Eden Treaty loses type inference with Better Auth plugin in modules

[alycoy]

What version of Elysia is running?

1.4.11

What platform is your computer?

Darwin 25.0.0 arm64 arm

What steps can reproduce the bug?

Reproduction repository: https://github.com/alycoy/eden-treaty-type-loss

1. Clone the reproduction repository

2. Install dependencies: bun install

3. The monorepo has backend/ and client/ packages with TypeScript path aliases in root tsconfig.json:

   {
     "compilerOptions": {
       "baseUrl": ".",
       "paths": {
         "@backend": ["./backend/src/index.ts"]
       }
     }
   }

4. Create a Better Auth plugin with .mount() and .macro():

   // backend/src/http/plugins/better-auth.ts
   export const betterAuthPlugin = new Elysia({ name: "better-auth" })
     .mount(auth.handler)
     .macro({
       auth: {
         async resolve({ status, request: { headers } }) {
           const session = await auth.api.getSession({ headers });
           if (!session) {
             return status(401, { message: "Unauthorized" });
           }
           return { user: session.user, session: session.session };
         },
       },
     });

5. Use the plugin inside a module (not at the root app level):

   // backend/src/modules/dashboard/index.ts
   export const dashboardModule = new Elysia({ prefix: "/api" })
     .use(betterAuthPlugin)  // Using plugin inside module
     .get("/dashboard", async ({ user }) => {
       return { example: "Help me Elysia!" };
     }, {
       auth: true,
       response: {
         200: t.Object({ example: t.String() })
       }
     });

6. Import the module in the main app and export the type:

   // backend/src/index.ts
   const app = new Elysia()
     .get("/", () => "Hello Elysia")
     .use(dashboardModule)
     .listen(3000);
   
   export type App = typeof app;

7. Import and use the type in the client package:

   // client/src/api.ts
   import { treaty } from "@elysiajs/eden";
   import type { App } from "@backend";
   
   export const api = treaty<App>("localhost:3000");
   const { data } = await api.broken.get();
   
   // This should error, but doesn't because data is 'any':
   data.thisPropertyDoesNotExist?.().whatever[123]();

8. Run bunx tsc --noEmit in the client folder - no TypeScript errors

9. Hover over data in your IDE - it shows any instead of the expected type

What is the expected behavior?

Eden Treaty should maintain full type inference when importing the App type across packages. The data variable should be properly typed with the response schema defined in the backend route ({ example: string }), and TypeScript should catch type errors when trying to access non-existent properties.

This works correctly when:

• The betterAuthPlugin is used at the root app level (not inside a module)
• The type is used within the same package (backend)

What do you see instead?

When betterAuthPlugin is used inside a module that gets imported into the main app, Eden Treaty loses all type inference for cross-package imports. The data variable becomes any, and TypeScript fails to catch obvious type errors.

Proof:

• bunx tsc --noEmit shows no errors even with nonsensical code like data.thisPropertyDoesNotExist?.().whatever[123]()
• IDE hover shows data: any instead of the proper response type
• Commenting out .use(betterAuthPlugin) from the dashboard module immediately restores full type inference

Additional information

Key Finding: This ONLY happens when the plugin is used inside a module. Using the exact same betterAuthPlugin at the root app level works fine with perfect type inference.

What I've tried:

• Using the plugin only at root level → Works, but then I lose type inference for user in the module
• Creating separate plugin instances → Same issue persists
• Manual type assertions → Defeats the purpose of Eden Treaty's end-to-end type safety

Reproduction: The issue is 100% reproducible by simply commenting/uncommenting the .use(betterAuthPlugin) line in the dashboard module and observing the type inference change.

This makes it impossible to use Better Auth (or any complex plugin with .mount() + .macro()) in a modular monorepo architecture while maintaining end-to-end type safety with Eden Treaty.

Have you try removing the node_modules and bun.lockb and try again yet?

Yes

[Metaphysics0]

I can confirm this is 100% an issue.
Also looks to be related to elysiajs/elysia#1468

[ickweb3]

+1

[Metaphysics0]

After spending some time with this issue, a workaround that fixed it for me was here -> #181 (comment)

[AAAAAAAAlan]

After spending some time with this issue, a workaround that fixed it for me was here -> #181 (comment)

Worked for me. Thanks!

[priosshrsth]

I too am stuck with this error. For me I am not using client in monorepo. I am simply using the eded treaty in the test in same repo. But it doesn't seem to work.

[rasyidly]

+1

[luiznegreiros]

+1

[SaltyAom]

This is not an Elysia bug

As mentioned in https://elysiajs.com/eden/installation.html#path-alias-monorepo and #181 (comment)

This happened when a monorepo with path aliases is setup but the frontend is unable to resolve backend path correctly, causing the type to be unresolved

Please kindly follow for fixing the problem https://elysiajs.com/eden/installation.html#path-alias-monorepo

Closing the issue, but discussion is welcome for any newer comments and people who has this problems in the future