diff --git a/backend/internal/service/openai_gateway_chat_completions.go b/backend/internal/service/openai_gateway_chat_completions.go
index ac7d28a7..663066a3 100644
--- a/backend/internal/service/openai_gateway_chat_completions.go
+++ b/backend/internal/service/openai_gateway_chat_completions.go
@@ -107,11 +107,15 @@ func (s *OpenAIGatewayService) ForwardAsChatCompletions(
 				responsesBody = stripped
 			}
 		}
+		responsesBody, normalizedServiceTier, err := normalizeResponsesBodyServiceTier(responsesBody)
+		if err != nil {
+			return nil, fmt.Errorf("normalize service_tier in responses-shape body: %w", err)
+		}
 		// Minimal stub populated from the raw body so downstream billing
 		// propagation (ServiceTier, ReasoningEffort) keeps working.
 		responsesReq = &apicompat.ResponsesRequest{
 			Model:       upstreamModel,
-			ServiceTier: gjson.GetBytes(responsesBody, "service_tier").String(),
+			ServiceTier: normalizedServiceTier,
 		}
 		if effort := gjson.GetBytes(responsesBody, "reasoning.effort").String(); effort != "" {
 			responsesReq.Reasoning = &apicompat.ResponsesReasoning{Effort: effort}
@@ -124,6 +128,7 @@ func (s *OpenAIGatewayService) ForwardAsChatCompletions(
 			return nil, fmt.Errorf("convert chat completions to responses: %w", err)
 		}
 		responsesReq.Model = upstreamModel
+		normalizeResponsesRequestServiceTier(responsesReq)
 		responsesBody, err = json.Marshal(responsesReq)
 		if err != nil {
 			return nil, fmt.Errorf("marshal responses request: %w", err)
@@ -274,6 +279,41 @@ func (s *OpenAIGatewayService) ForwardAsChatCompletions(
 	return result, handleErr
 }
 
+func normalizeResponsesRequestServiceTier(req *apicompat.ResponsesRequest) {
+	if req == nil {
+		return
+	}
+	req.ServiceTier = normalizedOpenAIServiceTierValue(req.ServiceTier)
+}
+
+func normalizeResponsesBodyServiceTier(body []byte) ([]byte, string, error) {
+	if len(body) == 0 {
+		return body, "", nil
+	}
+	rawServiceTier := gjson.GetBytes(body, "service_tier").String()
+	if rawServiceTier == "" {
+		return body, "", nil
+	}
+	normalizedServiceTier := normalizedOpenAIServiceTierValue(rawServiceTier)
+	if normalizedServiceTier == "" {
+		trimmed, err := sjson.DeleteBytes(body, "service_tier")
+		return trimmed, "", err
+	}
+	if normalizedServiceTier == rawServiceTier {
+		return body, normalizedServiceTier, nil
+	}
+	trimmed, err := sjson.SetBytes(body, "service_tier", normalizedServiceTier)
+	return trimmed, normalizedServiceTier, err
+}
+
+func normalizedOpenAIServiceTierValue(raw string) string {
+	normalized := normalizeOpenAIServiceTier(raw)
+	if normalized == nil {
+		return ""
+	}
+	return *normalized
+}
+
 // handleChatCompletionsErrorResponse reads an upstream error and returns it in
 // OpenAI Chat Completions error format.
 func (s *OpenAIGatewayService) handleChatCompletionsErrorResponse(
diff --git a/backend/internal/service/openai_gateway_chat_completions_test.go b/backend/internal/service/openai_gateway_chat_completions_test.go
new file mode 100644
index 00000000..a00fb71c
--- /dev/null
+++ b/backend/internal/service/openai_gateway_chat_completions_test.go
@@ -0,0 +1,44 @@
+package service
+
+import (
+	"testing"
+
+	"github.com/Wei-Shaw/sub2api/internal/pkg/apicompat"
+	"github.com/stretchr/testify/require"
+	"github.com/tidwall/gjson"
+)
+
+func TestNormalizeResponsesRequestServiceTier(t *testing.T) {
+	t.Parallel()
+
+	req := &apicompat.ResponsesRequest{ServiceTier: " fast "}
+	normalizeResponsesRequestServiceTier(req)
+	require.Equal(t, "priority", req.ServiceTier)
+
+	req.ServiceTier = "flex"
+	normalizeResponsesRequestServiceTier(req)
+	require.Equal(t, "flex", req.ServiceTier)
+
+	req.ServiceTier = "default"
+	normalizeResponsesRequestServiceTier(req)
+	require.Empty(t, req.ServiceTier)
+}
+
+func TestNormalizeResponsesBodyServiceTier(t *testing.T) {
+	t.Parallel()
+
+	body, tier, err := normalizeResponsesBodyServiceTier([]byte(`{"model":"gpt-5.1","service_tier":"fast"}`))
+	require.NoError(t, err)
+	require.Equal(t, "priority", tier)
+	require.Equal(t, "priority", gjson.GetBytes(body, "service_tier").String())
+
+	body, tier, err = normalizeResponsesBodyServiceTier([]byte(`{"model":"gpt-5.1","service_tier":"flex"}`))
+	require.NoError(t, err)
+	require.Equal(t, "flex", tier)
+	require.Equal(t, "flex", gjson.GetBytes(body, "service_tier").String())
+
+	body, tier, err = normalizeResponsesBodyServiceTier([]byte(`{"model":"gpt-5.1","service_tier":"default"}`))
+	require.NoError(t, err)
+	require.Empty(t, tier)
+	require.False(t, gjson.GetBytes(body, "service_tier").Exists())
+}