Simon Willison’s Weblog

9th December 2025 - Link Blog

mistralai/mistral-vibe. Here's the Apache 2.0 licensed source code for Mistral's new "Vibe" CLI coding agent, released today alongside Devstral 2.

It's a neat implementation of the now standard terminal coding agent pattern, built in Python on top of Pydantic and Rich/Textual (here are the dependencies.) Gemini CLI is TypeScript, Claude Code is closed source (TypeScript, now on top of Bun), OpenAI's Codex CLI is Rust. OpenHands is the other major Python coding agent I know of, but I'm likely missing some others. (UPDATE: Kimi CLI is another open source Apache 2 Python one.)

The Vibe source code is pleasant to read and the crucial prompts are neatly extracted out into Markdown files. Some key places to look:

• core/prompts/cli.md is the main system prompt ("You are operating as and within Mistral Vibe, a CLI coding-agent built by Mistral AI...")
• core/prompts/compact.md is the prompt used to generate compacted summaries of conversations ("Create a comprehensive summary of our entire conversation that will serve as complete context for continuing this work...")
• Each of the core tools has its own prompt file:
  • .../prompts/bash.md
  • .../prompts/grep.md
  • .../prompts/read_file.md
  • .../prompts/write_file.md
  • .../prompts/search_replace.md
  • .../prompts/todo.md

The Python implementations of those tools can be found here.

I tried it out and had it build me a Space Invaders game using three.js with the following prompt:

make me a space invaders game as HTML with three.js loaded from a CDN

Here's the source code and the live game (hosted in my new space-invaders-by-llms repo). It did OK.