diff --git a/backend/internal/pkg/antigravity/request_transformer.go b/backend/internal/pkg/antigravity/request_transformer.go
index 1b21bd58..63f6ee7c 100644
--- a/backend/internal/pkg/antigravity/request_transformer.go
+++ b/backend/internal/pkg/antigravity/request_transformer.go
@@ -367,8 +367,10 @@ func buildParts(content json.RawMessage, toolIDToName map[string]string, allowDu
 				Text:    block.Thinking,
 				Thought: true,
 			}
-			// 保留原有 signature（Claude 模型需要有效的 signature）
-			if block.Signature != "" {
+			// signature 处理：
+			// - Claude 模型（allowDummyThought=false）：必须是上游返回的真实 signature（dummy 视为缺失）
+			// - Gemini 模型（allowDummyThought=true）：优先透传真实 signature，缺失时使用 dummy signature
+			if block.Signature != "" && (allowDummyThought || block.Signature != dummyThoughtSignature) {
 				part.ThoughtSignature = block.Signature
 			} else if !allowDummyThought {
 				// Claude 模型需要有效 signature；在缺失时降级为普通文本，并在上层禁用 thinking mode。
@@ -407,12 +409,12 @@ func buildParts(content json.RawMessage, toolIDToName map[string]string, allowDu
 				},
 			}
 			// 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 {
+			// - Claude 模型（allowDummyThought=false）：必须是上游返回的真实 signature（dummy 视为缺失）
+			// - Gemini 模型（allowDummyThought=true）：优先透传真实 signature，缺失时使用 dummy signature
+			if block.Signature != "" && (allowDummyThought || block.Signature != dummyThoughtSignature) {
 				part.ThoughtSignature = block.Signature
+			} else if allowDummyThought {
+				part.ThoughtSignature = dummyThoughtSignature
 			}
 			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 60ee6f63..9d62a4a1 100644
--- a/backend/internal/pkg/antigravity/request_transformer_test.go
+++ b/backend/internal/pkg/antigravity/request_transformer_test.go
@@ -100,7 +100,7 @@ func TestBuildParts_ToolUseSignatureHandling(t *testing.T) {
 		{"type": "tool_use", "id": "t1", "name": "Bash", "input": {"command": "ls"}, "signature": "sig_tool_abc"}
 	]`
 
-	t.Run("Gemini uses dummy tool_use signature", func(t *testing.T) {
+	t.Run("Gemini preserves provided tool_use signature", func(t *testing.T) {
 		toolIDToName := make(map[string]string)
 		parts, _, err := buildParts(json.RawMessage(content), toolIDToName, true)
 		if err != nil {
@@ -109,6 +109,23 @@ func TestBuildParts_ToolUseSignatureHandling(t *testing.T) {
 		if len(parts) != 1 || parts[0].FunctionCall == nil {
 			t.Fatalf("expected 1 functionCall part, got %+v", parts)
 		}
+		if parts[0].ThoughtSignature != "sig_tool_abc" {
+			t.Fatalf("expected preserved tool signature %q, got %q", "sig_tool_abc", parts[0].ThoughtSignature)
+		}
+	})
+
+	t.Run("Gemini falls back to dummy tool_use signature when missing", func(t *testing.T) {
+		contentNoSig := `[
+			{"type": "tool_use", "id": "t1", "name": "Bash", "input": {"command": "ls"}}
+		]`
+		toolIDToName := make(map[string]string)
+		parts, _, err := buildParts(json.RawMessage(contentNoSig), toolIDToName, true)
+		if err != nil {
+			t.Fatalf("buildParts() error = %v", err)
+		}
+		if len(parts) != 1 || parts[0].FunctionCall == nil {
+			t.Fatalf("expected 1 functionCall part, got %+v", parts)
+		}
 		if parts[0].ThoughtSignature != dummyThoughtSignature {
 			t.Fatalf("expected dummy tool signature %q, got %q", dummyThoughtSignature, parts[0].ThoughtSignature)
 		}