FsMcp

Build MCP servers and clients in F# with typed cancellation, explicit errors, secure hosting composition, and honest runtime boundaries.

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 cancellationToken -> task {
        cancellationToken.ThrowIfCancellationRequested()
        let g = args.greeting |> Option.defaultValue "Hello"
        return Ok [ Content.text $"{g}, {args.name}!" ]
    }) |> unwrapResult)
}

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

Packages

Package	Install	Description
FsMcp.Core	dotnet add package FsMcp.Core	Domain types, validation, serialization
FsMcp.Server	dotnet add package FsMcp.Server	Server builder CE, cancellable typed handlers, stdio
FsMcp.Client	dotnet add package FsMcp.Client	Typed client and enterprise-managed authorization
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	Streamable HTTP and ASP.NET Core composition
FsMcp.Sampling	dotnet add package FsMcp.Sampling	Sampling types and explicit test helpers

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
• Request cancellation — the SDK request token reaches every primary handler
• Secure composition — caller-owned DI, SDK filters, and ASP.NET authorization
• Enterprise-managed authorization — validated ID-JAG client flow with bounded refresh/retry
• Fail-closed compatibility — disconnected 1.x runtime surfaces no longer pretend to work

FsMcp 2.0 is a breaking release. McpClient.readResource returns the complete ResourceContents list, handler signatures take a final CancellationToken, and transport selection belongs to Server.run or FsMcp.Server.Http, not the server computation expression.