feat: add support for custom OpenAI endpoint path

- Introduced a new endpoint option for OpenAI integrations, allowing users to specify a custom path.
- Updated relevant components and validation logic to accommodate the new endpoint.
- Enhanced documentation and error messages to reflect the addition of the custom endpoint option.

internal/modelchannel/identity.go

@@ -13,6 +13,7 @@ const ChannelIDHexLength = 16
 const (
 	OpenAIEndpointResponses       = "/v1/responses"
 	OpenAIEndpointChatCompletions = "/v1/chat/completions"
+	OpenAIEndpointCustom          = "/custom"
 )
 
 func NormalizeBaseURL(raw string) (string, error) {
@@ -39,20 +40,37 @@ func NormalizeBaseURL(raw string) (string, error) {
 	return normalized, nil
 }
 
+// NormalizeOpenAIEndpoint 归一化 OpenAI endpoint 路径。
+// 支持三个预设值：/v1/responses、/v1/chat/completions、/custom（自定义路径）。
+// 选 /custom 时，用户需在接口地址栏填写完整请求 URL。
 func NormalizeOpenAIEndpoint(providerType string, endpoint string) string {
 	if strings.TrimSpace(strings.ToLower(providerType)) != "openai" {
 		return ""
 	}
-	switch strings.ToLower(strings.TrimSpace(endpoint)) {
-	case "", OpenAIEndpointResponses:
+	normalized := strings.TrimSpace(endpoint)
+	switch normalized {
+	case "":
 		return OpenAIEndpointResponses
-	case OpenAIEndpointChatCompletions:
-		return OpenAIEndpointChatCompletions
+	case OpenAIEndpointResponses, OpenAIEndpointChatCompletions, OpenAIEndpointCustom:
+		return normalized
 	default:
 		return ""
 	}
 }
 
+// OpenAIEndpointShape 根据 endpoint 路径末段推断协议形态。
+// 返回 "responses"（Responses API）或 "chat/completions"（Chat Completions API）。
+// 这样 /v1/chat/completions、/v4/chat/completions、/chat/completions 都走同一协议分支。
+func OpenAIEndpointShape(endpoint string) string {
+	lower := strings.ToLower(strings.TrimSpace(endpoint))
+	switch {
+	case strings.HasSuffix(lower, "/responses"):
+		return "responses"
+	default:
+		return "chat/completions"
+	}
+}
+
 func BuildLegacyChannelID(baseURL string, modelID string, apiKey string, name string) string {
 	return buildChannelID([]string{
 		strings.TrimSpace(baseURL),

internal/modelchannel/identity_test.go

@@ -0,0 +1,70 @@
+package modelchannel
+
+import "testing"
+
+func TestNormalizeOpenAIEndpoint(t *testing.T) {
+	cases := []struct {
+		name         string
+		providerType string
+		endpoint     string
+		want         string
+	}{
+		// 非 openai 类型始终返回空
+		{"anthropic_ignored", "anthropic", "/v1/responses", ""},
+		{"empty_type_ignored", "", "/v1/responses", ""},
+
+		// 预设值
+		{"empty_defaults_to_responses", "openai", "", OpenAIEndpointResponses},
+		{"preset_responses", "openai", "/v1/responses", OpenAIEndpointResponses},
+		{"preset_chat_completions", "openai", "/v1/chat/completions", OpenAIEndpointChatCompletions},
+		{"preset_custom", "openai", "/custom", OpenAIEndpointCustom},
+
+		// 非法值：不再允许任意自定义路径，只接受三个预设值
+		{"arbitrary_path_rejected", "openai", "/v4/chat/completions", ""},
+		{"arbitrary_path_rejected_2", "openai", "/chat/completions", ""},
+		{"missing_slash", "openai", "v1/responses", ""},
+		{"slash_only", "openai", "/", ""},
+		{"whitespace", "openai", "   ", OpenAIEndpointResponses},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			got := NormalizeOpenAIEndpoint(tc.providerType, tc.endpoint)
+			if got != tc.want {
+				t.Fatalf("NormalizeOpenAIEndpoint(%q, %q) = %q, want %q", tc.providerType, tc.endpoint, got, tc.want)
+			}
+		})
+	}
+}
+
+func TestOpenAIEndpointShape(t *testing.T) {
+	cases := []struct {
+		endpoint string
+		want     string
+	}{
+		// Responses 形态
+		{"/v1/responses", "responses"},
+		{"/v4/responses", "responses"},
+		{"/responses", "responses"},
+
+		// Chat Completions 形态
+		{"/v1/chat/completions", "chat/completions"},
+		{"/v4/chat/completions", "chat/completions"},
+		{"/chat/completions", "chat/completions"},
+
+		// /custom 不含已知后缀 → 兜底走 chat/completions
+		{"/custom", "chat/completions"},
+
+		// 兜底默认
+		{"", "chat/completions"},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.endpoint, func(t *testing.T) {
+			got := OpenAIEndpointShape(tc.endpoint)
+			if got != tc.want {
+				t.Fatalf("OpenAIEndpointShape(%q) = %q, want %q", tc.endpoint, got, tc.want)
+			}
+		})
+	}
+}