Pi (coding agent)
Mario Zechner's minimal open-source coding agent, with four tools and a system prompt under 1,000 tokens.
Pi is Mario Zechner's minimal open-source coding agent, built on the rule "if I don't need it, it won't be built" (Zechner, 2025).
It ships four tools by default: read, write, edit and bash. The system prompt and tool definitions together come in under 1,000 tokens, plus the user's AGENTS.md files. It deliberately leaves out MCP, sub-agents, plan mode and built-in to-do lists. The model keeps plans and tasks in files like PLAN.md and TODO.md, and can start another copy of itself through bash. Zechner reports competitive Terminal-Bench 2.0 results with Claude Opus 4.5.
The harness in Building an agent harness from scratch to pass Ian Goodfellow's intelligence test follows its four-tool design. See also Thin Harnesses.
References
Mario Zechner. What i learned building an opinionated and minimal coding agent. 2025. Published 30 November 2025. Accessed 8 October 2026. URL: https://mariozechner.at/posts/2025-11-30-pi-coding-agent/ (visited on 2026-10-08). ↩