diff --git a/internal/runtime/executor/claude_executor.go b/internal/runtime/executor/claude_executor.go
index 7a9f1005..9c291328 100644
--- a/internal/runtime/executor/claude_executor.go
+++ b/internal/runtime/executor/claude_executor.go
@@ -733,6 +733,11 @@ func applyClaudeToolPrefix(body []byte, prefix string) []byte {
 
 	if tools := gjson.GetBytes(body, "tools"); tools.Exists() && tools.IsArray() {
 		tools.ForEach(func(index, tool gjson.Result) bool {
+			// Skip built-in tools (web_search, code_execution, etc.) which have
+			// a "type" field and require their name to remain unchanged.
+			if tool.Get("type").Exists() && tool.Get("type").String() != "" {
+				return true
+			}
 			name := tool.Get("name").String()
 			if name == "" || strings.HasPrefix(name, prefix) {
 				return true
diff --git a/internal/runtime/executor/claude_executor_test.go b/internal/runtime/executor/claude_executor_test.go
index 05f5b60c..36fb7ad4 100644
--- a/internal/runtime/executor/claude_executor_test.go
+++ b/internal/runtime/executor/claude_executor_test.go
@@ -25,6 +25,18 @@ func TestApplyClaudeToolPrefix(t *testing.T) {
 	}
 }
 
+func TestApplyClaudeToolPrefix_SkipsBuiltinTools(t *testing.T) {
+	input := []byte(`{"tools":[{"type":"web_search_20250305","name":"web_search"},{"name":"my_custom_tool","input_schema":{"type":"object"}}]}`)
+	out := applyClaudeToolPrefix(input, "proxy_")
+
+	if got := gjson.GetBytes(out, "tools.0.name").String(); got != "web_search" {
+		t.Fatalf("built-in tool name should not be prefixed: tools.0.name = %q, want %q", got, "web_search")
+	}
+	if got := gjson.GetBytes(out, "tools.1.name").String(); got != "proxy_my_custom_tool" {
+		t.Fatalf("custom tool should be prefixed: tools.1.name = %q, want %q", got, "proxy_my_custom_tool")
+	}
+}
+
 func TestStripClaudeToolPrefixFromResponse(t *testing.T) {
 	input := []byte(`{"content":[{"type":"tool_use","name":"proxy_alpha","id":"t1","input":{}},{"type":"tool_use","name":"bravo","id":"t2","input":{}}]}`)
 	out := stripClaudeToolPrefixFromResponse(input, "proxy_")