fix(composite): pass the requested model through when a prefix route leaves upstream_model empty

Composite model routes support a match_type: prefix mode with an optional
upstream_model. Leaving upstream_model empty is meant to pass the concrete
requested model through to the upstream, but every request matching the prefix
collapsed to the route's public_model instead. A route with
public_model=deepseek-v4, match_type=prefix and an empty upstream_model
forwarded both deepseek-v4-flash and deepseek-v4-pro upstream as deepseek-v4.

Root cause: normalizeCompositeRouteInput backfilled an empty upstream_model with
public_model at save time, so CompositeRouteResolver.Resolve never reached its
"upstreamModel == '' -> concrete requested model" fallback.

- Skip the backfill for prefix routes only; an empty upstream_model stays empty
  and Resolve passes the concrete requested model through.
- exact routes keep the backfill (model == public_model, so persisted/display
  contract is unchanged).
- Explicit upstream_model still forwards that fixed model in every mode.
- Frontend: add an upstream_model hint (zh/en) documenting empty = passthrough.

backend/internal/service/admin_service_group_test.go

@@ -1584,7 +1584,8 @@ func TestAdminService_CreateCompositeRoute_NormalizesAndPersists(t *testing.T) {
 	require.Equal(t, "router/gpt-", route.PublicModel)
 	require.Equal(t, CompositeRouteMatchPrefix, route.MatchType)
 	require.Equal(t, PlatformOpenAI, route.TargetPlatform)
-	require.Equal(t, "router/gpt-", route.UpstreamModel)
+	// prefix 路由留空 upstream_model 不再回填 public_model：留空表示透传原始请求模型。
+	require.Equal(t, "", route.UpstreamModel)
 	require.Equal(t, CompositeRouteEndpointResponses, route.Endpoint)
 	require.Equal(t, 100, route.Priority)
 	require.True(t, route.Enabled)
@@ -1592,6 +1593,29 @@ func TestAdminService_CreateCompositeRoute_NormalizesAndPersists(t *testing.T) {
 	require.Equal(t, route, routeRepo.created)
 }
 
+// TestAdminService_CreateCompositeRoute_ExactEmptyUpstreamBackfillsPublicModel 锁定
+// 保守行为：exact 路由留空 upstream_model 仍回填 public_model（持久化/展示契约不变）。
+func TestAdminService_CreateCompositeRoute_ExactEmptyUpstreamBackfillsPublicModel(t *testing.T) {
+	groupRepo := &groupRepoStubForAdmin{
+		getByID: &Group{ID: 7, Platform: PlatformComposite},
+	}
+	routeRepo := &compositeRouteRepoStubForAdmin{nextID: 99}
+	svc := &adminServiceImpl{groupRepo: groupRepo, compositeRouteRepo: routeRepo}
+
+	route, err := svc.CreateCompositeRoute(context.Background(), 7, CompositeRouteInput{
+		PublicModel:    "openrouter/gpt-5",
+		MatchType:      CompositeRouteMatchExact,
+		TargetPlatform: PlatformOpenAI,
+		Endpoint:       CompositeRouteEndpointResponses,
+		Enabled:        true,
+	})
+
+	require.NoError(t, err)
+	require.NotNil(t, route)
+	require.Equal(t, CompositeRouteMatchExact, route.MatchType)
+	require.Equal(t, "openrouter/gpt-5", route.UpstreamModel)
+}
+
 func TestAdminService_UpdateAndDeleteCompositeRouteRequireRouteOwnership(t *testing.T) {
 	groupRepo := &groupRepoStubForAdmin{
 		getByID: &Group{ID: 7, Platform: PlatformComposite},

backend/internal/service/composite_model_route.go

@@ -118,7 +118,12 @@ func normalizeCompositeRouteInput(input CompositeRouteInput) CompositeRouteInput
 	input.TargetPlatform = strings.TrimSpace(input.TargetPlatform)
 	input.UpstreamModel = strings.TrimSpace(input.UpstreamModel)
 	input.Endpoint = normalizeCompositeRouteEndpoint(input.Endpoint)
-	if input.UpstreamModel == "" {
+	// 仅对 exact 路由把空 upstream_model 回填成 public_model：exact 命中时请求模型
+	// 恒等于 public_model，回填只影响持久化/后台展示，保留原有契约不变。
+	// prefix 路由留空则不回填——Resolve 会回退到具体请求模型，从而透传原始模型
+	// （否则 public=deepseek-v4 的前缀路由会把 deepseek-v4-flash / deepseek-v4-pro
+	// 都塌缩成固定的 deepseek-v4）。显式填写 upstream_model 时任何模式都原样固定转发。
+	if input.UpstreamModel == "" && input.MatchType == CompositeRouteMatchExact {
 		input.UpstreamModel = input.PublicModel
 	}
 	input.Notes = strings.TrimSpace(input.Notes)

backend/internal/service/composite_route_resolver_test.go

@@ -107,6 +107,63 @@ func TestCompositeRouteResolverPrefersEndpointSpecificLongestPrefix(t *testing.T
 	require.Equal(t, int64(2), decision.Route.ID)
 }
 
+// TestCompositeRouteResolverPrefixEmptyUpstreamPassesThroughRequestedModel 验证：
+// 前缀匹配路由留空 upstream_model 时，转发的是具体请求模型（各自原样），而不是
+// 塌缩成 public_model。这是「留空 = 透传原始模型」语义的核心场景。
+func TestCompositeRouteResolverPrefixEmptyUpstreamPassesThroughRequestedModel(t *testing.T) {
+	resolver := NewCompositeRouteResolver(compositeRouteRepoStub{
+		routes: []CompositeModelRoute{
+			{
+				ID:             1,
+				GroupID:        7,
+				PublicModel:    "deepseek-v4",
+				MatchType:      CompositeRouteMatchPrefix,
+				TargetPlatform: PlatformOpenAI,
+				UpstreamModel:  "", // 留空 = 透传
+				Endpoint:       CompositeRouteEndpointAny,
+				Priority:       100,
+				Enabled:        true,
+			},
+		},
+	})
+
+	for _, model := range []string{"deepseek-v4-flash", "deepseek-v4-pro", "deepseek-v4"} {
+		decision, err := resolver.Resolve(context.Background(), 7, model, CompositeRouteEndpointChatCompletions)
+		require.NoError(t, err)
+		require.True(t, decision.Matched, "model %q should match prefix route", model)
+		require.Equal(t, CompositeRouteSourceExplicit, decision.Source)
+		require.Equal(t, PlatformOpenAI, decision.TargetPlatform)
+		require.Equal(t, model, decision.UpstreamModel, "model %q should pass through verbatim", model)
+	}
+}
+
+// TestCompositeRouteResolverPrefixExplicitUpstreamStillFixed 验证：前缀匹配路由显式
+// 填写 upstream_model 时，所有命中请求仍转发同一个固定上游模型（行为不变）。
+func TestCompositeRouteResolverPrefixExplicitUpstreamStillFixed(t *testing.T) {
+	resolver := NewCompositeRouteResolver(compositeRouteRepoStub{
+		routes: []CompositeModelRoute{
+			{
+				ID:             1,
+				GroupID:        7,
+				PublicModel:    "deepseek-v4",
+				MatchType:      CompositeRouteMatchPrefix,
+				TargetPlatform: PlatformOpenAI,
+				UpstreamModel:  "deepseek-chat",
+				Endpoint:       CompositeRouteEndpointAny,
+				Priority:       100,
+				Enabled:        true,
+			},
+		},
+	})
+
+	for _, model := range []string{"deepseek-v4-flash", "deepseek-v4-pro"} {
+		decision, err := resolver.Resolve(context.Background(), 7, model, CompositeRouteEndpointChatCompletions)
+		require.NoError(t, err)
+		require.True(t, decision.Matched)
+		require.Equal(t, "deepseek-chat", decision.UpstreamModel)
+	}
+}
+
 func TestCompositeRouteResolverIgnoresDisabledRoutesAndFallsBackToDetector(t *testing.T) {
 	resolver := NewCompositeRouteResolver(compositeRouteRepoStub{
 		routes: []CompositeModelRoute{

frontend/src/i18n/locales/en/admin/overview.ts

@@ -1031,6 +1031,7 @@ export default {
         endpoint: 'Endpoint',
         targetPlatform: 'Target Platform',
         upstreamModel: 'Upstream Model',
+        upstreamModelHint: 'Leave empty to pass the original requested model through: under prefix match each matched model forwards verbatim (e.g. deepseek-v4-flash and deepseek-v4-pro each forwarded as-is); set a value to forward every matched request to that fixed model.',
         notes: 'Notes',
         enabled: 'Enabled',
         preview: 'Preview',

frontend/src/i18n/locales/zh/admin/overview.ts

@@ -1028,6 +1028,7 @@ export default {
         endpoint: '端点',
         targetPlatform: '目标平台',
         upstreamModel: '上游模型',
+        upstreamModelHint: '留空表示透传原始请求模型：前缀匹配下每个命中模型各自原样转发（如 deepseek-v4-flash、deepseek-v4-pro 分别转发）；填写则所有命中请求都固定转发该模型。',
         notes: '备注',
         enabled: '启用',
         preview: '预览',

frontend/src/views/admin/GroupsView.vue

@@ -3809,6 +3809,9 @@
                 class="input"
                 placeholder="gpt-5"
               />
+              <p class="mt-1 text-xs text-gray-500 dark:text-gray-400">
+                {{ t("admin.groups.compositeRoutes.upstreamModelHint") }}
+              </p>
             </div>
 
             <div>