diff --git a/backend/internal/service/upstream_billing_probe.go b/backend/internal/service/upstream_billing_probe.go
index adf3d25277..b34900f4f5 100644
--- a/backend/internal/service/upstream_billing_probe.go
+++ b/backend/internal/service/upstream_billing_probe.go
@@ -1105,6 +1105,12 @@ func (s *UpstreamBillingProbeService) currentTime() time.Time {
 }
 
 func nextProbeDelay(intervalMinutes int, retryAfterDuration time.Duration) time.Duration {
+	if retryAfterDuration < 0 {
+		retryAfterDuration = 0
+	}
+	if retryAfterDuration > upstreamBillingProbeMaxDelay {
+		retryAfterDuration = upstreamBillingProbeMaxDelay
+	}
 	interval := time.Duration(intervalMinutes) * time.Minute
 	if interval < upstreamBillingProbeMinIntervalMinutes*time.Minute {
 		interval = upstreamBillingProbeMinIntervalMinutes * time.Minute
@@ -1120,8 +1126,8 @@ func nextProbeDelay(intervalMinutes int, retryAfterDuration time.Duration) time.
 		interval += time.Duration(rand.Int64N(int64(jitterRange)*2+1)) - jitterRange
 	}
 	if retryAfterDuration > interval {
-		// Retry-After is an explicit upstream instruction; do not shorten it
-		// with the local maximum delay.
+		// Retry-After is an explicit upstream instruction; honor it over the
+		// local backoff interval, clamped to the same 24h ceiling.
 		return retryAfterDuration
 	}
 	if interval > upstreamBillingProbeMaxDelay {
@@ -1133,8 +1139,7 @@ func nextProbeDelay(intervalMinutes int, retryAfterDuration time.Duration) time.
 // unsupportedProbeDelay 拉长 unsupported 账号的重探间隔，让无效候选自然退出
 // 热队列，不再和真正接入 sub2api 的中转账号抢每周期的探测名额。
 // 仍按 upstreamBillingProbeMaxDelay 封顶，保证上游后来接入 sub2api 时最迟一天
-// 内会被重新发现；base 本身已达上限（例如 Retry-After 明确要求更久）时原样返回，
-// 不缩短上游指令。
+// 内会被重新发现；base 本身亦受 24h 封顶保护。
 func unsupportedProbeDelay(intervalMinutes int, retryAfterDuration time.Duration) time.Duration {
 	base := nextProbeDelay(intervalMinutes, retryAfterDuration)
 	if base >= upstreamBillingProbeMaxDelay {
diff --git a/backend/internal/service/upstream_billing_probe_test.go b/backend/internal/service/upstream_billing_probe_test.go
index 9a5f774f4f..68df096efe 100644
--- a/backend/internal/service/upstream_billing_probe_test.go
+++ b/backend/internal/service/upstream_billing_probe_test.go
@@ -767,13 +767,13 @@ func TestUpstreamBillingProbeFailurePreservesLastSuccessAndRetryAfter(t *testing
 	require.Equal(t, initialRate, *account.RateMultiplier)
 }
 
-func TestUpstreamBillingProbeRetryAfterIsNotShortened(t *testing.T) {
+func TestUpstreamBillingProbeRetryAfterIsCappedAtMaxDelay(t *testing.T) {
 	delay := nextProbeDelay(30, 48*time.Hour)
-	require.Equal(t, 48*time.Hour, delay)
+	require.Equal(t, upstreamBillingProbeMaxDelay, delay)
 }
 
 // unsupported 的重探间隔明显长于普通失败，但始终有上界：上游后来接入 sub2api
-// 时最迟一天内会被重新发现，且不会缩短上游 Retry-After 指令。
+// 时最迟一天内会被重新发现，且上游超长 Retry-After 亦受 24h 封顶保护。
 func TestUpstreamBillingProbeUnsupportedDelayIsStretchedAndBounded(t *testing.T) {
 	// 默认 30 分钟 interval：普通失败 24~36 分钟，unsupported 为其 8 倍。
 	stretched := unsupportedProbeDelay(30, 0)
@@ -785,8 +785,8 @@ func TestUpstreamBillingProbeUnsupportedDelayIsStretchedAndBounded(t *testing.T)
 	require.LessOrEqual(t, unsupportedProbeDelay(upstreamBillingProbeMaxIntervalMinutes, 0), upstreamBillingProbeMaxDelay)
 	require.Positive(t, unsupportedProbeDelay(upstreamBillingProbeMinIntervalMinutes, 0))
 
-	// Retry-After 更长时原样保留，不被封顶缩短；更短时至少不早于该指令。
-	require.Equal(t, 48*time.Hour, unsupportedProbeDelay(30, 48*time.Hour))
+	// Retry-After 更长时受封顶保护，不超过 upstreamBillingProbeMaxDelay；更短时至少不早于该指令。
+	require.Equal(t, upstreamBillingProbeMaxDelay, unsupportedProbeDelay(30, 48*time.Hour))
 	require.GreaterOrEqual(t, unsupportedProbeDelay(30, time.Hour), time.Hour)
 }