FsLangMCP — semantic F# for your AI coding agent

Stop your agent grepping F#. Give it the real compiler: cross-project find, a trustworthy check verdict, types, rename, dead-code — over MCP.

Why

Text search misses the semantics F# depends on — partial application, aliased opens, shadowed bindings, cross-project uses, record-field set-sites, CE custom-operations — and over-matches comments and strings. FsLangMCP resolves symbols via the real compiler (FCS in-process + FSAC LSP), so your agent gets trustworthy answers instead of grep noise.

In a live-execution A/B, shaping the tool surface to agent intent cut agents' rg/grep-fallback from 56% → 28% and steps-to-completion from 10.4 → 5.8 (success rate 89% → 100%). Full story: docs/why-agents-grep-fsharp.md.

Quickstart

Prerequisites: .NET SDK 10+ on PATH.

dotnet tool install -g FsLangMcp
fslangmcp --bootstrap-tools   # one-time: fetches fsautocomplete + ionide.projinfo.tool

Add to your MCP client config:

{
  "mcpServers": {
    "fslangmcp": { "command": "fslangmcp" }
  }
}

Then call set_project with your .fsproj or .sln path, and start with find or check.

Full setup: docs/getting-started.md · Per-client configs: docs/configuration.md · Runnable examples: examples/.

Changelog

CHANGELOG.md

What You Get

35 tools in 8 groups. Full reference: docs/tools-reference.md.

Headline tools

Navigate / understand

Diagnose / fix

Refactor preview

Review / cleanup

Analyzers

NuGet / types

Raw LSP proxies

Require set_project. Prefer the semantic tools above for agent flows; these are useful for exact-position editor operations and FSAC debugging.

Meta

Example Agent Session

1. set_project  {"projectPath": "/abs/path/MyApp.sln"}
   → readiness.lsp=true, loadedProjects=[...], fslangmcpVersion="0.12.1"

2. check  {}
   → verdict="clean"

3. find  {"query": "OrderId", "kind": "definition"}
   → [Domain/Order.fs:12 definition `type OrderId = ...`, ...]

4. fcs_file_outline  {"path": "/abs/path/Domain/Order.fs"}
   → module/type headers, memberCounts by kind

5. fcs_rename_preview  {"path": "/abs/path/Domain/Order.fs", "line": 12, "character": 5, "newName": "OrderIdentifier"}
   → edits grouped by file, totalEdits=7, crossProject=false

Runtime Options

Command-line args:

• --project <path> / -p <path> — pre-load a project on startup
• --fsac-command <cmd> — override the fsautocomplete executable
• --fsac-args "<args>" — pass extra args to FSAC
• --bootstrap-tools — install/update fsautocomplete + ionide.projinfo.tool

Environment fallbacks: FSAC_COMMAND, FSAC_ARGS, FSA_PROJECT_PATH.

Garbage collector

FsLangMCP ships with Workstation GC + Concurrent GC baked in via runtimeconfig.template.json. This is the recommended profile for stdio MCP servers per FsMcp's runtime tuning guide: FCS workloads are bursty (load → idle → next request), and Server GC's "don't release until OS pressure" produces alarming RSS growth on dev laptops. Override if needed:

• DOTNET_gcServer=1 — opt into Server GC (higher throughput, holds memory longer)
• DOTNET_GCHeapHardLimitPercent=0xA — cap heap at 10% of RAM (either GC mode)

MCP Stdio Config

Installed tool:

{
  "mcpServers": {
    "fsharp": {
      "command": "fslangmcp",
      "args": ["--project", "/absolute/path/to/App.fsproj"]
    }
  }
}

Local dev (without install):

{
  "mcpServers": {
    "fsharp": {
      "command": "dotnet",
      "args": [
        "run", "--project", "/path/to/FsLangMcp.fsproj",
        "--", "--project", "/absolute/path/to/App.fsproj"
      ]
    }
  }
}

Parallel Agent Usage

Pass projectPath explicitly on every FCS tool call — FCS tools cache project-wide results per resolved .fsproj, so agents targeting different projects share no stale caches.

{ "path": "/absolute/path/to/File.fs", "projectPath": "/absolute/path/to/App.fsproj" }

Concurrency limits:

• FSLANGMCP_MAX_CONCURRENT_FCS=2
• LSP tools are always serialized. FSAC owns one mutable workspace, so LSP concurrency is intentionally not configurable.

Process and RPC timeouts (milliseconds):

• FSLANGMCP_LSP_STARTUP_TIMEOUT_MS=60000
• FSLANGMCP_LSP_REQUEST_TIMEOUT_MS=30000
• FSLANGMCP_PROJ_INFO_TIMEOUT_MS=120000
• FSLANGMCP_BOOTSTRAP_TIMEOUT_MS=300000

Response Shape

• Success: {"status": "ok", ...fields} or {"status": "ok", "result": <payload>}
• Not ready: {"status": "not_ready", "message": "..."} — FSAC workspace still loading
• Invalid args: {"status": "invalid_args", "message": "..."} — required arg blank/missing
• Error: MCP protocol error with {"errorKind": "...", "message": "..."} payload

LSP positions (line, character) are 0-based.

Development Commands

just restore
just check    # build + tests
just analyze  # F# analyzers via FSharp.Analyzers.Build

Wired analyzers: Ionide.Analyzers, G-Research.FSharp.Analyzers, FSharp.Analyzers.Build.

For local development, restore the local analyzer tool:

dotnet tool restore

Known Issues

• LSP proxy tools return {"status": "not_ready"} if called before fsautocomplete finishes loading. set_project waits up to 30 seconds for workspace readiness; the preceding RPC handshake has its own configurable 60-second timeout and kills/reset FSAC on expiry.
• FCS does not expose cancellation once synchronous project-option evaluation or GetAllUsesOfAllSymbols() has started. find still returns at its requested timeout and reuses one in-flight task per project/cache key on retries, but that computation may continue in the background until FCS returns. Restart FsLangMCP to terminate a genuinely stuck FCS operation.
• FCS tools fall back to script-style inference (GetProjectOptionsFromScript) only when no .fsproj is found. Diagnostics and symbol data can be incomplete for multi-project solutions in this mode.
• project_health is project-focused — it does not inspect whole solutions or resolve ambiguous directories.

About Tool Dependencies

dotnet tool packages cannot automatically install other global tools during installation. fslangmcp --bootstrap-tools runs dotnet tool update -g (or install if missing) for:

• fsautocomplete
• ionide.projinfo.tool

To install them manually instead:

dotnet tool install -g fsautocomplete
dotnet tool install -g ionide.projinfo.tool