feat: support upstream model fetch for advanced custom channels (#5971)

* feat: support upstream model fetch for advanced custom channels

* fix: add advanced custom routes as separate groups

* fix: select advanced custom route entry before adding

---------

Co-authored-by: CaIon <i@caion.me>

dto/channel_settings.go

@@ -112,6 +112,9 @@ const (
 	advancedCustomEndpointPathEmbeddings             = "/v1/embeddings"
 )
 
+// AdvancedCustomModelListPath identifies the optional OpenAI Models discovery route.
+const AdvancedCustomModelListPath = "/v1/models"
+
 // MatchPath returns the first route whose IncomingPath matches requestPath.
 // Matching mirrors the relay adaptor: exact match, {model} placeholder, and
 // :generateContent <-> :streamGenerateContent equivalence.
@@ -143,6 +146,20 @@ func (c *AdvancedCustomConfig) MatchPathForModel(requestPath string, model strin
 	return AdvancedCustomRoute{}, false
 }
 
+// ModelListRoute returns the explicitly configured OpenAI Models discovery route.
+// Template routes that merely happen to match /v1/models are not discovery routes.
+func (c *AdvancedCustomConfig) ModelListRoute() (AdvancedCustomRoute, bool) {
+	if c == nil {
+		return AdvancedCustomRoute{}, false
+	}
+	for _, route := range c.Routes {
+		if strings.TrimSpace(route.IncomingPath) == AdvancedCustomModelListPath {
+			return route, true
+		}
+	}
+	return AdvancedCustomRoute{}, false
+}
+
 // SupportsPath reports whether any route matches requestPath.
 func (c *AdvancedCustomConfig) SupportsPath(requestPath string) bool {
 	_, ok := c.MatchPath(requestPath)
@@ -307,6 +324,7 @@ func (c *AdvancedCustomConfig) Validate() error {
 	}
 
 	paths := make(map[string]*advancedCustomPathModelState, len(c.Routes))
+	modelListRouteIndex := -1
 	for i := range c.Routes {
 		route := c.Routes[i]
 		route.IncomingPath = strings.TrimSpace(route.IncomingPath)
@@ -325,6 +343,21 @@ func (c *AdvancedCustomConfig) Validate() error {
 		if strings.Contains(route.IncomingPath, "?") {
 			return fmt.Errorf("advanced_custom.advanced_routes[%d].incoming_path must not include query", i)
 		}
+		if route.IncomingPath == AdvancedCustomModelListPath {
+			if modelListRouteIndex >= 0 {
+				return fmt.Errorf("advanced_custom.advanced_routes[%d] duplicates the /v1/models route at advanced_routes[%d]", i, modelListRouteIndex)
+			}
+			modelListRouteIndex = i
+			if len(normalizeAdvancedCustomRouteModels(route.Models)) > 0 {
+				return fmt.Errorf("advanced_custom.advanced_routes[%d].models must be empty for /v1/models", i)
+			}
+			if route.Converter != advancedCustomConverterNone {
+				return fmt.Errorf("advanced_custom.advanced_routes[%d].converter must be none for /v1/models", i)
+			}
+			if strings.Contains(upstreamPath, advancedCustomModelPlaceholder) {
+				return fmt.Errorf("advanced_custom.advanced_routes[%d].upstream_path must not contain %s for /v1/models", i, advancedCustomModelPlaceholder)
+			}
+		}
 		if err := validateAdvancedCustomRouteModels(i, route.IncomingPath, route.Models, paths); err != nil {
 			return err
 		}

dto/channel_settings_test.go

@@ -58,6 +58,94 @@ func TestAdvancedCustomValidateResponsesToChatConverterPath(t *testing.T) {
 	}
 }
 
+func TestAdvancedCustomValidateModelListRouteConstraints(t *testing.T) {
+	valid := &AdvancedCustomConfig{
+		Routes: []AdvancedCustomRoute{
+			{
+				IncomingPath: AdvancedCustomModelListPath,
+				UpstreamPath: "https://upstream.example/custom/models",
+				Converter:    advancedCustomConverterNone,
+			},
+		},
+	}
+	require.NoError(t, valid.Validate())
+
+	tests := []struct {
+		name   string
+		routes []AdvancedCustomRoute
+		want   string
+	}{
+		{
+			name: "model matching rules",
+			routes: []AdvancedCustomRoute{
+				{
+					IncomingPath: AdvancedCustomModelListPath,
+					UpstreamPath: "/v1/models",
+					Models:       []string{"gpt-4o"},
+				},
+			},
+			want: "models must be empty",
+		},
+		{
+			name: "converter",
+			routes: []AdvancedCustomRoute{
+				{
+					IncomingPath: AdvancedCustomModelListPath,
+					UpstreamPath: "/v1/models",
+					Converter:    advancedCustomConverterOpenAIChatToOpenAIResponses,
+				},
+			},
+			want: "converter must be none",
+		},
+		{
+			name: "model placeholder",
+			routes: []AdvancedCustomRoute{
+				{
+					IncomingPath: AdvancedCustomModelListPath,
+					UpstreamPath: "/v1/models/{model}",
+				},
+			},
+			want: "upstream_path must not contain {model}",
+		},
+		{
+			name: "duplicate routes",
+			routes: []AdvancedCustomRoute{
+				{IncomingPath: AdvancedCustomModelListPath, UpstreamPath: "/v1/models"},
+				{IncomingPath: AdvancedCustomModelListPath, UpstreamPath: "/provider/models"},
+			},
+			want: "duplicates the /v1/models route",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := (&AdvancedCustomConfig{Routes: tt.routes}).Validate()
+			require.Error(t, err)
+			assert.Contains(t, err.Error(), tt.want)
+		})
+	}
+}
+
+func TestAdvancedCustomModelListRouteRequiresExactIncomingPath(t *testing.T) {
+	config := &AdvancedCustomConfig{
+		Routes: []AdvancedCustomRoute{
+			{
+				IncomingPath: "/v1/{model}",
+				UpstreamPath: "/generic/{model}",
+			},
+			{
+				IncomingPath: AdvancedCustomModelListPath,
+				UpstreamPath: "/provider/models",
+			},
+		},
+	}
+	require.NoError(t, config.Validate())
+
+	route, ok := config.ModelListRoute()
+	require.True(t, ok)
+	assert.Equal(t, "/provider/models", route.UpstreamPath)
+}
+
 func TestAdvancedCustomValidateDuplicateIncomingPathWithDisjointModels(t *testing.T) {
 	config := &AdvancedCustomConfig{
 		Routes: []AdvancedCustomRoute{