Sage Mode for F# development — REPL with solution or project loading, Live Testing for FREE, Hot Reload, and session management.
{
"mcpServers": {
"sagefs": {
"url": "http://localhost:37749/",
"type": "streamable-http"
}
}
}Are you the author?
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A live F# engine — hot reload, live testing, AI-native — for every editor, for free.
Is it safe?
No package registry to scan.
No authentication — any process on your machine can connect.
MIT. View license →
Is it maintained?
Last commit 2 days ago. 60 stars.
Will it work with my client?
Transport: stdio, sse, http. Works with Claude Desktop, Cursor, Claude Code, and most MCP clients.
This server supports HTTP transport. Be the first to test it — help the community know if it works.
No known vulnerabilities.
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A live F# engine — hot reload, live testing, AI-native — for every editor, for free.
SageFs is a live F# development engine. Start it once, connect any editor — VS Code, Neovim, Visual Studio, or the built-in TUI — and get sub-500ms feedback on every save: inline results, live test markers, hot reload, and AI agent access via MCP. It runs as a daemon with isolated session workers, so multiple editors and AI agents can share the same live state simultaneously.
How is SageFs different from Ionide? Ionide provides IntelliSense, diagnostics, and project support through the F# Compiler Service. SageFs adds live execution: eval any expression and see results inline, continuous test feedback on every save, and hot reload that patches your running app. Use both together — Ionide for editing, SageFs for running.
Platforms: Windows, macOS, Linux. Requires .NET 10 SDK.
Status: Active development. Used in production by the author.
🆕 Never written F#? You're in the right place.
Pick your language — each guide maps familiar concepts to F#, with runnable examples that show results the instant you press Alt+Enter.
🐍 Python · 📓 Jupyter · 🔷 C# · ☕ Java · 🟨 JS/TS · 🦀 Rust · 🧘 F# Koans
Or just dive in:
dotnet tool install --global SageFs && sagefs— then open any.fsxfile and hit Alt+Enter.
Save a .fs file. SageFs reloads it in ~100ms via Harmony runtime patching. No rebuild. No restart. Connected browsers auto-refresh via SSE. Your web app is already showing the new code before your fingers leave the keyboard.
SageFs exposes a Model Context Protocol server with an affordance-driven state machine and a deliberately small tool surface — AI agents only see tools valid for the current session state, and the core MCP path stays focused on session trust, F# evaluation, exact test execution, and failure explanation. No wasted tokens guessing. Copilot, Claude, and any MCP client can execute F# code, type-check, verify a changed behavior, and run tests against your real project.
Start SageFs once. Connect from VS Code, Neovim, Visual Studio, a terminal TUI, a GPU-rendered Raylib GUI, a web dashboard, o