feat: support resource subscriptions and list-changed capabilities - #296

Closes #185.

Adds support for MCP resource subscriptions and makes the server's list-changed capabilities discoverable.

What's added

• Capabilities: advertise resources: { listChanged: true, subscribe: true } and prompts: { listChanged: true } when those features are enabled. The server already emitted notifications/resources/list_changed and notifications/prompts/list_changed, but never advertised the capability, so spec-compliant clients didn't know to expect them.
• Subscribe/unsubscribe: handlers for resources/subscribe and resources/unsubscribe, tracked per session.
• Update notifications: server.sendResourceUpdated(uri) emits notifications/resources/updated only to clients that subscribed to that URI (spec).

Example

server.addResource({
  uri: "file:///logs/app.log",
  name: "Application Logs",
  mimeType: "text/plain",
  async load() {
    return { text: await readLogFile() };
  },
});

// When the underlying data changes, notify subscribed clients:
await server.sendResourceUpdated("file:///logs/app.log");

sendResourceUpdated is a no-op for URIs no client has subscribed to, so it's safe to call whenever data changes.

Design note

subscribe: true is advertised for every server that exposes resources rather than being gated behind an opt-in flag, since the subscribe/update plumbing is generic and sendResourceUpdated is a no-op for URIs nobody subscribed to. If you'd rather make it opt-in, happy to gate it behind a flag.

Spec references

• resources/subscribe, resources/unsubscribe, notifications/resources/updated: https://modelcontextprotocol.io/specification/2025-06-18/server/resources#subscriptions
• listChanged notifications for resources and prompts.

Tests

New src/FastMCP.resource-subscriptions.test.ts covers capability advertisement (resources + prompts), notification delivery to a subscribed client, no delivery without a subscription, and unsubscribe stopping delivery. Full suite green: 388 tests passing, plus prettier, eslint, and tsc --noEmit all clean.