diff --git a/backend/internal/service/upstream_models.go b/backend/internal/service/upstream_models.go
index e9fa7de451..4f6a305b25 100644
--- a/backend/internal/service/upstream_models.go
+++ b/backend/internal/service/upstream_models.go
@@ -391,14 +391,7 @@ func buildV1ModelsURL(base string) string {
 }
 
 func buildOpenAIModelsURL(base string) string {
-	normalized := strings.TrimRight(strings.TrimSpace(base), "/")
-	if strings.HasSuffix(normalized, "/v1/models") {
-		return normalized
-	}
-	if strings.HasSuffix(normalized, "/v1") {
-		return normalized + "/models"
-	}
-	return normalized + "/v1/models"
+	return buildOpenAIEndpointURL(base, "/v1/models")
 }
 
 func buildGeminiModelsURL(base string) string {
diff --git a/backend/internal/service/upstream_models_test.go b/backend/internal/service/upstream_models_test.go
index 1fe9415d34..3904194ffa 100644
--- a/backend/internal/service/upstream_models_test.go
+++ b/backend/internal/service/upstream_models_test.go
@@ -29,6 +29,61 @@ func TestBuildV1ModelsURL(t *testing.T) {
 	require.Equal(t, "https://gateway.example.com/antigravity/v1/models", buildV1ModelsURL("https://gateway.example.com/antigravity/"))
 }
 
+func TestBuildOpenAIModelsURL(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name string
+		base string
+		want string
+	}{
+		{
+			name: "zhipu v4 coding base url",
+			base: "https://open.bigmodel.cn/api/coding/paas/v4",
+			want: "https://open.bigmodel.cn/api/coding/paas/v4/models",
+		},
+		{
+			name: "openai v1 base url",
+			base: "https://api.openai.com/v1",
+			want: "https://api.openai.com/v1/models",
+		},
+		{
+			name: "models url unchanged",
+			base: "https://api.openai.com/v1/models",
+			want: "https://api.openai.com/v1/models",
+		},
+		{
+			name: "host fallback uses v1",
+			base: "https://api.openai.com",
+			want: "https://api.openai.com/v1/models",
+		},
+		{
+			name: "trailing slash on v4",
+			base: "https://open.bigmodel.cn/api/coding/paas/v4/",
+			want: "https://open.bigmodel.cn/api/coding/paas/v4/models",
+		},
+		{
+			name: "v2 base url",
+			base: "https://gateway.example.com/openai/v2",
+			want: "https://gateway.example.com/openai/v2/models",
+		},
+		{
+			name: "v3 base url",
+			base: "https://gateway.example.com/openai/v3",
+			want: "https://gateway.example.com/openai/v3/models",
+		},
+	}
+
+	for _, tt := range tests {
+		tt := tt
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			require.Equal(t, tt.want, buildOpenAIModelsURL(tt.base))
+		})
+	}
+}
+
 func TestBuildGeminiModelsURL(t *testing.T) {
 	t.Parallel()