API reference · picomcp

import { pico, McpError, PROTOCOL_VERSIONS, text, image, audio, link, withJsonSchema } from "picomcp";
import type { Server, ToolConfig, ResourceConfig, PromptConfig, Context, CallToolResult } from "picomcp";

pico(info)#

Creates a server.

function pico(info: ServerInfo): Server;

interface ServerInfo {
  name: string;                // machine-friendly name, e.g. "weather"
  version?: string;            // defaults to "0.0.0"
  title?: string;              // display name
  instructions?: string;       // how the model should use the server as a whole
  origins?: string[] | "*";    // allowed browser origins for HTTP; defaults to localhost
}

server.tool(name, config)#

Registers a tool and returns the server. See Tools.

interface ToolConfig<I, O> {
  title?: string;
  description?: string;
  input?: I;                   // Standard Schema
  output?: O;                  // Standard Schema; enables structuredContent
  annotations?: ToolAnnotations;
  run(input: Infer<I>, ctx: Context): Return | Promise<Return>;
}

interface ToolAnnotations {
  title?: string;
  readOnlyHint?: boolean;
  destructiveHint?: boolean;
  idempotentHint?: boolean;
  openWorldHint?: boolean;
}

Throws a TypeError for an invalid or duplicate name, or a schema without JSON Schema support.

server.resource(uri, config)#

Registers a resource, or a resource template when uri contains {variables}. Returns the server. See Resources.

interface ResourceConfig<U extends string> {
  name?: string;
  title?: string;
  description?: string;
  mimeType?: string;
  annotations?: Annotations;
  read(params: UriParams<U>, ctx: Context & { uri: string }): ResourceReturn | Promise<ResourceReturn>;
}

server.prompt(name, config)#

Registers a prompt and returns the server. See Prompts.

interface PromptConfig<A> {
  title?: string;
  description?: string;
  args?: A;                    // Standard Schema of an object with string fields
  run(args: Infer<A>, ctx: Context): string | PromptMessage[] | { description?: string; messages: PromptMessage[] };
}

server.call(name, input?)#

Runs a tool in-process and returns its CallToolResult. Type-checked when tools were registered by chaining. Rejects for an unknown tool name. See Testing.

call(name: string, input?: unknown): Promise<CallToolResult>;

interface CallToolResult {
  content: ContentBlock[];
  structuredContent?: Record<string, unknown>;
  isError?: boolean;
}

server.serve(options?)#

Starts the server over stdio, or over Streamable HTTP when http is set. See Transports.

serve(options?: { http?: number | { port?: number; hostname?: string; path?: string } }): Promise<{ url?: string; close(): Promise<void> }>;

server.fetch(request)#

Web-standard Streamable HTTP handler. Bound to the server, so it can be passed around as a value.

fetch(request: Request): Promise<Response>;

server.handle(message)#

Processes one JSON-RPC message or a batch. Returns the response, or undefined for notifications.

handle(message: unknown): Promise<JsonRpcResponse | JsonRpcResponse[] | undefined>;

Context#

The second argument of every handler.

interface Context {
  signal: AbortSignal;                       // aborted when the client cancels
  meta: Record<string, unknown> | undefined; // the request's _meta
}

Content helpers#

text(text: string, annotations?: Annotations): ContentBlock;
image(data: string | Uint8Array, mimeType: string): ContentBlock;
audio(data: string | Uint8Array, mimeType: string): ContentBlock;
link(uri: string, name: string, extra?: { title?: string; description?: string; mimeType?: string }): ContentBlock;

data is base64 or raw bytes. Bytes are encoded for you on every runtime.

withJsonSchema(schema, jsonSchema, output?)#

Attaches a JSON Schema to a Standard Schema that cannot generate one. See Schemas.

withJsonSchema<S>(schema: S, jsonSchema: JsonSchema, output?: JsonSchema): StandardSchemaV1<InferInput<S>, Infer<S>>;

McpError#

Throw it from a handler to answer with a JSON-RPC error instead of a result.

new McpError(code: number, message: string, data?: unknown);

Code	Meaning
`-32602`	Invalid params
`-32002`	Resource not found
`-32000` to `-32099`	Free for your own server errors

PROTOCOL_VERSIONS#

The MCP protocol versions picomcp speaks, newest first: ["2025-11-25", "2025-06-18", "2025-03-26", "2024-11-05"].

Type helpers#

Type	Description
`Infer<S>`	Output type of a schema
`InferInput<S>`	Input type of a schema
`UriParams<"a://{x}/{+y}">`	`{ x: string; y: string }`
`StandardSchemaV1`	The Standard Schema interface, inlined