feat: MCP plugin (#2666)

* chore: wip

* wip

* feat: mcp plugin

* wip

* chore: fix lock file

* clean up

* schema

* docs

* chore: lint

* chore: release v1.2.9-beta.1

* blog

* chore: lint

examples/nextjs-mcp/app/api/[transport]/route.ts

@@ -0,0 +1,38 @@
+import { auth } from "@/lib/auth";
+import { createMcpHandler } from "@vercel/mcp-adapter";
+import { withMcpAuth } from "better-auth/plugins";
+import { z } from "zod";
+
+const handler = withMcpAuth(auth, (req, sesssion) => {
+	return createMcpHandler(
+		(server) => {
+			server.tool(
+				"echo",
+				"Echo a message",
+				{ message: z.string() },
+				async ({ message }) => {
+					return {
+						content: [{ type: "text", text: `Tool echo: ${message}` }],
+					};
+				},
+			);
+		},
+		{
+			capabilities: {
+				tools: {
+					echo: {
+						description: "Echo a message",
+					},
+				},
+			},
+		},
+		{
+			redisUrl: process.env.REDIS_URL,
+			basePath: "/api",
+			verboseLogs: true,
+			maxDuration: 60,
+		},
+	)(req);
+});
+
+export { handler as GET, handler as POST, handler as DELETE };

examples/nextjs-mcp/app/api/auth/[...all]/route.ts

@@ -0,0 +1,4 @@
+import { auth } from "@/lib/auth";
+import { toNextJsHandler } from "better-auth/next-js";
+
+export const { GET, POST } = toNextJsHandler(auth);