diff --git a/service/text_quota.go b/service/text_quota.go
index 0d452fba..4f6e4c11 100644
--- a/service/text_quota.go
+++ b/service/text_quota.go
@@ -165,9 +165,14 @@ func composeTieredTextQuota(relayInfo *relaycommon.RelayInfo, summary textQuotaS
 		}
 	}
 
-	surcharge, clamp := common.QuotaFromDecimalChecked(summary.ToolCallSurchargeQuota)
+	// Saturate the final sum, not just the surcharge: tieredQuota can be near
+	// MaxQuota and adding the surcharge could push the total past the int32
+	// quota policy bound (persisted quota columns are 32-bit).
+	total, clamp := common.QuotaFromDecimalChecked(
+		decimal.NewFromInt(int64(tieredQuota)).Add(summary.ToolCallSurchargeQuota),
+	)
 	noteQuotaClamp(relayInfo, clamp)
-	return tieredQuota + surcharge
+	return total
 }
 
 func calculateTextQuotaSummary(ctx *gin.Context, relayInfo *relaycommon.RelayInfo, usage *dto.Usage) textQuotaSummary {