diff --git a/backend/internal/pkg/antigravity/request_transformer.go b/backend/internal/pkg/antigravity/request_transformer.go
index 1750848348..5f9e74bc6f 100644
--- a/backend/internal/pkg/antigravity/request_transformer.go
+++ b/backend/internal/pkg/antigravity/request_transformer.go
@@ -148,21 +148,20 @@ func TransformClaudeToGeminiWithOptions(claudeReq *ClaudeRequest, projectID, map
 	}
 
 	// 针对 Gemini Reasoning 模型（如 gemini-3.1-pro-high等）过滤强制空 ToolConfig
-	isReasoning := IsGeminiReasoningModel(targetModel)
-	if !isReasoning || len(tools) > 0 {
-		// 总是设置 toolConfig，与官方客户端一致
-		innerRequest.ToolConfig = &GeminiToolConfig{
-			FunctionCallingConfig: &GeminiFunctionCallingConfig{
-				Mode: "VALIDATED",
-			},
-		}
-		// 内置工具（googleSearch）与函数调用混用时，上游要求显式开启
-		// includeServerSideToolInvocations，否则返回 400（issue #5709）。
-		// 与 raw 透传路的 enableMixedGeminiToolInvocations 注入保持同一语义。
-		if hasMixedToolInvocations(tools) {
-			enabled := true
-			innerRequest.ToolConfig.IncludeServerSideToolInvocations = &enabled
-		}
+	// toolConfig must always be present: upstream rejects requests without it,
+	// including reasoning models called without any tools.
+	// 总是设置 toolConfig，与官方客户端一致
+	innerRequest.ToolConfig = &GeminiToolConfig{
+		FunctionCallingConfig: &GeminiFunctionCallingConfig{
+			Mode: "VALIDATED",
+		},
+	}
+	// 内置工具（googleSearch）与函数调用混用时，上游要求显式开启
+	// includeServerSideToolInvocations，否则返回 400（issue #5709）。
+	// 与 raw 透传路的 enableMixedGeminiToolInvocations 注入保持同一语义。
+	if hasMixedToolInvocations(tools) {
+		enabled := true
+		innerRequest.ToolConfig.IncludeServerSideToolInvocations = &enabled
 	}
 
 	if systemInstruction != nil {
diff --git a/backend/internal/pkg/antigravity/request_transformer_test.go b/backend/internal/pkg/antigravity/request_transformer_test.go
index 05764ef7ff..62d038640c 100644
--- a/backend/internal/pkg/antigravity/request_transformer_test.go
+++ b/backend/internal/pkg/antigravity/request_transformer_test.go
@@ -621,3 +621,38 @@ func TestGeminiToolConfig_IncludeServerSideToolInvocations(t *testing.T) {
 		require.NotContains(t, raw, "includeServerSideToolInvocations")
 	})
 }
+
+// TestToolConfigAlwaysPresent ensures toolConfig is always emitted, including for
+// reasoning models without any tools: upstream rejects requests that omit it.
+func TestToolConfigAlwaysPresent(t *testing.T) {
+	cases := []struct {
+		name  string
+		model string
+		tools []ClaudeTool
+	}{
+		{name: "reasoning model without tools", model: "gemini-3.1-pro-high"},
+		{name: "reasoning model with tools", model: "gemini-3.1-pro-high", tools: []ClaudeTool{{
+			Name:        "web_search",
+			Description: "Search the web",
+			InputSchema: map[string]any{"type": "object", "properties": map[string]any{}},
+		}}},
+		{name: "non-reasoning model without tools", model: "gemini-3.1-pro"},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			claudeReq := &ClaudeRequest{
+				Model:    tc.model,
+				Messages: []ClaudeMessage{{Role: "user", Content: json.RawMessage(`"Hello"`)}},
+				Tools:    tc.tools,
+			}
+			body, err := TransformClaudeToGeminiWithOptions(claudeReq, "project-1", tc.model, DefaultTransformOptions())
+			require.NoError(t, err)
+
+			var req V1InternalRequest
+			require.NoError(t, json.Unmarshal(body, &req))
+			require.NotNil(t, req.Request.ToolConfig, "toolConfig must be present")
+			require.NotNil(t, req.Request.ToolConfig.FunctionCallingConfig)
+			require.Equal(t, "VALIDATED", req.Request.ToolConfig.FunctionCallingConfig.Mode)
+		})
+	}
+}