diff --git a/backend/internal/pkg/antigravity/request_transformer.go b/backend/internal/pkg/antigravity/request_transformer.go
index adafa196..9b703187 100644
--- a/backend/internal/pkg/antigravity/request_transformer.go
+++ b/backend/internal/pkg/antigravity/request_transformer.go
@@ -389,10 +389,13 @@ func buildParts(content json.RawMessage, toolIDToName map[string]string, allowDu
 					ID:   block.ID,
 				},
 			}
-			// 只有 Gemini 模型使用 dummy signature
-			// Claude 模型不设置 signature（避免验证问题）
+			// tool_use 的 signature 处理：
+			// - Gemini 模型：使用 dummy signature（跳过 thought_signature 校验）
+			// - Claude 模型：透传上游返回的真实 signature（Vertex/Google 需要完整签名链路）
 			if allowDummyThought {
 				part.ThoughtSignature = dummyThoughtSignature
+			} else if block.Signature != "" && block.Signature != dummyThoughtSignature {
+				part.ThoughtSignature = block.Signature
 			}
 			parts = append(parts, part)
 
diff --git a/backend/internal/pkg/antigravity/request_transformer_test.go b/backend/internal/pkg/antigravity/request_transformer_test.go
index eca3107e..60ee6f63 100644
--- a/backend/internal/pkg/antigravity/request_transformer_test.go
+++ b/backend/internal/pkg/antigravity/request_transformer_test.go
@@ -114,7 +114,7 @@ func TestBuildParts_ToolUseSignatureHandling(t *testing.T) {
 		}
 	})
 
-	t.Run("Claude model - no signature for tool_use", func(t *testing.T) {
+	t.Run("Claude model - preserve valid signature for tool_use", func(t *testing.T) {
 		toolIDToName := make(map[string]string)
 		parts, _, err := buildParts(json.RawMessage(content), toolIDToName, false)
 		if err != nil {
@@ -123,9 +123,9 @@ func TestBuildParts_ToolUseSignatureHandling(t *testing.T) {
 		if len(parts) != 1 || parts[0].FunctionCall == nil {
 			t.Fatalf("expected 1 functionCall part, got %+v", parts)
 		}
-		// Claude 模型不设置 signature
-		if parts[0].ThoughtSignature != "" {
-			t.Fatalf("expected no tool signature for Claude, got %q", parts[0].ThoughtSignature)
+		// Claude 模型应透传有效的 signature（Vertex/Google 需要完整签名链路）
+		if parts[0].ThoughtSignature != "sig_tool_abc" {
+			t.Fatalf("expected preserved tool signature %q, got %q", "sig_tool_abc", parts[0].ThoughtSignature)
 		}
 	})
 }