fatalf("read response: %v", err)
+			}
+			if stream {
+				if !strings.Contains(string(data), "event: response.completed") {
+					t.Fatalf("stream response = %q", data)
+				}
+			} else if !strings.Contains(string(data), `"id":"resp_native"`) {
+				t.Fatalf("native response = %s", data)
+			}
+
+			if env.upstream.lastPath != "/v1/responses" {
+				t.Errorf("upstream path = %q, want /v1/responses", env.upstream.lastPath)
+			}
+			if env.upstream.lastAuth != "Bearer up-key" {
+				t.Errorf("upstream auth = %q", env.upstream.lastAuth)
+			}
+			if env.upstream.lastModel != "native-model" {
+				t.Errorf("upstream model = %q, want model mapping", env.upstream.lastModel)
+			}
+			if env.upstream.lastRequest["input"] != "What model are you?" || env.upstream.lastRequest["messages"] != nil {
+				t.Errorf("request was converted instead of passed through: %v", env.upstream.lastRequest)
+			}
+			if env.upstream.lastRequest["reasoning"] == nil {
+				t.Errorf("native reasoning field was dropped: %v", env.upstream.lastRequest)
+			}
+			tools := env.upstream.lastRequest["tools"].([]any)
+			if len(tools) != 2 || tools[0].(map[string]any)["type"] != "web_search_preview" {
+				t.Errorf("native tools were changed: %v", tools)
+			}
+		})
+	}
+}
+
 func TestResponsesRejectsBadToken(t *testing.T) {
 	env := newCodexTestEnv(t)
 	resp := env.post(t, "/v1/responses", responsesBody("gpt-5-codex", false), "crl_wrong")