pi-from-scratch

Pi-from-scratch rebuilds a working coding agent, one that reads files, edits code, and runs shell commands, in about 600 lines of TypeScript. It's a deliberate deconstruction of the pi agent framework, delivered as an interactive web tutorial where the code appears progressively as you read, with a trace debugger for stepping through the agent loop. MIT licensed, free, bring an OpenAI-compatible API key.

Setup is Node 22 and npm install, or just read the hosted version. The tutorial prose is primarily in Chinese, so non-Chinese readers will lean on translation.

Read it if you use coding agents daily and can't explain what happens between your prompt and the file edit. It also works as a skeleton for a custom agent. The upstream pi framework is the production version; Anthropic's and OpenAI's agent SDKs are what you'd actually build on.

The catch: this is pedagogy, not infrastructure. Its entire value is that it stays small enough to read in a sitting, which is exactly why nobody should ship it.