feat(unstable): Deno.McpServer API for writing MCP servers

[bartlomieju]

Experiment: a built-in API for writing Model Context Protocol servers in
Deno with zero dependencies, behind --unstable-mcp.

const server = new Deno.McpServer({ name: "calculator", version: "1.0.0" });

server.tool("add", {
  description: "Add two numbers",
  inputSchema: {
    type: "object",
    properties: { a: { type: "number" }, b: { type: "number" } },
    required: ["a", "b"],
  },
}, ({ a, b }) => a + b);

await server.serve();            // stdio transport
// or: Deno.serve(server.fetch)  // streamable HTTP transport

It is implemented as a JS-only lazy-loaded extension (ext/mcp, no ops)
that handles the MCP JSON-RPC lifecycle (initialize handshake with
protocol version negotiation, ping, tools, resources, prompts), reports
tool execution errors in-band per spec, and supports the stdio transport
(newline-delimited JSON-RPC) plus a stateless streamable HTTP transport.
Handlers can return plain values, which are auto-wrapped into MCP result
shapes, or full spec-shaped result objects. Verified against the official
MCP Inspector client over stdio and with raw JSON-RPC on both transports.

Not implemented yet (follow-ups if this is pursued): server-initiated
notifications (listChanged), SSE streaming and sessions for the HTTP
transport, resource templates, argument completion, schema validation of
tool arguments, and spec tests.

[sigmaSd]

Seems like this is going the bun way of adding everything under the main scope , maybe another global variable called Extra that have all this and S3 and what not should be considered and if something is truly used frequently with time it will be upgraded to deno scooe