Runprompt is a command-line tool that executes .prompt files (a Google-defined prompt specification format) against various LLM providers. It allows developers to define prompts with metadata, model selection, and schema in YAML-frontmatter files, then run them from the terminal with JSON data input. The tool supports multiple providers including OpenAI, Anthropic, Google Gemini, Ollama, and OpenRouter, with built-in features like request caching.