Add AI chat feature

server/index.ts

@@ -1,4 +1,5 @@
 import Elysia from "elysia";
 import { health } from "./modules/health";
+import { chat } from "./modules/chat";
 
-export const app = new Elysia({ prefix: "/api" }).use(health);
+export const app = new Elysia({ prefix: "/api" }).use(health).use(chat);

server/modules/chat/index.ts

@@ -0,0 +1,15 @@
+import { Elysia } from "elysia";
+
+import { ChatModel } from "./model";
+import { Chat } from "./service";
+
+export const chat = new Elysia({ prefix: "/chat" }).post(
+  "/",
+  async ({ body }) => {
+    const stream = await Chat.streamText(body);
+    return stream.toUIMessageStreamResponse();
+  },
+  {
+    body: ChatModel.chat,
+  },
+);

server/modules/chat/model.ts

@@ -0,0 +1,21 @@
+import { t, type UnwrapSchema } from "elysia";
+
+export const ChatModel = {
+  chat: t.Object({
+    messages: t.Array(
+      t.Object({
+        id: t.String(),
+        role: t.Union([
+          t.Literal("user"),
+          t.Literal("assistant"),
+          t.Literal("system"),
+        ]),
+        parts: t.Array(t.Any()),
+      }),
+    ),
+  }),
+} as const;
+
+export type ChatModel = {
+  [k in keyof typeof ChatModel]: UnwrapSchema<(typeof ChatModel)[k]>;
+};

server/modules/chat/service.ts

@@ -0,0 +1,13 @@
+import { convertToModelMessages, streamText } from "ai";
+import type { ChatModel } from "./model";
+import { openai } from "@ai-sdk/openai";
+
+export abstract class Chat {
+  static async streamText(chat: ChatModel["chat"]) {
+    return streamText({
+      model: openai("gpt-5.5"),
+      system: "You are Yae Miko from Genshin Impact",
+      messages: await convertToModelMessages(chat.messages),
+    });
+  }
+}