Arnold
Review your agent's F# with the compiler at your side.
Arnold is a desktop app for macOS and Windows, made for the work AI agents leave behind. Every diff is a real F# editor (errors, hover, go to definition, rename, quick fixes and Fantomas) while you commit the agent's changes line by line, review the branch and see its Azure DevOps pull request through to completion. GitHub Copilot, through your organization's own subscription, explains any of it on request.
Highlights
F#, first class. Most diff viewers show F# as colored text. Arnold runs FsAutoComplete, the language server behind Ionide, in every diff: in Commit mode, in a branch review and in a pull request. Errors as you read, hover, go to definition, find references, rename everywhere, quick fixes, completion and semantic colors. In repositories that use Fantomas, committing checks the formatting first and can format exactly what is committed.
Zero setup. Nothing to install but the .NET SDK. Arnold downloads FsAutoComplete itself, and branches and pull requests get a real checkout (a worktree of their own when they need one), with tools and packages restored, so the language server sees your projects as they are, once you turn F# features on and trust the repository's build code (see Trusting a repository's build code).
The whole way, in one window. Commit what the agent left uncommitted, review the branch, push, open the pull request and review it until it is completed.
Commit, line by line. An agent's uncommitted changes in Unstaged and Staged lists, editable in the diff, with S, U and R to stage, unstage and reset files or single lines, as in Git Extensions. Changes made by the agent, an editor or a terminal show up as they happen.
Compare branches. Any branch against the branch it goes into, as its pull request will show it: merge-base → the branch's last commit, one file at a time or every file in one scrolling list, narrowed to some of its commits if you like, with F7 to step from change to change across files. A branch checked out nowhere else is checked out in your folder when you pick it, so you can always fix things right in the diff; if your folder has uncommitted changes, Arnold asks first (see Checking out).
Azure DevOps pull requests. Create them with a Markdown description and a linked work item; then the active pull requests, their discussions in the diff, comments on lines and files, suggestions, votes, and completing with the merge type of your choice.
Explain and improve with your organization's Copilot. Select code, right-click a file or folder, or ask about all the changes, and GitHub Copilot explains them in a panel beside the diff, with follow-up questions. Suggest improvements has Copilot review the changes: where they depart from the repository's own patterns, or there is a well-proven reason, a suggestion shows under the lines in the diff, saying why, for you to accept or discard. Arnold has no AI service of its own: it uses the Copilot your organization already provides, so code goes only to GitHub Copilot, under your organization's agreement and policies.
Safe by default. Checking out never throws work away unasked: when the opened folder has uncommitted changes, Arnold offers a worktree of its own or stashing them instead, and discarding them takes a second, red confirmation. Otherwise only Commit mode writes, and only when you ask. Nothing is ever force-pushed.
Keyboard first. F7 for the next change across files, S/U/R to stage, unstage and reset, and the F# keys you know from Rider and Visual Studio.
Contents: Install · Getting started · Committing · Comparing branches · Azure DevOps pull requests · F# · Explaining and improving code with AI · Keyboard shortcuts · What Arnold writes · Status · License · Development
Install
Download the installer from arnoldapp.dev or Releases: the .pkg for macOS (Apple silicon), the -setup.exe or .msi for Windows (x64). Arnold needs git on the PATH, and the .NET SDK (8 or newer) for F# language features.
- macOS: the build is not signed with a developer certificate yet. Try to open the
.pkg once, then choose Open Anyway under System Settings → Privacy & Security.
- Windows: the installers are signed by Reaptor AB. While Arnold is new, SmartScreen may still show Windows protected your PC; choose More info → Run anyway.
Start from a terminal
The installers put two commands on the PATH, arnold and the shorter ace:
arnold path/to/repository
ace .
Without a folder they open the current one. Terminals that were open during the installation have to be reopened first. Started from the Dock or the Start menu instead, Arnold reopens the last repository.
| Where the commands are | Uninstalling |
|---|
| macOS | links in /usr/local/bin to a script inside Arnold.app | delete /Applications/Arnold.app, /usr/local/bin/arnold and /usr/local/bin/ace |
| Windows | bin inside the installation folder, which is added to PATH (the user's for the -setup.exe, the machine's for the .msi) | the uninstaller takes the folder off PATH again |
This is the beginning of the README. Read the rest at the original. Read the full README ↗
