diff --git a/backend/internal/handler/openai_chat_completions.go b/backend/internal/handler/openai_chat_completions.go
index 7fd24f97..23844508 100644
--- a/backend/internal/handler/openai_chat_completions.go
+++ b/backend/internal/handler/openai_chat_completions.go
@@ -121,7 +121,6 @@ func (h *OpenAIGatewayHandler) ChatCompletions(c *gin.Context) {
 	var lastFailoverErr *service.UpstreamFailoverError
 
 	for {
-		c.Set("openai_chat_completions_fallback_model", "")
 		reqLog.Debug("openai_chat_completions.account_selecting", zap.Int("excluded_account_count", len(failedAccountIDs)))
 		selection, scheduleDecision, err := h.gatewayService.SelectAccountWithScheduler(
 			c.Request.Context(),
@@ -139,32 +138,8 @@ func (h *OpenAIGatewayHandler) ChatCompletions(c *gin.Context) {
 				zap.Int("excluded_account_count", len(failedAccountIDs)),
 			)
 			if len(failedAccountIDs) == 0 {
-				defaultModel := ""
-				if apiKey.Group != nil {
-					defaultModel = apiKey.Group.DefaultMappedModel
-				}
-				if defaultModel != "" && defaultModel != reqModel {
-					reqLog.Info("openai_chat_completions.fallback_to_default_model",
-						zap.String("default_mapped_model", defaultModel),
-					)
-					selection, scheduleDecision, err = h.gatewayService.SelectAccountWithScheduler(
-						c.Request.Context(),
-						apiKey.GroupID,
-						"",
-						sessionHash,
-						defaultModel,
-						failedAccountIDs,
-						service.OpenAIUpstreamTransportAny,
-						false,
-					)
-					if err == nil && selection != nil {
-						c.Set("openai_chat_completions_fallback_model", defaultModel)
-					}
-				}
-				if err != nil {
-					h.handleStreamingAwareError(c, http.StatusServiceUnavailable, "api_error", "Service temporarily unavailable", streamStarted)
-					return
-				}
+				h.handleStreamingAwareError(c, http.StatusServiceUnavailable, "api_error", "Service temporarily unavailable", streamStarted)
+				return
 			} else {
 				if lastFailoverErr != nil {
 					h.handleFailoverExhausted(c, lastFailoverErr, streamStarted)
@@ -192,12 +167,11 @@ func (h *OpenAIGatewayHandler) ChatCompletions(c *gin.Context) {
 		service.SetOpsLatencyMs(c, service.OpsRoutingLatencyMsKey, time.Since(routingStart).Milliseconds())
 		forwardStart := time.Now()
 
-		defaultMappedModel := resolveOpenAIForwardDefaultMappedModel(apiKey, c.GetString("openai_chat_completions_fallback_model"))
 		forwardBody := body
 		if channelMapping.Mapped {
 			forwardBody = h.gatewayService.ReplaceModelInBody(body, channelMapping.MappedModel)
 		}
-		result, err := h.gatewayService.ForwardAsChatCompletions(c.Request.Context(), c, account, forwardBody, promptCacheKey, defaultMappedModel)
+		result, err := h.gatewayService.ForwardAsChatCompletions(c.Request.Context(), c, account, forwardBody, promptCacheKey, "")
 
 		forwardDurationMs := time.Since(forwardStart).Milliseconds()
 		if accountReleaseFunc != nil {
diff --git a/backend/internal/handler/openai_gateway_handler.go b/backend/internal/handler/openai_gateway_handler.go
index c0de4476..b5eec393 100644
--- a/backend/internal/handler/openai_gateway_handler.go
+++ b/backend/internal/handler/openai_gateway_handler.go
@@ -37,16 +37,6 @@ type OpenAIGatewayHandler struct {
 	cfg                     *config.Config
 }
 
-func resolveOpenAIForwardDefaultMappedModel(apiKey *service.APIKey, fallbackModel string) string {
-	if fallbackModel = strings.TrimSpace(fallbackModel); fallbackModel != "" {
-		return fallbackModel
-	}
-	if apiKey == nil || apiKey.Group == nil {
-		return ""
-	}
-	return strings.TrimSpace(apiKey.Group.DefaultMappedModel)
-}
-
 func resolveOpenAIMessagesDispatchMappedModel(apiKey *service.APIKey, requestedModel string) string {
 	if apiKey == nil || apiKey.Group == nil {
 		return ""
diff --git a/backend/internal/handler/openai_gateway_handler_test.go b/backend/internal/handler/openai_gateway_handler_test.go
index 0e21dc08..2744e0cc 100644
--- a/backend/internal/handler/openai_gateway_handler_test.go
+++ b/backend/internal/handler/openai_gateway_handler_test.go
@@ -353,30 +353,6 @@ func TestOpenAIEnsureResponsesDependencies(t *testing.T) {
 	})
 }
 
-func TestResolveOpenAIForwardDefaultMappedModel(t *testing.T) {
-	t.Run("prefers_explicit_fallback_model", func(t *testing.T) {
-		apiKey := &service.APIKey{
-			Group: &service.Group{DefaultMappedModel: "gpt-5.4"},
-		}
-		require.Equal(t, "gpt-5.2", resolveOpenAIForwardDefaultMappedModel(apiKey, " gpt-5.2 "))
-	})
-
-	t.Run("uses_group_default_when_explicit_fallback_absent", func(t *testing.T) {
-		apiKey := &service.APIKey{
-			Group: &service.Group{DefaultMappedModel: "gpt-5.4"},
-		}
-		require.Equal(t, "gpt-5.4", resolveOpenAIForwardDefaultMappedModel(apiKey, ""))
-	})
-
-	t.Run("returns_empty_without_group_default", func(t *testing.T) {
-		require.Empty(t, resolveOpenAIForwardDefaultMappedModel(nil, ""))
-		require.Empty(t, resolveOpenAIForwardDefaultMappedModel(&service.APIKey{}, ""))
-		require.Empty(t, resolveOpenAIForwardDefaultMappedModel(&service.APIKey{
-			Group: &service.Group{},
-		}, ""))
-	})
-}
-
 func TestResolveOpenAIMessagesDispatchMappedModel(t *testing.T) {
 	t.Run("exact_claude_model_override_wins", func(t *testing.T) {
 		apiKey := &service.APIKey{
diff --git a/backend/internal/service/billing_service.go b/backend/internal/service/billing_service.go
index 392b3e0b..cb502a2e 100644
--- a/backend/internal/service/billing_service.go
+++ b/backend/internal/service/billing_service.go
@@ -226,6 +226,12 @@ func (s *BillingService) initFallbackPricing() {
 		CacheReadPricePerToken: 7.5e-8,
 		SupportsCacheBreakdown: false,
 	}
+	s.fallbackPrices["gpt-5.4-nano"] = &ModelPricing{
+		InputPricePerToken:     2e-7,
+		OutputPricePerToken:    1.25e-6,
+		CacheReadPricePerToken: 2e-8,
+		SupportsCacheBreakdown: false,
+	}
 	// OpenAI GPT-5.2（本地兜底）
 	s.fallbackPrices["gpt-5.2"] = &ModelPricing{
 		InputPricePerToken:             1.75e-6,
@@ -295,6 +301,8 @@ func (s *BillingService) getFallbackPricing(model string) *ModelPricing {
 			return s.fallbackPrices["gpt-5.5"]
 		case "gpt-5.4-mini":
 			return s.fallbackPrices["gpt-5.4-mini"]
+		case "gpt-5.4-nano":
+			return s.fallbackPrices["gpt-5.4-nano"]
 		case "gpt-5.4":
 			return s.fallbackPrices["gpt-5.4"]
 		case "gpt-5.2":
diff --git a/backend/internal/service/openai_codex_transform.go b/backend/internal/service/openai_codex_transform.go
index b256f1c7..de98b50d 100644
--- a/backend/internal/service/openai_codex_transform.go
+++ b/backend/internal/service/openai_codex_transform.go
@@ -38,6 +38,29 @@ var codexModelMap = map[string]string{
 	"gpt-5.2-medium":             "gpt-5.2",
 	"gpt-5.2-high":               "gpt-5.2",
 	"gpt-5.2-xhigh":              "gpt-5.2",
+	"gpt-5":                      "gpt-5.4",
+	"gpt-5-mini":                 "gpt-5.4",
+	"gpt-5-nano":                 "gpt-5.4",
+	"gpt-5.1":                    "gpt-5.4",
+	"gpt-5.1-codex":              "gpt-5.3-codex",
+	"gpt-5.1-codex-max":          "gpt-5.3-codex",
+	"gpt-5.1-codex-mini":         "gpt-5.3-codex",
+	"gpt-5.2-codex":              "gpt-5.2",
+	"codex-mini-latest":          "gpt-5.3-codex",
+	"gpt-5-codex":                "gpt-5.3-codex",
+}
+
+var codexVersionModelPrefixes = []struct {
+	prefix string
+	target string
+}{
+	{prefix: "gpt-5.3-codex-spark", target: "gpt-5.3-codex-spark"},
+	{prefix: "gpt-5.3-codex", target: "gpt-5.3-codex"},
+	{prefix: "gpt-5.4-mini", target: "gpt-5.4-mini"},
+	{prefix: "gpt-5.4-nano", target: "gpt-5.4-nano"},
+	{prefix: "gpt-5.5", target: "gpt-5.5"},
+	{prefix: "gpt-5.4", target: "gpt-5.4"},
+	{prefix: "gpt-5.2", target: "gpt-5.2"},
 }
 
 type codexTransformResult struct {
@@ -447,8 +470,19 @@ func normalizeCodexModel(model string) string {
 	if model == "" {
 		return "gpt-5.4"
 	}
+	if mapped, ok := normalizeKnownCodexModel(model); ok {
+		return mapped
+	}
+	return model
+}
+
+func normalizeKnownCodexModel(model string) (string, bool) {
+	model = strings.TrimSpace(model)
+	if model == "" {
+		return "", false
+	}
 	if isOpenAIImageGenerationModel(model) {
-		return model
+		return model, true
 	}
 
 	modelID := model
@@ -457,41 +491,58 @@ func normalizeCodexModel(model string) string {
 		modelID = parts[len(parts)-1]
 	}
 
-	if mapped := getNormalizedCodexModel(modelID); mapped != "" {
-		return mapped
+	key := codexModelLookupKey(modelID)
+	if key == "" {
+		return "", false
 	}
+	if mapped := getNormalizedCodexModel(key); mapped != "" {
+		return mapped, true
+	}
+	for _, item := range codexVersionModelPrefixes {
+		if key == item.prefix {
+			return item.target, true
+		}
+		suffix, ok := strings.CutPrefix(key, item.prefix+"-")
+		if ok && isKnownCodexModelSuffix(suffix) {
+			return item.target, true
+		}
+	}
+	return "", false
+}
 
-	normalized := strings.ToLower(modelID)
+func codexModelLookupKey(modelID string) string {
+	modelID = strings.TrimSpace(modelID)
+	if modelID == "" {
+		return ""
+	}
+	if strings.Contains(modelID, "/") {
+		parts := strings.Split(modelID, "/")
+		modelID = parts[len(parts)-1]
+	}
+	return strings.ToLower(strings.Join(strings.Fields(modelID), "-"))
+}
 
-	if strings.Contains(normalized, "gpt-5.5") || strings.Contains(normalized, "gpt 5.5") {
-		return "gpt-5.5"
-	}
-	if strings.Contains(normalized, "gpt-5.4-mini") || strings.Contains(normalized, "gpt 5.4 mini") {
-		return "gpt-5.4-mini"
-	}
-	if strings.Contains(normalized, "gpt-5.4") || strings.Contains(normalized, "gpt 5.4") {
-		return "gpt-5.4"
-	}
-	if strings.Contains(normalized, "gpt-5.2") || strings.Contains(normalized, "gpt 5.2") {
-		return "gpt-5.2"
-	}
-	if strings.Contains(normalized, "gpt-5.3-codex-spark") || strings.Contains(normalized, "gpt 5.3 codex spark") {
-		return "gpt-5.3-codex-spark"
-	}
-	if strings.Contains(normalized, "gpt-5.3-codex") || strings.Contains(normalized, "gpt 5.3 codex") {
-		return "gpt-5.3-codex"
-	}
-	if strings.Contains(normalized, "gpt-5.3") || strings.Contains(normalized, "gpt 5.3") {
-		return "gpt-5.3-codex"
-	}
-	if strings.Contains(normalized, "codex") {
-		return "gpt-5.3-codex"
-	}
-	if strings.Contains(normalized, "gpt-5") || strings.Contains(normalized, "gpt 5") {
-		return "gpt-5.4"
+func isKnownCodexModelSuffix(suffix string) bool {
+	switch suffix {
+	case "none", "minimal", "low", "medium", "high", "xhigh":
+		return true
 	}
+	return isCodexDateSuffix(suffix)
+}
 
-	return "gpt-5.4"
+func isCodexDateSuffix(suffix string) bool {
+	parts := strings.Split(suffix, "-")
+	if len(parts) != 3 || len(parts[0]) != 4 || len(parts[1]) != 2 || len(parts[2]) != 2 {
+		return false
+	}
+	for _, part := range parts {
+		for _, r := range part {
+			if r < '0' || r > '9' {
+				return false
+			}
+		}
+	}
+	return true
 }
 
 func isCodexSparkModel(model string) bool {
@@ -789,18 +840,13 @@ func SupportsVerbosity(model string) bool {
 }
 
 func getNormalizedCodexModel(modelID string) string {
-	if modelID == "" {
+	key := codexModelLookupKey(modelID)
+	if key == "" {
 		return ""
 	}
-	if mapped, ok := codexModelMap[modelID]; ok {
+	if mapped, ok := codexModelMap[key]; ok {
 		return mapped
 	}
-	lower := strings.ToLower(modelID)
-	for key, value := range codexModelMap {
-		if strings.ToLower(key) == lower {
-			return value
-		}
-	}
 	return ""
 }
 
diff --git a/backend/internal/service/openai_gateway_chat_completions_test.go b/backend/internal/service/openai_gateway_chat_completions_test.go
index c129a4df..b0d1fa31 100644
--- a/backend/internal/service/openai_gateway_chat_completions_test.go
+++ b/backend/internal/service/openai_gateway_chat_completions_test.go
@@ -97,6 +97,42 @@ func TestNormalizeResponsesBodyServiceTier(t *testing.T) {
 	require.False(t, gjson.GetBytes(body, "service_tier").Exists())
 }
 
+func TestForwardAsChatCompletions_UnknownModelDoesNotUseDefaultMappedModel(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+
+	rec := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(rec)
+	body := []byte(`{"model":"gpt6","messages":[{"role":"user","content":"hello"}],"stream":false}`)
+	c.Request = httptest.NewRequest(http.MethodPost, "/v1/chat/completions", bytes.NewReader(body))
+	c.Request.Header.Set("Content-Type", "application/json")
+
+	upstream := &httpUpstreamRecorder{resp: &http.Response{
+		StatusCode: http.StatusBadRequest,
+		Header:     http.Header{"Content-Type": []string{"application/json"}, "x-request-id": []string{"rid_chat_unknown_model"}},
+		Body:       io.NopCloser(strings.NewReader(`{"error":{"type":"invalid_request_error","message":"model not found"}}`)),
+	}}
+
+	svc := &OpenAIGatewayService{httpUpstream: upstream}
+	account := &Account{
+		ID:          1,
+		Name:        "openai-oauth",
+		Platform:    PlatformOpenAI,
+		Type:        AccountTypeOAuth,
+		Concurrency: 1,
+		Credentials: map[string]any{
+			"access_token":       "oauth-token",
+			"chatgpt_account_id": "chatgpt-acc",
+		},
+	}
+
+	result, err := svc.ForwardAsChatCompletions(context.Background(), c, account, body, "", "gpt-5.4")
+	require.Error(t, err)
+	require.Nil(t, result)
+	require.Equal(t, "gpt6", gjson.GetBytes(upstream.lastBody, "model").String())
+	require.NotEqual(t, "gpt-5.4", gjson.GetBytes(upstream.lastBody, "model").String())
+	require.Equal(t, http.StatusBadRequest, rec.Code)
+}
+
 func TestForwardAsChatCompletions_ClientDisconnectDrainsUpstreamUsage(t *testing.T) {
 	gin.SetMode(gin.TestMode)
 
diff --git a/backend/internal/service/openai_gateway_record_usage_test.go b/backend/internal/service/openai_gateway_record_usage_test.go
index 76fbb794..4722c82d 100644
--- a/backend/internal/service/openai_gateway_record_usage_test.go
+++ b/backend/internal/service/openai_gateway_record_usage_test.go
@@ -1006,9 +1006,8 @@ func TestOpenAIGatewayServiceRecordUsage_ChannelMappedDoesNotOverrideBillingMode
 	svc := newOpenAIRecordUsageServiceForTest(usageRepo, userRepo, subRepo, nil)
 	usage := OpenAIUsage{InputTokens: 20, OutputTokens: 10}
 
-	// When channel did NOT map the model (ChannelMappedModel == OriginalModel),
-	// billing should use result.BillingModel (the actual model used after group
-	// DefaultMappedModel resolution), not the unmapped original model.
+	// 渠道未发生模型映射时，应使用 result.BillingModel 中记录的实际上游计费模型，
+	// 而不是未映射的原始请求模型。
 	expectedCost, err := svc.billingService.CalculateCost("gpt-5.1", UsageTokens{
 		InputTokens:  20,
 		OutputTokens: 10,
diff --git a/backend/internal/service/openai_model_mapping.go b/backend/internal/service/openai_model_mapping.go
index f332633c..7cec5212 100644
--- a/backend/internal/service/openai_model_mapping.go
+++ b/backend/internal/service/openai_model_mapping.go
@@ -2,44 +2,24 @@ package service
 
 import "strings"
 
-// resolveOpenAIForwardModel determines the upstream model for OpenAI-compatible
-// forwarding. Group-level default mapping only applies when the account itself
-// did not match any explicit model_mapping rule.
+// resolveOpenAIForwardModel 解析 OpenAI 兼容转发使用的模型。
+// defaultMappedModel 只服务于 /v1/messages 的 Claude 系列显式调度映射，
+// 不作为普通 OpenAI 请求的未知模型兜底。
 func resolveOpenAIForwardModel(account *Account, requestedModel, defaultMappedModel string) string {
 	if account == nil {
-		if defaultMappedModel != "" {
+		if defaultMappedModel != "" && claudeMessagesDispatchFamily(requestedModel) != "" {
 			return defaultMappedModel
 		}
 		return requestedModel
 	}
 
 	mappedModel, matched := account.ResolveMappedModel(requestedModel)
-	if !matched && defaultMappedModel != "" && !isExplicitCodexModel(requestedModel) {
+	if !matched && defaultMappedModel != "" && claudeMessagesDispatchFamily(requestedModel) != "" {
 		return defaultMappedModel
 	}
 	return mappedModel
 }
 
-func isExplicitCodexModel(model string) bool {
-	model = strings.TrimSpace(model)
-	if model == "" {
-		return false
-	}
-	if strings.Contains(model, "/") {
-		parts := strings.Split(model, "/")
-		model = parts[len(parts)-1]
-	}
-	model = strings.ToLower(strings.TrimSpace(model))
-	if getNormalizedCodexModel(model) != "" {
-		return true
-	}
-	if strings.HasSuffix(model, "-openai-compact") {
-		base := strings.TrimSuffix(model, "-openai-compact")
-		return getNormalizedCodexModel(base) != ""
-	}
-	return false
-}
-
 // resolveOpenAICompactForwardModel determines the compact-only upstream model
 // for /responses/compact requests. It never affects normal /responses traffic.
 // When no compact-specific mapping matches, the input model is returned as-is.
diff --git a/backend/internal/service/openai_model_mapping_test.go b/backend/internal/service/openai_model_mapping_test.go
index 4802c089..5c3e1ae0 100644
--- a/backend/internal/service/openai_model_mapping_test.go
+++ b/backend/internal/service/openai_model_mapping_test.go
@@ -11,7 +11,7 @@ func TestResolveOpenAIForwardModel(t *testing.T) {
 		expectedModel      string
 	}{
 		{
-			name: "falls back to group default when account has no mapping",
+			name: "uses messages dispatch default for claude model",
 			account: &Account{
 				Credentials: map[string]any{},
 			},
@@ -19,6 +19,15 @@ func TestResolveOpenAIForwardModel(t *testing.T) {
 			defaultMappedModel: "gpt-4o-mini",
 			expectedModel:      "gpt-4o-mini",
 		},
+		{
+			name: "does not fall back to group default for invalid gpt model",
+			account: &Account{
+				Credentials: map[string]any{},
+			},
+			requestedModel:     "gpt6",
+			defaultMappedModel: "gpt-5.4",
+			expectedModel:      "gpt6",
+		},
 		{
 			name: "preserves explicit gpt-5.4 instead of group default",
 			account: &Account{
@@ -119,14 +128,14 @@ func TestResolveOpenAIForwardModel_PreventsClaudeModelFromFallingBackToGpt54(t *
 		Credentials: map[string]any{},
 	}
 
-	withoutDefault := normalizeCodexModel(resolveOpenAIForwardModel(account, "claude-opus-4-6", ""))
-	if withoutDefault != "gpt-5.4" {
-		t.Fatalf("normalizeCodexModel(...) = %q, want %q", withoutDefault, "gpt-5.4")
+	withoutDefault := resolveOpenAIForwardModel(account, "claude-opus-4-6", "")
+	if withoutDefault != "claude-opus-4-6" {
+		t.Fatalf("resolveOpenAIForwardModel(...) = %q, want %q", withoutDefault, "claude-opus-4-6")
 	}
 
-	withDefault := normalizeCodexModel(resolveOpenAIForwardModel(account, "claude-opus-4-6", "gpt-5.4"))
+	withDefault := resolveOpenAIForwardModel(account, "claude-opus-4-6", "gpt-5.4")
 	if withDefault != "gpt-5.4" {
-		t.Fatalf("normalizeCodexModel(...) = %q, want %q", withDefault, "gpt-5.4")
+		t.Fatalf("resolveOpenAIForwardModel(...) = %q, want %q", withDefault, "gpt-5.4")
 	}
 }
 
@@ -205,6 +214,10 @@ func TestNormalizeCodexModel(t *testing.T) {
 		"gpt-5.3-codex-spark-xhigh": "gpt-5.3-codex-spark",
 		"gpt-5.3":                   "gpt-5.3-codex",
 		"gpt-image-2":               "gpt-image-2",
+		"gpt-5.4-nano":              "gpt-5.4-nano",
+		"gpt-5.4-nano-high":         "gpt-5.4-nano",
+		"gpt6":                      "gpt6",
+		"claude-opus-4-6":           "claude-opus-4-6",
 	}
 
 	for input, expected := range cases {
@@ -222,9 +235,21 @@ func TestNormalizeOpenAIModelForUpstream(t *testing.T) {
 		want    string
 	}{
 		{
-			name:    "oauth keeps codex normalization behavior",
+			name:    "oauth preserves unknown non codex model",
 			account: &Account{Type: AccountTypeOAuth},
 			model:   "gemini-3-flash-preview",
+			want:    "gemini-3-flash-preview",
+		},
+		{
+			name:    "oauth preserves invalid gpt model",
+			account: &Account{Type: AccountTypeOAuth},
+			model:   "gpt6",
+			want:    "gpt6",
+		},
+		{
+			name:    "oauth normalizes known codex alias",
+			account: &Account{Type: AccountTypeOAuth},
+			model:   "gpt-5.4-high",
 			want:    "gpt-5.4",
 		},
 		{
diff --git a/backend/internal/service/openai_oauth_passthrough_test.go b/backend/internal/service/openai_oauth_passthrough_test.go
index 87a05b14..cc9fc572 100644
--- a/backend/internal/service/openai_oauth_passthrough_test.go
+++ b/backend/internal/service/openai_oauth_passthrough_test.go
@@ -48,6 +48,49 @@ func (u *httpUpstreamRecorder) DoWithTLS(req *http.Request, proxyURL string, acc
 	return u.Do(req, proxyURL, accountID, accountConcurrency)
 }
 
+func TestOpenAIGatewayService_ResponsesUnknownModelDoesNotFallbackToGPT54(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+
+	rec := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(rec)
+	originalBody := []byte(`{"model":"gpt6","stream":false,"instructions":"local-test-instructions","input":[{"type":"text","text":"hi"}]}`)
+	c.Request = httptest.NewRequest(http.MethodPost, "/v1/responses", bytes.NewReader(originalBody))
+	c.Request.Header.Set("Content-Type", "application/json")
+
+	upstream := &httpUpstreamRecorder{resp: &http.Response{
+		StatusCode: http.StatusBadRequest,
+		Header:     http.Header{"Content-Type": []string{"application/json"}, "x-request-id": []string{"rid_unknown_model"}},
+		Body:       io.NopCloser(strings.NewReader(`{"error":{"type":"invalid_request_error","message":"model not found"}}`)),
+	}}
+
+	svc := &OpenAIGatewayService{
+		cfg:          &config.Config{},
+		httpUpstream: upstream,
+	}
+	account := &Account{
+		ID:          123,
+		Name:        "acc",
+		Platform:    PlatformOpenAI,
+		Type:        AccountTypeOAuth,
+		Concurrency: 1,
+		Credentials: map[string]any{
+			"access_token":       "oauth-token",
+			"chatgpt_account_id": "chatgpt-acc",
+		},
+		Status:      StatusActive,
+		Schedulable: true,
+	}
+
+	result, err := svc.Forward(context.Background(), c, account, originalBody)
+	require.Error(t, err)
+	require.Nil(t, result)
+	require.NotNil(t, upstream.lastReq)
+	require.Equal(t, "https://chatgpt.com/backend-api/codex/responses", upstream.lastReq.URL.String())
+	require.Equal(t, "gpt6", gjson.GetBytes(upstream.lastBody, "model").String())
+	require.NotEqual(t, "gpt-5.4", gjson.GetBytes(upstream.lastBody, "model").String())
+	require.True(t, rec.Code >= http.StatusBadRequest)
+}
+
 type openAIPassthroughFailoverRepo struct {
 	stubOpenAIAccountRepo
 	rateLimitCalls []time.Time