fixed on main (b8886e77) by commit cc7a95ad, which added a Server.setRequestHandler override using ResultTypeMap[M] — {} is now correctly rejected for typed handlers. still present on origin/v1.x (bf1e022b) — needs backport.

root cause: packages/core/src/shared/protocol.ts:1010 typed handler return as Result | Promise<Result> where Result = z.looseObject({ _meta?: ... }), which {} satisfies. no per-method narrowing existed until the Server override at packages/server/src/server/server.ts:227 was introduced.

workaround on v1.x: explicitly annotate handler return types (e.g. async (_req): Promise<ListToolsResult> => ...) to get the type error at the call site instead.

repro + verification

repro (packages/server/repro-typecheck.ts):

import { Server } from "./src/index.js";
const server = new Server({ name: "repro", version: "1.0" });
server.setRequestHandler("tools/list",     async (_req) => { return {}; });
server.setRequestHandler("resources/list", async (_req) => { return {}; });
server.setRequestHandler("prompts/list",   async (_req) => { return {}; });

main (b8886e77) — tsc -p tsconfig.json --noEmit from packages/server/:

repro-typecheck.ts(2,40): error TS2345: Property 'tools' is missing in type '{}'
repro-typecheck.ts(3,44): error TS2345: Property 'resources' is missing in type '{}'
repro-typecheck.ts(4,42): error TS2345: Property 'prompts' is missing in type '{}'

✅ correctly rejected

v1.x (bf1e022b) — tsc -p tsconfig-repro.json --noEmit: exit 0, no errors
❌ bug still present — needs backport of the ResultTypeMap[M] approach

fix location: packages/server/src/server/server.ts:225-228 (override) + packages/core/src/types/types.ts:383 (ResultTypeMap)

Uh oh!

Typechecking should fail on returning {} for all requests #42

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