Migrate package to use ESM

jest.config.ts

@@ -4,6 +4,9 @@ const config: Config = {
   clearMocks: true,
   coverageProvider: "v8",
   testEnvironment: "node",
+  moduleNameMapper: {
+    "^(\\.{1,2}/.*)\\.js$": "$1",
+  },
   transform: {
     "^.+.tsx?$": ["ts-jest", {}],
   },

package.json

@@ -1,6 +1,7 @@
 {
   "name": "fastify-mcp",
   "version": "2.1.0",
+  "type": "module",
   "packageManager": "yarn@4.7.0",
   "author": {
     "name": "Kshitij Chauhan",
@@ -17,14 +18,18 @@
     "ai",
     "anthropic"
   ],
-  "main": "dist/index.js",
-  "types": "dist/index.d.ts",
+  "exports": {
+    ".": {
+      "import": "./dist/index.js",
+      "types": "./dist/index.d.ts"
+    }
+  },
   "files": [
     "dist"
   ],
   "scripts": {
     "lint": "eslint .",
-    "test": "jest",
+    "test": "NODE_OPTIONS=--experimental-vm-modules jest",
     "build": "tsc",
     "format": "prettier --write .",
     "format:check": "prettier --check ."

src/index.ts

@@ -1,3 +1,3 @@
-export { fastifyMCPSSE } from "./mcp-sse-plugin";
-export { Sessions } from "./session-storage";
-export { streamableHttp } from "./streamable-http";
+export { fastifyMCPSSE } from "./mcp-sse-plugin.js";
+export { Sessions } from "./session-storage.js";
+export { streamableHttp } from "./streamable-http.js";

src/mcp-sse-plugin.ts

@@ -1,7 +1,7 @@
 import type { Server } from "@modelcontextprotocol/sdk/server/index.js";
 import { SSEServerTransport } from "@modelcontextprotocol/sdk/server/sse.js";
 import { FastifyPluginCallback, FastifyRequest } from "fastify";
-import { Sessions } from "./session-storage";
+import { Sessions } from "./session-storage.js";
 
 type MCPSSEPluginOptions = {
   server: Server;
@@ -39,7 +39,7 @@ export const fastifyMCPSSE: FastifyPluginCallback<MCPSSEPluginOptions> = (
       sessions.remove(sessionId);
     });
 
-    fastify.log.info("Starting new session", { sessionId });
+    fastify.log.info({ sessionId }, "Starting new session");
     await server.connect(transport);
   });
 

src/session-storage.test.ts

@@ -1,3 +1,4 @@
+import { jest } from "@jest/globals";
 import { SSEServerTransport } from "@modelcontextprotocol/sdk/server/sse.js";
 import { Sessions } from "./session-storage";
 import { setTimeout } from "node:timers/promises";

src/session-storage.ts

@@ -1,4 +1,4 @@
-import type { Transport } from "@modelcontextprotocol/sdk/shared/transport";
+import type { Transport } from "@modelcontextprotocol/sdk/shared/transport.js";
 import { EventEmitter } from "node:events";
 
 type SessionEvents = {

src/streamable-http.ts

@@ -3,7 +3,7 @@ import { StreamableHTTPServerTransport } from "@modelcontextprotocol/sdk/server/
 import { isInitializeRequest } from "@modelcontextprotocol/sdk/types.js";
 import { FastifyPluginAsync, FastifyReply } from "fastify";
 import { randomUUID } from "node:crypto";
-import { Sessions } from "./session-storage";
+import { Sessions } from "./session-storage.js";
 
 type StreamableHttpPluginOptions =
   | StatefulStreamableHttpPluginOptions

tsconfig.json

@@ -1,8 +1,7 @@
 {
   "compilerOptions": {
-    "target": "es2016",
-    "module": "commonjs",
-    "esModuleInterop": true,
+    "target": "es2022",
+    "module": "nodenext",
     "forceConsistentCasingInFileNames": true,
     "strict": true,
     "skipLibCheck": true,