diff --git a/internal/adapter/gemini/handler_generate.go b/internal/adapter/gemini/handler_generate.go
index 5703d0b..a7de92d 100644
--- a/internal/adapter/gemini/handler_generate.go
+++ b/internal/adapter/gemini/handler_generate.go
@@ -90,6 +90,11 @@ func (h *Handler) proxyViaOpenAI(w http.ResponseWriter, r *http.Request, stream
 	defer res.Body.Close()
 	body, _ := io.ReadAll(res.Body)
 	if res.StatusCode < 200 || res.StatusCode >= 300 {
+		for k, vv := range res.Header {
+			for _, v := range vv {
+				w.Header().Add(k, v)
+			}
+		}
 		writeGeminiErrorFromOpenAI(w, res.StatusCode, body)
 		return true
 	}
diff --git a/internal/adapter/gemini/handler_test.go b/internal/adapter/gemini/handler_test.go
index 20cc0e6..fdb4b79 100644
--- a/internal/adapter/gemini/handler_test.go
+++ b/internal/adapter/gemini/handler_test.go
@@ -64,9 +64,13 @@ func (m testGeminiDS) CallCompletion(_ context.Context, _ *auth.RequestAuth, _ m
 type geminiOpenAIErrorStub struct {
 	status int
 	body   string
+	headers map[string]string
 }
 
 func (s geminiOpenAIErrorStub) ChatCompletions(w http.ResponseWriter, _ *http.Request) {
+	for k, v := range s.headers {
+		w.Header().Set(k, v)
+	}
 	w.Header().Set("Content-Type", "application/json")
 	w.WriteHeader(s.status)
 	_, _ = w.Write([]byte(s.body))
@@ -244,7 +248,15 @@ func TestStreamGenerateContentEmitsSSE(t *testing.T) {
 func TestGenerateContentOpenAIProxyErrorUsesGeminiEnvelope(t *testing.T) {
 	h := &Handler{
 		Store:  testGeminiConfig{},
-		OpenAI: geminiOpenAIErrorStub{status: http.StatusUnauthorized, body: `{"error":{"message":"invalid api key"}}`},
+		OpenAI: geminiOpenAIErrorStub{
+			status: http.StatusUnauthorized,
+			body:   `{"error":{"message":"invalid api key"}}`,
+			headers: map[string]string{
+				"WWW-Authenticate":      `Bearer realm="example"`,
+				"Retry-After":           "30",
+				"X-RateLimit-Remaining": "0",
+			},
+		},
 	}
 	r := chi.NewRouter()
 	RegisterRoutes(r, h)
@@ -267,6 +279,15 @@ func TestGenerateContentOpenAIProxyErrorUsesGeminiEnvelope(t *testing.T) {
 	if errObj["message"] != "invalid api key" {
 		t.Fatalf("expected parsed error message, got=%v", errObj["message"])
 	}
+	if got := rec.Header().Get("WWW-Authenticate"); got == "" {
+		t.Fatalf("expected WWW-Authenticate header to be preserved")
+	}
+	if got := rec.Header().Get("Retry-After"); got != "30" {
+		t.Fatalf("expected Retry-After header 30, got=%q", got)
+	}
+	if got := rec.Header().Get("X-RateLimit-Remaining"); got != "0" {
+		t.Fatalf("expected X-RateLimit-Remaining header 0, got=%q", got)
+	}
 }
 
 func extractGeminiSSEFrames(t *testing.T, body string) []map[string]any {