feat: allow agents to go back-and-forth with AI chat in a thread

.gitignore

@@ -1,5 +1,7 @@
 dist/
 node_modules/
+.env
+db.json
 
 # System
 .DS_Store

package.json

@@ -2,19 +2,22 @@
   "name": "fern-mcp-server",
   "description": "Model Context Protocol (MCP) server for the Fern API.",
   "version": "0.1.0",
+  "type": "module",
   "bin": "dist/index.js",
   "files": [
     "dist"
   ],
   "scripts": {
     "start": "concurrently \"npm run build:watch\" \"npm run inspector\"",
     "inspector": "npx @modelcontextprotocol/inspector@latest -- nodemon --env-file=.env -q --watch dist dist/index.js",
-    "build": "tsup src/index.ts --dts --clean",
-    "build:watch": "tsup src/index.ts --dts --watch",
+    "build": "tsup src/index.ts --format esm --dts --clean",
+    "build:watch": "tsup src/index.ts --format esm --dts --watch",
     "test": "jest"
   },
   "dependencies": {
     "@modelcontextprotocol/sdk": "^1.8.0",
+    "lowdb": "^7.0.1",
+    "uuid": "^11.1.0",
     "zod": "^3.24.2"
   },
   "devDependencies": {

src/api.test.ts

@@ -6,7 +6,9 @@ describe("postChat", () => {
   it(
     "returns a non-empty string",
     async () => {
-      const result = await postChat("What is Fern AI Chat?");
+      const result = await postChat([
+        { role: "user", content: "What is Fern AI Chat?" },
+      ]);
       console.log({ result });
 
       expect(result).not.toBe("");

src/api.ts

@@ -1,13 +1,18 @@
 const BASE_URL = "https://buildwithfern.com/learn";
 
-export async function postChat(message: string) {
+interface ChatMessage {
+  role: "user" | "assistant";
+  content: string;
+}
+
+export async function postChat(messages: ChatMessage[]) {
   const response = await fetch(`${BASE_URL}/api/fern-docs/search/v2/chat`, {
     method: "POST",
     headers: {
       "content-type": "application/json",
     },
     body: JSON.stringify({
-      messages: [{ role: "user", content: message }],
+      messages: messages,
       url: "https://buildwithfern.com/learn",
       filters: [],
     }),

src/db.ts

@@ -0,0 +1,20 @@
+import { Low, Memory } from "lowdb";
+
+export type Thread = Array<{
+  role: "user" | "assistant";
+  content: string;
+}>;
+
+export type Data = {
+  threads: Record<string, Thread>;
+};
+
+export type DB = Low<Data>;
+
+const defaultData: Data = { threads: {} };
+
+export function createDb() {
+  // TODO: Use JSONFilePreset instead
+  const db = new Low<Data>(new Memory(), defaultData);
+  return db;
+}

src/index.ts

@@ -1,7 +1,8 @@
 #!/usr/bin/env node
 
 import { StdioServerTransport } from "@modelcontextprotocol/sdk/server/stdio.js";
-import { createMcpServer, registerMcpTools } from "./mcp";
+import { createMcpServer, registerMcpTools } from "./mcp.js";
+import { createDb } from "./db.js";
 
 const packageJson = require("../package.json") as any;
 
@@ -12,7 +13,8 @@ async function run() {
   }
 
   const server = createMcpServer(packageJson.name, packageJson.version);
-  registerMcpTools(server);
+  const db = createDb();
+  registerMcpTools(server, db);
 
   const transport = new StdioServerTransport();
   await server.connect(transport);

src/mcp.ts

@@ -1,8 +1,10 @@
 #!/usr/bin/env node
 
 import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
+import { v4 as uuidv4 } from "uuid";
 import { z } from "zod";
-import * as api from "./api";
+import * as api from "./api.js";
+import { DB } from "./db.js";
 
 // Create an MCP server
 export function createMcpServer(name: string, version: string) {
@@ -19,15 +21,46 @@ ABOUT FERN (builtwithfern.com): Start with OpenAPI. Generate SDKs in multiple la
 }
 
 // Register MCP tools
-export function registerMcpTools(server: McpServer) {
+export function registerMcpTools(server: McpServer, db: DB) {
   server.tool(
     "ask_fern_ai",
-    "Ask Fern AI about anything related to Fern.",
-    { message: z.string() },
-    async ({ message }) => {
-      const result = await api.postChat(message);
+    `Ask Fern AI about anything related to Fern.
+Don't include a threadId in your initial message.
+If a message requires a follow-up, include the threadId in your follow-up messages until the thread is resolved.`,
+    { question: z.string(), threadId: z.string().optional() },
+    async ({ question, threadId: _threadId }) => {
+      // Issue a new thread ID if none is provided
+      const threadId = _threadId ?? uuidv4();
+      await db.read();
+      const thread = [...(db.data.threads[threadId] ?? [])];
+
+      // Create a user message
+      thread.push({ role: "user" as const, content: question });
+
+      // Post the question to the API
+      const answer = await api.postChat(thread);
+
+      // Create an assistant message
+      thread.push({ role: "assistant" as const, content: answer });
+
+      console.error(thread);
+
+      // Update the thread state
+      db.update(({ threads }) => {
+        threads[threadId] = thread;
+      });
+
+      // Return the answer (last message in the thread)
       return {
-        content: [{ type: "text", text: result }],
+        content: [
+          {
+            type: "text",
+            text: JSON.stringify({
+              threadId: threadId,
+              thread: thread,
+            }),
+          },
+        ],
       };
     }
   );