Types Reference

All public domain types in FsMcp with their definitions, constructors, and usage patterns.

Content types

Content

Content carried by tool results, resource reads, and prompt messages:

type Content =
    | Text of text: string
    | Image of data: byte[] * mimeType: MimeType
    | EmbeddedResource of resource: ResourceContents

Convenience constructors:

Content.text "hello"                                    // Text
Content.image [| 0x89uy; 0x50uy |] mimeType            // Image
Content.embeddedResource (TextResource (uri, mime, "")) // EmbeddedResource

ResourceContents

Resource data -- either text with a MIME type or raw binary:

type ResourceContents =
    | TextResource of uri: ResourceUri * mimeType: MimeType * text: string
    | BlobResource of uri: ResourceUri * mimeType: MimeType * data: byte[]

Message types

McpRole

type McpRole =
    | User
    | Assistant

McpMessage

type McpMessage = {
    Role: McpRole
    Content: Content
}

Error types

McpError

All errors that can occur in the FsMcp toolkit:

type McpError =
    | ValidationFailed of errors: ValidationError list
    | ToolNotFound of name: ToolName
    | ResourceNotFound of uri: ResourceUri
    | PromptNotFound of name: PromptName
    | HandlerException of exn: exn
    | TransportError of message: string
    | ProtocolError of code: int * message: string

ValidationError

Errors from smart constructor validation:

type ValidationError =
    | EmptyValue of fieldName: string
    | InvalidFormat of fieldName: string * value: string * expected: string
    | DuplicateEntry of entryType: string * name: string

Format as a human-readable string:

ValidationError.format (EmptyValue "ToolName")
// "ToolName cannot be empty"

ValidationError.format (InvalidFormat ("ResourceUri", "bad", "valid absolute URI with scheme"))
// "ResourceUri 'bad' is invalid, expected valid absolute URI with scheme"

ValidationError.format (DuplicateEntry ("Tool", "echo"))
// "Duplicate Tool: 'echo'"

Definition types

ToolDefinition

type ToolDefinition = {
    Name: ToolName
    Description: string
    InputSchema: JsonElement option
    Handler: Map<string, JsonElement> -> Task<Result<Content list, McpError>>
}

ResourceDefinition

type ResourceDefinition = {
    Uri: ResourceUri
    Name: string
    Description: string option
    MimeType: MimeType option
    Handler: Map<string, string> -> Task<Result<ResourceContents, McpError>>
}

PromptDefinition

type PromptDefinition = {
    Name: PromptName
    Description: string option
    Arguments: PromptArgument list
    Handler: Map<string, string> -> Task<Result<McpMessage list, McpError>>
}

PromptArgument

type PromptArgument = {
    Name: string
    Description: string option
    Required: bool
}

Identifier types (smart constructors)

All identifier types are single-case DUs with private constructors. They guarantee validity at construction time.

ToolName

A validated, non-empty, trimmed tool name.

// Create (validates)
let tn : Result<ToolName, ValidationError> = ToolName.create "my-tool"

// Extract the raw string
let s : string = ToolName.value tn

// Typical usage
let toolName = ToolName.create "echo" |> unwrapResult

ResourceUri

A validated absolute URI string with a scheme:

let uri = ResourceUri.create "https://example.com/resource"   // Ok
let uri2 = ResourceUri.create "file:///tmp/test.txt"          // Ok
let uri3 = ResourceUri.create "not-a-uri"                     // Error

let s = ResourceUri.value uri  // "https://example.com/resource"

PromptName

A validated, non-empty, trimmed prompt name:

let pn = PromptName.create "summarize" |> unwrapResult
let s = PromptName.value pn  // "summarize"

MimeType

A validated MIME type string. Empty or null defaults to "application/octet-stream":

let mime = MimeType.create "text/plain" |> unwrapResult
let mime2 = MimeType.create "" |> unwrapResult          // application/octet-stream
let mime3 = MimeType.create "invalid" |> unwrapResult   // Error

let s = MimeType.value mime  // "text/plain"
MimeType.defaultMimeType     // "application/octet-stream"

ServerName

A validated, non-empty server name:

let sn = ServerName.create "MyServer" |> unwrapResult
let s = ServerName.value sn

ServerVersion

A validated, non-empty server version:

let sv = ServerVersion.create "1.0.0" |> unwrapResult
let s = ServerVersion.value sv

Smart constructor patterns

Every identifier type follows the same pattern:

// ModuleName.create : string -> Result<Type, ValidationError>
// ModuleName.value  : Type -> string

create validates and returns Result. value extracts the inner string. Use unwrapResult to convert Result to a value or throw with a formatted message:

let unwrapResult (result: Result<'T, ValidationError>) : 'T =
    match result with
    | Ok v -> v
    | Error e -> failwith (ValidationError.format e)

unwrapResult is [<AutoOpen>] in the FsMcp.Core namespace, so it is available everywhere you open FsMcp.Core.

Server configuration types

ServerConfig

type ServerConfig = {
    Name: ServerName
    Version: ServerVersion
    Tools: ToolDefinition list
    Resources: ResourceDefinition list
    Prompts: PromptDefinition list
    Middleware: McpMiddleware list
    Transport: Transport
}

Transport

type Transport =
    | Stdio
    | Http of endpoint: string option

McpMiddleware

type McpMiddleware =
    McpContext -> (McpContext -> Task<McpResponse>) -> Task<McpResponse>

McpContext

type McpContext = {
    Method: string
    Params: JsonElement option
    CancellationToken: CancellationToken
}

McpResponse

type McpResponse =
    | Success of JsonElement
    | McpResponseError of McpError

Namespace summary

Namespace	Contains
FsMcp.Core	Content, ResourceContents, McpRole, McpMessage, McpError, ToolDefinition, ResourceDefinition, PromptDefinition, PromptArgument, unwrapResult
FsMcp.Core.Validation	ValidationError, ToolName, ResourceUri, PromptName, MimeType, ServerName, ServerVersion
FsMcp.Server	ServerConfig, Transport, McpMiddleware, McpContext, McpResponse, FsMcpConfigException, McpServerBuilder, mcpServer, mcpTool, Tool, Resource, Prompt, TypedTool, TypedResource, TypedPrompt, TypedHandler, StreamingTool, Middleware, ValidationMiddleware, Telemetry, DynamicServer, Notifications
FsMcp.Client	ClientTransport, ClientConfig, McpClient, McpClientAsync, ToolInfo, ResourceInfo, PromptInfo
FsMcp.TaskApi	ClientPipeline
FsMcp.Testing	TestServer, Expect, McpArbitraries
FsMcp.Sampling	SamplingRequest, SamplingResult, SamplingError, SamplingContext, SamplingTool, SampleFunc
FsMcp.Server.Http	HttpServer