FsMcp

Build MCP servers in F# with type safety, computation expressions, and zero boilerplate.

FsMcp wraps Microsoft's ModelContextProtocol .NET SDK with an idiomatic F# API.

type GreetArgs = { name: string; greeting: string option }

let server = mcpServer {
    name "MyServer"
    version "1.0.0"

    tool (TypedTool.define<GreetArgs> "greet" "Greets" (fun args -> task {
        let g = args.greeting |> Option.defaultValue "Hello"
        return Ok [ Content.text $"{g}, {args.name}!" ]
    }) |> unwrapResult)

    useStdio
}

Server.run server |> fun t -> t.GetAwaiter().GetResult()

Packages

Package	Install	Description
FsMcp.Server	dotnet add package FsMcp.Server	Server builder CE, typed handlers, middleware, stdio
FsMcp.Client	dotnet add package FsMcp.Client	Typed client with Result-based errors
FsMcp.Testing	dotnet add package FsMcp.Testing	TestServer, assertions, FsCheck generators
FsMcp.TaskApi	dotnet add package FsMcp.TaskApi	FsToolkit.ErrorHandling pipeline
FsMcp.Server.Http	dotnet add package FsMcp.Server.Http	HTTP/SSE transport (opt-in ASP.NET)
FsMcp.Sampling	dotnet add package FsMcp.Sampling	LLM sampling from server tools

Why FsMcp?

• mcpServer { } CE — declare tools, resources, prompts in one block
• TypedTool.define<'T> — F# record as input, JSON Schema auto-generated via TypeShape
• Result<'T, McpError> — typed errors, no exceptions in expected paths
• Smart constructors — ToolName.create validates at construction, not at runtime
• Composable middleware — logging, validation, telemetry via pipeline
• 306 tests — Expecto + FsCheck property tests on every domain type