[verified] fix: align sync cache billing after failover

backend/internal/handler/gateway_handler.go

@@ -797,6 +797,9 @@ func (h *GatewayHandler) Messages(c *gin.Context) {
 			if fs.SwitchCount > 0 {
 				requestCtx = service.WithAccountSwitchCount(requestCtx, fs.SwitchCount, h.metadataBridgeEnabled())
 			}
+			if fs.ForceCacheBilling {
+				requestCtx = service.WithForceCacheBilling(requestCtx)
+			}
 			// 记录 Forward 前已写入字节数，Forward 后若增加则说明 SSE 内容已发，禁止 failover
 			writerSizeBeforeForward := c.Writer.Size()
 			if account.Platform == service.PlatformAntigravity && account.Type != service.AccountTypeAPIKey {

backend/internal/service/gateway_anthropic_passthrough.go

@@ -20,6 +20,7 @@ import (
 	"github.com/Wei-Shaw/sub2api/internal/pkg/logger"
 	"github.com/Wei-Shaw/sub2api/internal/util/responseheaders"
 	"github.com/tidwall/gjson"
+	"github.com/tidwall/sjson"
 
 	"github.com/gin-gonic/gin"
 )
@@ -782,6 +783,12 @@ func (s *GatewayService) handleNonStreamingResponseAnthropicAPIKeyPassthrough(
 	}
 
 	usage := parseClaudeUsageFromResponseBody(body)
+	if IsForceCacheBilling(ctx) && usage.InputTokens > 0 {
+		body, err = classifyAnthropicResponseInputAsCacheRead(body, usage)
+		if err != nil {
+			return nil, err
+		}
+	}
 
 	writeAnthropicPassthroughResponseHeaders(c.Writer.Header(), resp.Header, s.responseHeaderFilter)
 	contentType := strings.TrimSpace(resp.Header.Get("Content-Type"))
@@ -793,6 +800,18 @@ func (s *GatewayService) handleNonStreamingResponseAnthropicAPIKeyPassthrough(
 	return usage, nil
 }
 
+func classifyAnthropicResponseInputAsCacheRead(body []byte, usage *ClaudeUsage) ([]byte, error) {
+	classified, err := sjson.SetBytes(body, "usage.input_tokens", 0)
+	if err != nil {
+		return nil, fmt.Errorf("classify forced cache billing input tokens: %w", err)
+	}
+	classified, err = sjson.SetBytes(classified, "usage.cache_read_input_tokens", usage.CacheReadInputTokens+usage.InputTokens)
+	if err != nil {
+		return nil, fmt.Errorf("classify forced cache billing cache read tokens: %w", err)
+	}
+	return classified, nil
+}
+
 func writeAnthropicPassthroughResponseHeaders(dst http.Header, src http.Header, filter *responseheaders.CompiledHeaderFilter) {
 	if dst == nil || src == nil {
 		return

backend/internal/service/gateway_non_streaming_response_test.go

@@ -13,6 +13,7 @@ import (
 	"github.com/Wei-Shaw/sub2api/internal/config"
 	"github.com/gin-gonic/gin"
 	"github.com/stretchr/testify/require"
+	"github.com/tidwall/gjson"
 )
 
 type nonJSONTempUnschedAccountRepo struct {
@@ -143,6 +144,52 @@ func TestHandleNonStreamingResponseAnthropicAPIKeyPassthrough_ValidJSONUnchanged
 	require.JSONEq(t, string(body), rec.Body.String())
 }
 
+func TestHandleNonStreamingResponseAnthropicAPIKeyPassthrough_ForceCacheBillingResponse(t *testing.T) {
+	tests := []struct {
+		name string
+		body string
+		want string
+	}{
+		{
+			name: "converts input tokens for downstream billing",
+			body: `{"id":"msg_1","type":"message","content":[{"type":"text","text":"unchanged"}],"usage":{"input_tokens":5,"output_tokens":3}}`,
+			want: `{"id":"msg_1","type":"message","content":[{"type":"text","text":"unchanged"}],"usage":{"input_tokens":0,"output_tokens":3,"cache_read_input_tokens":5}}`,
+		},
+		{
+			name: "adds to genuine cache reads",
+			body: `{"id":"msg_2","type":"message","usage":{"input_tokens":5,"output_tokens":3,"cache_read_input_tokens":7,"cache_creation_input_tokens":11}}`,
+			want: `{"id":"msg_2","type":"message","usage":{"input_tokens":0,"output_tokens":3,"cache_read_input_tokens":12,"cache_creation_input_tokens":11}}`,
+		},
+		{
+			name: "zero input leaves response unchanged",
+			body: `{"id":"msg_3","type":"message","usage":{"input_tokens":0,"output_tokens":3,"cache_read_input_tokens":7}}`,
+			want: `{"id":"msg_3","type":"message","usage":{"input_tokens":0,"output_tokens":3,"cache_read_input_tokens":7}}`,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			gin.SetMode(gin.TestMode)
+			rec := httptest.NewRecorder()
+			c, _ := gin.CreateTestContext(rec)
+			c.Request = httptest.NewRequest(http.MethodPost, "/v1/messages", nil)
+			resp := &http.Response{
+				StatusCode: http.StatusOK,
+				Header:     http.Header{"Content-Type": []string{"application/json"}},
+				Body:       io.NopCloser(bytes.NewBufferString(tt.body)),
+			}
+			svc := &GatewayService{cfg: &config.Config{}}
+
+			usage, err := svc.handleNonStreamingResponseAnthropicAPIKeyPassthrough(WithForceCacheBilling(context.Background()), resp, c, &Account{ID: 2})
+
+			require.NoError(t, err)
+			require.Equal(t, int(gjson.Get(tt.body, "usage.input_tokens").Int()), usage.InputTokens, "local accounting must retain the unclassified usage")
+			require.Equal(t, int(gjson.Get(tt.body, "usage.cache_read_input_tokens").Int()), usage.CacheReadInputTokens, "local accounting must convert exactly once in RecordUsage")
+			require.JSONEq(t, tt.want, rec.Body.String())
+		})
+	}
+}
+
 func TestHandleNonStreamingResponse_NonJSON2xxMatchesModelScopedTempUnschedulableRule(t *testing.T) {
 	gin.SetMode(gin.TestMode)
 	rec := httptest.NewRecorder()