CodeFission

Tree-structured AI coding assistant for exploring and experimenting with code. Each conversation node is an isolated git worktree — branch to try alternative approaches, and every branch gets its own filesystem sandbox.

Example: research workflow

The canvas works for open-ended research: branch from a root prompt, iterate on sub-questions, and keep parallel threads visible — sticky notes and deep trees included.

Example: duplicating a session

Use Duplicate on a node to spin up another branch from the same starting point — handy when you want the same prompt but different tool runs, follow-ups, or outcomes side by side.

Example: start a tree from commit history

Open the git graph view for a project to see branches and commits. Pick any commit and use Plant new tree here to spin up a new conversation tree rooted at that exact revision — useful for revisiting an older state or exploring “what if” from a specific point in history.

Prerequisites

• uv — Python package manager (used to install CodeFission)
• An AI backend — Claude Code or Codex CLI (you only need one)
• git — worktree isolation. Usually pre-installed.

Install uv

uv is a fast Python package manager. If you don't have it:

# macOS / Linux
curl -LsSf https://astral.sh/uv/install.sh | sh

# Windows
powershell -ExecutionPolicy ByPass -c "irm https://astral.sh/uv/install.ps1 | iex"

You don't need to install Python separately — uv manages that for you.

Authenticate Claude Code

Install and log in with your Anthropic account (Claude Pro/Max) or API key:

npm install -g @anthropic-ai/claude-code
claude login

Alternatively, set ANTHROPIC_API_KEY in your environment.

Authenticate Codex CLI

Install and log in with your OpenAI account:

npm install -g @openai/codex

Then authenticate using one of:

codex login              # OpenAI API key (paid API access)
codex login --chatgpt    # ChatGPT Plus or Pro account

Alternatively, set OPENAI_API_KEY in your environment.

Install

uv tool install codefission

This installs fission as a standalone command on your system. uv creates an isolated environment for it automatically — no Python environment management needed on your end.

Prefer pip? pip install codefission works too.

Then run:

fission

Opens as a desktop app on first launch (downloads ~80 MB once). Use fission --browser to open in a browser instead.

fission              # desktop app (default)
fission --browser    # browser mode
fission --desktop    # force desktop app

Updates

When a new version is available, fission will notify you on launch and offer to upgrade automatically. You can also check manually:

fission --update

Install from source

Requires uv and Node.js 20.19+ or 22.12+.

git clone https://github.com/codefission-ai/CodeFission.git
cd CodeFission
make install
fission

For development with auto frontend rebuild:

make dev

How it works

Create a tree in the sidebar, type a message, and CodeFission spawns a Claude Code session in an isolated git worktree. Branch any node to explore alternatives — each branch forks the conversation context and the filesystem state.

         [root]
        /      \
   [add auth]  [add auth]     <- same prompt, different approaches
      |            |
 [fix tests]  [add logging]   <- independent follow-ups

Every node tracks its git branch, commit, and Claude session. Child nodes fork from the parent's prompt cache so context carries over without re-sending history.

Authentication

Configurable in the Settings panel (gear icon in sidebar):

• CLI (OAuth) — default. Uses your claude login session. No API key needed.
• API Key — provide an Anthropic API key in settings. Useful for headless/remote setups.

Both modes require the Claude Code CLI binary to be installed.

Configuration

Open Settings (gear icon) to configure:

• Global defaults — provider, model, max turns, auth mode. Applies to all trees.
• Per-tree overrides — provider, model, max turns. Leave as "Default" to inherit global settings.

Settings persist in the backend database across sessions and devices.

Development

make dev          # install + build frontend + run server
make test         # run tests
make build        # build wheel for PyPI
make publish      # build + upload to PyPI
make clean        # remove build artifacts

Frontend dev server (hot reload):

cd ui
npm run dev

Data is stored in ~/.codefission/ (SQLite database, git worktrees). Override with CODEFISSION_DATA_DIR env var.

Platform support

Windows users: if you hit issues, open a GitHub issue and I'll respond within 3 hours.

Support

Found a bug or something not working? Open an issue — I respond to all issues within 3 hours.