FastMCP - FastMCP TS

Constructors

Constructor

new FastMCP(options): FastMCP;

Defined in: server/FastMCP.ts:501

Parameters

options

FastMCPOptions

Returns

FastMCP

Properties

name

readonly name: string;

Defined in: server/FastMCP.ts:442

version

readonly version: string;

Defined in: server/FastMCP.ts:443

Accessors

address

Get Signature

get address(): ServerAddress | null;

Defined in: server/FastMCP.ts:1881 The bound address after run() resolves for the http transport. Null for stdio or before run().

Returns

ServerAddress | null

isRunning

Get Signature

get isRunning(): boolean;

Defined in: server/FastMCP.ts:1890 Whether run() or connect() has been called on this instance. Used by the CLI’s entrypoint loader to detect servers that already started themselves via top-level code, so it doesn’t attempt to start them a second time.

Returns

boolean

Methods

_addCloseCallback()

_addCloseCallback(cb): void;

Defined in: server/FastMCP.ts:1713 Register a callback invoked when this server is closed — used by proxy connections.

Parameters

cb

() => Promise<void>

Returns

void

_dispatchPrompt()

_dispatchPrompt(
   name, 
   args, 
   ctx): Promise<
  | InputRequiredResult
  | {
  description?: string;
  messages: PromptMessage[];
}>;

Defined in: server/FastMCP.ts:1573 Dispatch a prompt render through this server’s middleware chain using an inherited context.

Parameters

name

string

args

Record<string, string>

ctx

McpContext

Returns

Promise< | InputRequiredResult | { description?: string; messages: PromptMessage[]; }>

_dispatchResource()

_dispatchResource(
   uri, 
   params, 
   ctx): Promise<unknown>;

Defined in: server/FastMCP.ts:1533 Run a resource read through this server’s middleware chain using an inherited context. Returns the raw handler result so the parent can apply convertResourceResult with the actual requested URI (important for template resources).

Parameters

uri

string

params

Record<string, string> | undefined

ctx

McpContext

Returns

Promise<unknown>

_dispatchTool()

_dispatchTool(
   name, 
   rawArgs, 
   ctx): Promise<
  | InputRequiredResult
  | {
[key: string]: unknown;
}>;

Defined in: server/FastMCP.ts:1474 Dispatch a tool call through this server’s middleware chain using an inherited context. Used by parent servers to honour child-level middleware when routing mounted tool calls.

Parameters

name

string

rawArgs

unknown

ctx

McpContext

Returns

Promise< | InputRequiredResult | { [key: string]: unknown; }>

_modernFetch()

_modernFetch(request, options?): Promise<Response>;

Defined in: server/FastMCP.ts:1937 Modern-era (2026-07-28) in-process fetch entrypoint — the McpServerLike duck-type hook fastmcp-ts/client’s Client looks for when a caller pins modern era for an in-process server (versionNegotiation: { mode: { pin: '2026-07-28' } } }). Delegates directly to the same createMcpHandler instance the real HTTP modern path uses (_getModernHandler), so this is the identical dispatch a real network connection would get — no sockets, same code. Modern-only (legacy: 'reject'): a client using 'auto' or 'legacy' negotiation instead goes through connect() + InMemoryTransport (2025-era only), unaffected by this method.

Parameters

request

Request

options?

McpHandlerRequestOptions

Returns

Promise<Response>

_removePrompt()

_removePrompt(name): boolean;

Defined in: server/FastMCP.ts:1457

Parameters

name

string

Returns

boolean

_removeResource()

_removeResource(uri): boolean;

Defined in: server/FastMCP.ts:1443

Parameters

uri

string

Returns

boolean

_removeTool()

_removeTool(name): boolean;

Defined in: server/FastMCP.ts:1436

Parameters

name

string

Returns

boolean

addProvider()

addProvider(provider): this;

Defined in: server/FastMCP.ts:1722 Register a provider (FastMCPApp or FastMCP) on this server. All tools, resources, and prompts from the provider are mounted without a prefix. For FastMCPApp, pass provider.server; for FastMCP, pass directly.

Parameters

provider

| FastMCP | { server: FastMCP; }

Returns

this

close()

close(): Promise<void>;

Defined in: server/FastMCP.ts:2396

Returns

Promise<void>

connect()

connect(transport): Promise<void>;

Defined in: server/FastMCP.ts:1894

Parameters

transport

Transport

Returns

Promise<void>

customRoute()

customRoute(config, handler): void;

Defined in: server/FastMCP.ts:1432 Register a custom HTTP route on the listener run() starts — e.g. a Kubernetes liveness/readiness probe. The handler owns the whole exchange: no MCP auth, no DNS-rebinding guards, and no CORS headers run in front of it. Matching is exact-path (query string ignored); methods default to GET. Routes never serve on stdio or through fetch() — a framework embedding fetch() owns its own routing (same posture as dnsRebinding).

Parameters

config

CustomRouteConfig

handler

CustomRouteHandler

Returns

void

fetch()

fetch(request, options?): Promise<Response>;

Defined in: server/FastMCP.ts:1913 Serve an MCP request through a web-standard, fetch-native HTTP entrypoint. This entrypoint is always stateless and does not require run(): legacy (2025-era) requests use a fresh server instance per request, while modern (2026-07-28) requests share the instance’s modern handler and event bus. Authentication is deliberately external — options.authInfo is trusted and passed unchanged into the protocol handler, where it is exposed as ctx.auth. The embedding framework also owns Host/Origin validation and should abort its carrying requests during shutdown; stateless legacy response streams follow that request lifecycle, while close() terminates modern exchanges and streams.

Parameters

request

Request

options?

McpHandlerRequestOptions

Returns

Promise<Response>

getContext()

getContext(): McpContext;

Defined in: server/FastMCP.ts:1874

Returns

McpContext

mount()

mount(child, prefix?): this;

Defined in: server/FastMCP.ts:1689 Mount a child server onto this server. All tools, resources, and prompts from the child become accessible via this server. Pass a prefix to namespace names and prevent collisions.

Parameters

child

FastMCP

prefix?

string

Returns

this

notifyResourceUpdated()

notifyResourceUpdated(uri): void;

Defined in: server/FastMCP.ts:1311 Signal that a resource changed and any subscriber should re-read it. This is the SINGLE change signal for resource updates across both eras — the legacy per-session push and the modern subscriptions/listen bus are driven from here:

Legacy connections (primary / stdio / legacy HTTP sessions) that called resources/subscribe for uri are pushed notifications/resources/updated directly on their live session (subscription set kept in per-session state).
The modern handler’s bus (W3) is published to as well; it does its own per-stream filtering, delivering only to subscriptions/listen streams that opted in to this URI. Optional chaining keeps it a no-op until the first modern HTTP request has actually built the handler.
A modern stdio connection is sent the notification unconditionally; the SDK’s stdio listen router intercepts it and fans out per honored subscription (or drops it when no stream opted in to the URI).

Unsubscribed sessions (and the modern streams that never opted in) receive nothing, so calling this for a URI with no subscribers is safe and cheap.

Parameters

uri

string

Returns

void

prompt()

prompt(config, handler): void;

Defined in: server/FastMCP.ts:1402

Parameters

config

PromptConfig

handler

(args?) => unknown

Returns

void

resource()

resource(config, handler): void;

Defined in: server/FastMCP.ts:1413

Parameters

config

ResourceConfig

handler

(params?) => unknown

Returns

void

run()

run(options?): Promise<void>;

Defined in: server/FastMCP.ts:1941

Parameters

options?

RunOptions

Returns

Promise<void>

tool()

Call Signature

tool<S>(config, handler): void;

Defined in: server/FastMCP.ts:1387

Type Parameters

S

S extends StandardSchemaV1<unknown, unknown>

Parameters

config

Omit<ToolConfig, "input"> & { input: S; }

handler

(args) => unknown

Returns

void

Call Signature

tool(config, handler): void;

Defined in: server/FastMCP.ts:1392

Parameters

config

ToolConfig

handler

(args) => unknown

Returns

void

transform()

transform(t): this;

Defined in: server/FastMCP.ts:1465 Add a transform to the pipeline. Applied to list responses in registration order.

Parameters

t

Transform

Returns

this

use()

use(mw): this;

Defined in: server/FastMCP.ts:1867 Add a middleware to the pipeline. Can be called at any point before the first request arrives. setup() is called immediately on the primary server so that notification handlers and other server-level side-effects are active before connect() / run() is called. HTTP sessions created after use() will also have setup() called via _makeServer().

Parameters

mw

Middleware

Returns

this

​Constructors

​Constructor

​Parameters

options

​Returns

​Properties

​name

​version

​Accessors

​address

​Get Signature

Returns

​isRunning

​Get Signature

Returns

​Methods

​_addCloseCallback()

​Parameters

cb

​Returns

​_dispatchPrompt()

​Parameters

name

args

ctx

​Returns

​_dispatchResource()

​Parameters

uri

params

ctx

​Returns

​_dispatchTool()

​Parameters

name

rawArgs

ctx

​Returns

​_modernFetch()

​Parameters

request

options?

​Returns

​_removePrompt()

​Parameters

name

​Returns

​_removeResource()

​Parameters

uri

​Returns

​_removeTool()

​Parameters

name

​Returns

​addProvider()

​Parameters

provider

​Returns

​close()

​Returns

​connect()

​Parameters

transport

​Returns

​customRoute()

​Parameters

config

handler

​Returns

​fetch()

​Parameters

request

options?

​Returns

​getContext()

​Returns

​mount()

​Parameters

child

Constructors

Constructor

Parameters

Returns

Properties

name

version

Accessors

address

Get Signature

isRunning

Get Signature

Methods

_addCloseCallback()

Parameters

Returns

_dispatchPrompt()

Parameters

Returns

_dispatchResource()

Parameters

Returns

_dispatchTool()

Parameters

Returns

_modernFetch()

Parameters

Returns

_removePrompt()

Parameters

Returns

_removeResource()

Parameters

Returns

_removeTool()

Parameters

Returns

addProvider()

Parameters

Returns

close()

Returns

connect()

Parameters

Returns

customRoute()

Parameters

Returns

fetch()

Parameters

Returns

getContext()

Returns

mount()

Parameters

Returns

notifyResourceUpdated()

Parameters

Returns

prompt()

Parameters

Returns

resource()

Parameters

Returns

run()

Parameters

Returns

tool()

Call Signature

Call Signature

transform()

Parameters

Returns

use()

Parameters

Returns