Butterfish Shell is built for people who live in the terminal. Installing it via Homebrew or go install and running butterfish shell spawns a new instance of your normal bash or zsh, with the tool intercepting standard input and output. From then on, typing a command that begins with a capital letter routes it to an AI model instead of the shell — you can ask "Why didn't that work?" and the model sees your recent history and command output to answer in context. No copy-pasting between browser and terminal.
Beyond prompting, the tool offers GPT-powered autosuggestions, similar to GitHub Copilot but inside the shell, which you accept with Tab. Prompts are kept visible and editable in a YAML file rather than hidden, so you control the instructional text sent to the model. Models are configurable too: it defaults to an OpenAI model but accepts any OpenAI-compatible endpoint through a base URL flag, including local model servers.
Goal Mode is the agentic piece. Starting a command with ! hands a goal to the model, which proposes commands you confirm with Enter; starting with !! lets it execute them without confirmation. The project is honest that this mode is hit-or-miss and works best for simple, well-guided goals such as finding large files or listing Go files recursively. An accompanying Neovim plugin, butterfish.nvim, brings the same prompting style into the editor.
