Ability to pass custom context to tool handlers

[jordan-burnett]

Is your feature request related to a problem? Please describe.

It would be helpful if it were possible to pass arbitrary context to all tools containing information that was derived from the original request.

For my use case, I need to run an additional check to get the current role for a user within my application, but I imagine there would also be other use cases that would be best served by a generic mechanism.

Describe the solution you'd like

Frameworks like GraphQL and TRPC support creating a context object per-request, which is then passed to all methods. A similar implementation for McpServer could look like:

const mcpServer = new McpServer(
  { name: "my-server", version: "1.0.0" },
  {
    async getContext({ req, res }) {
      const role = await getUserRole(req.auth);
      return { role };
    },
  }
);

mcpServer.registerTool("get_info", {}, (input, { ctx }) => {
  if (ctx.role !== "admin") {
    throw new Error("Unauthorized");
  }
  return await getInfo(input);
});

Describe alternatives you've considered
#166 added support for accessing AuthInfo from a tool call, however this is limited to what is returned from the bearer token middleware.

Additional context
Add any other context or screenshots about the feature request here.

[KKonstantinov]

Today you could achieve this with some design patterns - e.g. if you provided your own class for tool registration which wraps the SDK registration, you could pass any additional data you want to your callback.

There's a few open source implementations out there that do this, happy to send you a few - drop me a DM if you wish some concrete examples.

[jordan-burnett]

The key requirement I have is I need the context to be unique per request. For my use case I need to make a call to a DB every time a tool is called and I'd like to avoid repeating the logic in every callback if possible.

Context in other frameworks works as a sort of middleware, except more portable as it's agnostic of the transport.