fix(gateway): avoid duplicate Accept-Encoding headers

backend/internal/service/gateway_accept_encoding_test.go

@@ -0,0 +1,81 @@
+package service
+
+import (
+	"context"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"net/url"
+	"testing"
+	"time"
+
+	"github.com/Wei-Shaw/sub2api/internal/config"
+	"github.com/gin-gonic/gin"
+	"github.com/stretchr/testify/require"
+)
+
+func TestGatewayService_AcceptEncodingOnWire(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	for _, protocol := range []struct {
+		name  string
+		major int
+	}{
+		{name: "HTTP1", major: 1},
+		{name: "HTTP2", major: 2},
+	} {
+		t.Run(protocol.name, func(t *testing.T) {
+			type receivedRequest struct {
+				encodings []string
+				proto     int
+			}
+			received := make(chan receivedRequest, 1)
+			server := httptest.NewUnstartedServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+				received <- receivedRequest{encodings: r.Header.Values("Accept-Encoding"), proto: r.ProtoMajor}
+				w.WriteHeader(http.StatusNoContent)
+			}))
+			server.EnableHTTP2 = protocol.major == 2
+			server.StartTLS()
+			defer server.Close()
+			client := server.Client()
+			client.Timeout = 5 * time.Second
+			upstreamURL, err := url.Parse(server.URL)
+			require.NoError(t, err)
+
+			for _, tc := range []struct {
+				name     string
+				encoding string
+				want     string
+			}{
+				{name: "gzip", encoding: "gzip", want: "gzip"},
+				{name: "multiple encodings", encoding: "gzip, deflate, br", want: "gzip, deflate, br"},
+				{name: "identity", encoding: "identity", want: "identity"},
+				{name: "omitted", want: "gzip"},
+			} {
+				t.Run(tc.name, func(t *testing.T) {
+					c, _ := gin.CreateTestContext(httptest.NewRecorder())
+					c.Request = httptest.NewRequest(http.MethodPost, "/v1/messages", nil)
+					if tc.encoding != "" {
+						c.Request.Header.Set("Accept-Encoding", tc.encoding)
+					}
+					svc := &GatewayService{cfg: &config.Config{}}
+					account := &Account{Platform: PlatformAnthropic, Type: AccountTypeOAuth}
+					req, _, err := svc.buildUpstreamRequest(context.Background(), c, account,
+						[]byte(`{"model":"claude-sonnet-4-6","messages":[]}`), "test-token", "oauth", "claude-sonnet-4-6", false, false)
+					require.NoError(t, err)
+					req.URL.Scheme = upstreamURL.Scheme
+					req.URL.Host = upstreamURL.Host
+					req.Host = ""
+
+					resp, err := client.Do(req)
+					require.NoError(t, err)
+					_, err = io.Copy(io.Discard, resp.Body)
+					require.NoError(t, err)
+					require.NoError(t, resp.Body.Close())
+					got := <-received
+					require.Equal(t, protocol.major, got.proto)
+					require.Equal(t, []string{tc.want}, got.encodings)
+				})
+			}
+		})
+	}
+}

backend/internal/service/header_util.go

@@ -6,6 +6,7 @@ import (
 )
 
 // headerWireCasing 定义每个白名单 header 在真实 Claude CLI 抓包中的准确大小写。
+// Accept-Encoding keeps canonical casing so net/http recognizes explicit compression negotiation.
 // Go 的 HTTP server 解析请求时会将所有 header key 转为 Canonical 形式（如 x-app → X-App），
 // 此 map 用于在转发时恢复到真实的 wire format。
 //
@@ -34,7 +35,7 @@ var headerWireCasing = map[string]string{
 	"content-type":                              "content-type",
 	"accept-language":                           "accept-language",
 	"sec-fetch-mode":                            "sec-fetch-mode",
-	"accept-encoding":                           "accept-encoding",
+	"accept-encoding":                           "Accept-Encoding",
 	"authorization":                             "authorization",
 
 	// Claude Code 2.1.87+ 新增 header