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FSI-MCP-Server Setup for Neovim

This guide explains how to use fsi-mcp-server in your Neovim setup for F# development.

What is fsi-mcp-server?

fsi-mcp-server is an enhanced F# Interactive (FSI) server that provides Model Context Protocol (MCP) capabilities, offering better integration and features compared to the standard dotnet fsi.

Installation

Install fsi-mcp-server

First, you need to install fsi-mcp-server. You can typically install it via:

# Using dotnet tool (if available)
dotnet tool install -g fsi-mcp-server

# Or via npm (if it's packaged that way)
npm install -g fsi-mcp-server

Check if it's installed correctly:

fsi-mcp-server --version

How It Works

Your Neovim configuration now automatically detects and prefers fsi-mcp-server when available:

  1. Automatic Detection: When you send F# code to the REPL (using <M-CR>), Neovim checks:

    • Is fsi-mcp-server available? → Use it
    • Otherwise → Fall back to standard dotnet fsi via Ionide

  2. Manual Terminal Control: You can explicitly open FSI terminals:

    • <leader>tf - Auto-detect and use best available FSI
    • <leader>tm - Force use of fsi-mcp-server
    • <leader>td - Force use of standard dotnet fsi

Key Mappings

Existing (now enhanced)

  • <M-CR> (Alt+Enter) in normal mode: Send current line to FSI REPL
  • <M-CR> (Alt+Enter) in visual mode: Send selected lines to FSI REPL

These now prefer fsi-mcp-server when available!

New Terminal Toggles

  • <leader>tf: Toggle FSI REPL (auto-detects best option)
  • <leader>tm: Toggle FSI-MCP REPL (forces fsi-mcp-server)
  • <leader>td: Toggle Dotnet FSI REPL (forces standard fsi)

Usage Example

Basic Workflow

  1. Open an F# file (.fs or .fsx)
  2. Press <leader>tf to open the FSI REPL (will use fsi-mcp-server if installed)
  3. Write some F# code: 
    let add x y = x + y
let result = add 5 3
  4. In normal mode, put cursor on a line and press <Alt-Enter> to send it to REPL
  5. Or select multiple lines in visual mode and press <Alt-Enter> to evaluate them

Forcing Specific FSI Version

If you want to test with standard FSI even when fsi-mcp-server is installed:

  1. Press <leader>td to open standard dotnet fsi
  2. Then use <Alt-Enter> as usual (will use the open terminal)

Configuration Files Modified

The following files were created/modified:

  1. lua/plugins/fsi-mcp.lua (NEW)

    • Core functionality for fsi-mcp-server integration
    • Auto-detection logic
    • Terminal management
    • Send-to-REPL functions

  2. lua/config/keymaps.lua (MODIFIED)

    • Updated <M-CR> mappings to prefer fsi-mcp-server
    • Falls back gracefully to Ionide's FSI if fsi-mcp-server isn't available

Troubleshooting

"fsi-mcp-server not found"

This means the executable isn't in your PATH. To fix:

  1. Install fsi-mcp-server (see Installation section)
  2. Ensure it's in your PATH: 
    $env:PATH
  3. Restart Neovim

Falls back to dotnet fsi

This is expected if fsi-mcp-server isn't installed. The setup gracefully falls back to standard FSI.

Neither FSI works

Ensure you have .NET SDK installed:

dotnet --version

Advantages of fsi-mcp-server

When using fsi-mcp-server over standard FSI, you get:

  • Better integration with MCP-compatible tools
  • Enhanced REPL capabilities
  • Improved code completion context
  • Better error reporting

Reverting to Standard FSI

If you want to completely disable fsi-mcp-server preference:

  1. Edit lua/config/keymaps.lua
  2. Find the lines checking vim.fn.executable("fsi-mcp-server")
  3. Comment them out to always use Ionide's standard FSI

Or simply don't install fsi-mcp-server - the configuration will automatically use standard FSI.

Additional Resources