PTC-Decoder is a training-free, plug-and-play decoder framework for improving the reliability of small language models (SLMs) on offline, resource-constrained edge devices. It uses a Plan-to-Act paradigm and a deterministic finite automaton to impose token-level hard constraints on tool names, retaining SLM reasoning capability and improving performance by 1.21 mean overall score gain on 200 remote-sensing satellite tasks.