Programmable logic controllers (PLCs) run much of the world’s industrial automation, but writing and validating their IEC~61131-3 Structured Text (ST) is time-consuming and expert-intensive. Large language models (LLMs) can draft ST from a natural-language description, but existing open-source PLC editors give users no way to check that the generated program behaves as specified, or to simulate it before deployment. We present SemaPLC, an open-source IDE that closes this gap. From a natural-language description, a model-agnostic LLM agent drafts ST and SemaPLC runs a closed loop over it: it compiles, deploys, and verifies the user’s scenarios into a pass/fail behavioral report, repairing the code from feedbacks until it compiles and every scenario passes. It then renders a live in-browser process simulation alongside the ST source, an automatically derived ladder view, and real-time variables, all in synchronized panels. SemaPLC ships as two reusable open-source components, a web IDE and a PLC tool suite. Across classic control scenarios, SemaPLC turns natural-language prompts into behaviorally verified and simulated programs, putting a trustworthy, hardware-free PLC programming agent into a single browser tab.