While current LLM agents utilizing paradigms like ReAct or Plan-and-Solve have established a strong foundation for step-by-step reasoning, they remain brittle in open-ended environments due to two intrinsic limitations: (1) A closed action space: These frameworks are confined to static, pre-defined toolsets, rendering them unable to adapt when required tools are missing or obsolete. (2) Myopic error recovery: Existing agents often get trapped in repetitive local retries, failing to diagnose and rectify root causes within the high-level plan. To overcome these limitations, we introduce CAR (Create And Replan), a novel architecture that incorporates a meta-tool synthesizer to dynamically augment the action space and a reflective replanning mechanism to revise global strategies. To rigorously evaluate our approach, we release ToolHop-Pro, a diagnostic benchmark with systematically pruned toolsets to simulate tool scarcity. Experiments demonstrate that CAR significantly outperforms representative baselines, validating its superior robustness where static agents fail. Code and data are available at https://github.com/Zaiz-77/car.