diff --git a/backend/internal/service/openai_endpoint_url.go b/backend/internal/service/openai_endpoint_url.go
new file mode 100644
index 00000000..93ae9b95
--- /dev/null
+++ b/backend/internal/service/openai_endpoint_url.go
@@ -0,0 +1,78 @@
+package service
+
+import (
+	"net/url"
+	"strings"
+)
+
+func buildOpenAIEndpointURL(base string, endpoint string) string {
+	normalized := strings.TrimRight(strings.TrimSpace(base), "/")
+	endpoint = "/" + strings.TrimLeft(strings.TrimSpace(endpoint), "/")
+	relative := strings.TrimPrefix(endpoint, "/v1")
+	if strings.HasSuffix(normalized, endpoint) || strings.HasSuffix(normalized, relative) {
+		return normalized
+	}
+	if openAIBaseURLHasVersionSuffix(normalized) {
+		return normalized + relative
+	}
+	return normalized + endpoint
+}
+
+func openAIBaseURLHasVersionSuffix(raw string) bool {
+	trimmed := strings.TrimSpace(raw)
+	if trimmed == "" {
+		return false
+	}
+
+	pathValue := ""
+	if parsed, err := url.Parse(trimmed); err == nil && parsed.Scheme != "" && parsed.Host != "" {
+		pathValue = parsed.Path
+	} else if slash := strings.Index(trimmed, "/"); slash >= 0 {
+		pathValue = trimmed[slash:]
+	}
+
+	pathValue = strings.TrimRight(pathValue, "/")
+	if pathValue == "" {
+		return false
+	}
+	lastSlash := strings.LastIndex(pathValue, "/")
+	segment := pathValue
+	if lastSlash >= 0 {
+		segment = pathValue[lastSlash+1:]
+	}
+	return isOpenAIAPIVersionSegment(segment)
+}
+
+func isOpenAIAPIVersionSegment(segment string) bool {
+	s := strings.ToLower(strings.TrimSpace(segment))
+	if len(s) < 2 || s[0] != 'v' || !isASCIIDigit(s[1]) {
+		return false
+	}
+
+	i := 1
+	for i < len(s) && isASCIIDigit(s[i]) {
+		i++
+	}
+	if i == len(s) {
+		return true
+	}
+	if s[i] == '.' {
+		i++
+		if i == len(s) || !isASCIIDigit(s[i]) {
+			return false
+		}
+		for i < len(s) && isASCIIDigit(s[i]) {
+			i++
+		}
+		return i == len(s)
+	}
+
+	suffix := s[i:]
+	return strings.HasPrefix(suffix, "alpha") ||
+		strings.HasPrefix(suffix, "beta") ||
+		strings.HasPrefix(suffix, "preview")
+}
+
+func isASCIIDigit(b byte) bool {
+	return b >= '0' && b <= '9'
+}
diff --git a/backend/internal/service/openai_gateway_chat_completions.go b/backend/internal/service/openai_gateway_chat_completions.go
index 84d85c74..921948b0 100644
--- a/backend/internal/service/openai_gateway_chat_completions.go
+++ b/backend/internal/service/openai_gateway_chat_completions.go
@@ -247,6 +247,16 @@ func (s *OpenAIGatewayService) ForwardAsChatCompletions(
 
 		upstreamMsg := strings.TrimSpace(extractUpstreamErrorMessage(respBody))
 		upstreamMsg = sanitizeUpstreamErrorMessage(upstreamMsg)
+		if account.Type == AccountTypeAPIKey &&
+			openai_compat.ResolveResponsesSupport(account.Extra) == openai_compat.ResponsesSupportUnknown &&
+			!isResponsesEndpointSupportedByStatus(resp.StatusCode) {
+			logger.L().Info("openai chat_completions: /responses unsupported, falling back to raw chat completions",
+				zap.Int64("account_id", account.ID),
+				zap.Int("upstream_status", resp.StatusCode),
+				zap.String("upstream_message", upstreamMsg),
+			)
+			return s.forwardAsRawChatCompletions(ctx, c, account, body, defaultMappedModel)
+		}
 		if s.shouldFailoverOpenAIUpstreamResponse(resp.StatusCode, upstreamMsg, respBody) {
 			upstreamDetail := ""
 			if s.cfg != nil && s.cfg.Gateway.LogUpstreamErrorBody {
diff --git a/backend/internal/service/openai_gateway_chat_completions_raw.go b/backend/internal/service/openai_gateway_chat_completions_raw.go
index 3be765a2..0203b94a 100644
--- a/backend/internal/service/openai_gateway_chat_completions_raw.go
+++ b/backend/internal/service/openai_gateway_chat_completions_raw.go
@@ -422,16 +422,10 @@ func (s *OpenAIGatewayService) bufferRawChatCompletions(
 //
 //   - base 已是 /chat/completions：原样返回
 //   - base 以 /v1 结尾：追加 /chat/completions
+//   - base 以其他版本段结尾（如 /v4）：追加 /chat/completions
 //   - 其他情况：追加 /v1/chat/completions
 //
 // 与 buildOpenAIResponsesURL 是姐妹函数。
 func buildOpenAIChatCompletionsURL(base string) string {
-	normalized := strings.TrimRight(strings.TrimSpace(base), "/")
-	if strings.HasSuffix(normalized, "/chat/completions") {
-		return normalized
-	}
-	if strings.HasSuffix(normalized, "/v1") {
-		return normalized + "/chat/completions"
-	}
-	return normalized + "/v1/chat/completions"
+	return buildOpenAIEndpointURL(base, "/v1/chat/completions")
 }
diff --git a/backend/internal/service/openai_gateway_chat_completions_raw_test.go b/backend/internal/service/openai_gateway_chat_completions_raw_test.go
index 1be07fd7..46ddbc09 100644
--- a/backend/internal/service/openai_gateway_chat_completions_raw_test.go
+++ b/backend/internal/service/openai_gateway_chat_completions_raw_test.go
@@ -36,6 +36,7 @@ func TestBuildOpenAIChatCompletionsURL(t *testing.T) {
 		// 第三方上游常见形式
 		{"third-party bare domain", "https://api.deepseek.com", "https://api.deepseek.com/v1/chat/completions"},
 		{"third-party with path prefix", "https://api.gptgod.online/api", "https://api.gptgod.online/api/v1/chat/completions"},
+		{"third-party versioned path", "https://open.bigmodel.cn/api/paas/v4", "https://open.bigmodel.cn/api/paas/v4/chat/completions"},
 		// 带空白字符
 		{"whitespace trimmed", "  https://api.openai.com/v1  ", "https://api.openai.com/v1/chat/completions"},
 	}
@@ -64,6 +65,7 @@ func TestBuildOpenAIResponsesURL_ProbeURL(t *testing.T) {
 		{"bare /v1", "https://api.openai.com/v1", "https://api.openai.com/v1/responses"},
 		{"already /responses", "https://api.openai.com/v1/responses", "https://api.openai.com/v1/responses"},
 		{"third-party bare domain", "https://api.deepseek.com", "https://api.deepseek.com/v1/responses"},
+		{"third-party versioned path", "https://open.bigmodel.cn/api/paas/v4", "https://open.bigmodel.cn/api/paas/v4/responses"},
 		{"only domain, no scheme", "api.gptgod.online", "api.gptgod.online/v1/responses"},
 	}
 
@@ -193,6 +195,49 @@ func TestForwardAsRawChatCompletions_UpstreamRequestIgnoresClientCancel(t *testi
 	require.NoError(t, upstream.lastReq.Context().Err())
 }
 
+func TestForwardAsChatCompletions_UnknownResponsesSupportFallbackUsesVersionedChatURL(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+
+	body := []byte(`{"model":"glm-4.5-air","messages":[{"role":"user","content":"hello"}],"stream":false}`)
+	rec := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(rec)
+	c.Request = httptest.NewRequest(http.MethodPost, "/v1/chat/completions", bytes.NewReader(body))
+	c.Request.Header.Set("Content-Type", "application/json")
+
+	upstream := &httpUpstreamRecorder{responses: []*http.Response{
+		{
+			StatusCode: http.StatusNotFound,
+			Header:     http.Header{"Content-Type": []string{"application/json"}},
+			Body:       io.NopCloser(strings.NewReader(`{"error":{"message":"not found"}}`)),
+		},
+		{
+			StatusCode: http.StatusOK,
+			Header:     http.Header{"Content-Type": []string{"application/json"}, "x-request-id": []string{"rid_raw_fallback"}},
+			Body: io.NopCloser(strings.NewReader(
+				`{"id":"chatcmpl_1","object":"chat.completion","model":"glm-4.5-air","choices":[{"index":0,"message":{"role":"assistant","content":"ok"},"finish_reason":"stop"}],"usage":{"prompt_tokens":1,"completion_tokens":2,"total_tokens":3}}`,
+			)),
+		},
+	}}
+
+	svc := &OpenAIGatewayService{
+		cfg:          rawChatCompletionsTestConfig(),
+		httpUpstream: upstream,
+	}
+	account := rawChatCompletionsTestAccount()
+	account.Credentials["base_url"] = "https://open.bigmodel.cn/api/paas/v4"
+
+	result, err := svc.ForwardAsChatCompletions(context.Background(), c, account, body, "", "")
+	require.NoError(t, err)
+	require.NotNil(t, result)
+	require.Equal(t, 1, result.Usage.InputTokens)
+	require.Equal(t, 2, result.Usage.OutputTokens)
+	require.Len(t, upstream.requests, 2)
+	require.Equal(t, "https://open.bigmodel.cn/api/paas/v4/responses", upstream.requests[0].URL.String())
+	require.Equal(t, "https://open.bigmodel.cn/api/paas/v4/chat/completions", upstream.requests[1].URL.String())
+	require.Equal(t, http.StatusOK, rec.Code)
+	require.Contains(t, rec.Body.String(), `"content":"ok"`)
+}
+
 func TestIsOpenAIChatUsageOnlyStreamChunk(t *testing.T) {
 	t.Parallel()
 
diff --git a/backend/internal/service/openai_gateway_service.go b/backend/internal/service/openai_gateway_service.go
index e12b208e..c1bdd9b5 100644
--- a/backend/internal/service/openai_gateway_service.go
+++ b/backend/internal/service/openai_gateway_service.go
@@ -4955,17 +4955,11 @@ func (s *OpenAIGatewayService) validateUpstreamBaseURL(raw string) (string, erro
 
 // buildOpenAIResponsesURL 组装 OpenAI Responses 端点。
 // - base 以 /v1 结尾：追加 /responses
+// - base 以其他版本段结尾（如 /v4）：追加 /responses
 // - base 已是 /responses：原样返回
 // - 其他情况：追加 /v1/responses
 func buildOpenAIResponsesURL(base string) string {
-	normalized := strings.TrimRight(strings.TrimSpace(base), "/")
-	if strings.HasSuffix(normalized, "/responses") {
-		return normalized
-	}
-	if strings.HasSuffix(normalized, "/v1") {
-		return normalized + "/responses"
-	}
-	return normalized + "/v1/responses"
+	return buildOpenAIEndpointURL(base, "/v1/responses")
 }
 
 func trimOpenAIEncryptedReasoningItems(reqBody map[string]any) bool {
diff --git a/backend/internal/service/openai_images.go b/backend/internal/service/openai_images.go
index afa94156..1ed4d7d1 100644
--- a/backend/internal/service/openai_images.go
+++ b/backend/internal/service/openai_images.go
@@ -795,15 +795,7 @@ func (s *OpenAIGatewayService) buildOpenAIImagesRequest(
 }
 
 func buildOpenAIImagesURL(base string, endpoint string) string {
-	normalized := strings.TrimRight(strings.TrimSpace(base), "/")
-	relative := strings.TrimPrefix(strings.TrimSpace(endpoint), "/v1")
-	if strings.HasSuffix(normalized, endpoint) || strings.HasSuffix(normalized, relative) {
-		return normalized
-	}
-	if strings.HasSuffix(normalized, "/v1") {
-		return normalized + relative
-	}
-	return normalized + endpoint
+	return buildOpenAIEndpointURL(base, endpoint)
 }
 
 func rewriteOpenAIImagesModel(body []byte, contentType string, model string) ([]byte, string, error) {
diff --git a/backend/internal/service/openai_images_test.go b/backend/internal/service/openai_images_test.go
index 45fb24e9..4f7c2fe7 100644
--- a/backend/internal/service/openai_images_test.go
+++ b/backend/internal/service/openai_images_test.go
@@ -418,6 +418,10 @@ func TestBuildOpenAIImagesURL_HandlesVersionedBaseURL(t *testing.T) {
 		"https://image-upstream.example/v1/images/generations",
 		buildOpenAIImagesURL("https://image-upstream.example/v1", openAIImagesGenerationsEndpoint),
 	)
+	require.Equal(t,
+		"https://open.bigmodel.cn/api/paas/v4/images/generations",
+		buildOpenAIImagesURL("https://open.bigmodel.cn/api/paas/v4", openAIImagesGenerationsEndpoint),
+	)
 	require.Equal(t,
 		"https://image-upstream.example/v1/images/edits",
 		buildOpenAIImagesURL("https://image-upstream.example/v1/", openAIImagesEditsEndpoint),