Fantomas review
A focused open-source formatter for F# projects across .NET editors and CI workflows.
Reviewed by iTechGuides Editors · Editorial team · Updated Oct 2026
Fantomas is an open-source formatter for F# source code, distributed as a .NET tool with an online formatter. It formats individual files or folders from the command line and supports configuration through .editorconfig. Developers working in F# projects can also check formatting without writing changes, making it suitable for CI workflows and style enforcement. Its focus is particularly relevant to teams using .NET-based development environments and F# codebases that need repeatable formatting rules.
Its strongest fit is the combination of command-line, editor, and automation features. Fantomas integrates with Ionide, FSAutoComplete, Visual Studio, Visual Studio Code, and JetBrains Rider, while an LSP-like daemon supports editor tooling. JSON output is available for scripts and coding agents, and .fantomasignore files use gitignore syntax to exclude selected paths. These options give teams several ways to place formatting into local development and automated workflows without limiting the tool to a single editor.
Fantomas also extends beyond a standalone formatter through Fantomas.Core and the CodeFormatter API, allowing programmatic formatting. That makes it a practical choice for developers who need both an established command-line workflow and a library interface. The trade-off is its narrow language scope: projects that require formatting for languages other than F# will need a separate tool. Choose Fantomas when F# consistency, editor integration, configuration, and CI checking are the priority; choose an alternative when broad multi-language coverage is central.
Fantomas pros and cons
- Where it wins
- Formats F# files and folders with .editorconfig configuration
- Checks formatting without changing files for CI workflows
- Supports editors, JSON output, ignore rules, and a library API
- Where it doesn't
- Focused on F# rather than multi-language formatting
- Platform coverage is limited to web and self-hosted use
- Editor support depends on the listed integrations
Fantomas fact sheet, pricing and score →
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