diff --git a/backend/internal/pkg/apicompat/responses_to_anthropic.go b/backend/internal/pkg/apicompat/responses_to_anthropic.go
index 376f0d97da..2863fe7302 100644
--- a/backend/internal/pkg/apicompat/responses_to_anthropic.go
+++ b/backend/internal/pkg/apicompat/responses_to_anthropic.go
@@ -452,10 +452,20 @@ func resToAnthHandleFuncArgsDone(evt *ResponsesStreamEvent, state *ResponsesEven
 		raw = string(sanitized)
 	}
 
-	idx := state.ContentBlockIndex
+	// 从事件的 OutputIndex 解析正确的 block index，与 resToAnthHandleFuncArgsDelta 对齐
+	blockIdx, ok := state.OutputIndexToBlockIdx[evt.OutputIndex]
+	if !ok {
+		blockIdx = state.ContentBlockIndex
+	}
+
+	// 如果 block 已关闭（ContentBlockIndex 已越过它），说明 arguments 已通过 delta 流式发完，不再补发
+	if !state.ContentBlockOpen || blockIdx != state.ContentBlockIndex {
+		return nil
+	}
+
 	events := []AnthropicStreamEvent{{
 		Type:  "content_block_delta",
-		Index: &idx,
+		Index: &blockIdx,
 		Delta: &AnthropicDelta{
 			Type:        "input_json_delta",
 			PartialJSON: raw,