Type inference lost for callback parameter

[Azzerty23]

Steps to reproduce

This issue occurs in the better-auth Next.js demo, specifically in the organization plugin callback.

Original Repository: https://github.com/better-auth/better-auth/tree/main/demo/nextjs Repro (extracted demo folder with tsgo config, without any other changes): https://github.com/Azzerty23/tsgo-issue-repro

File: lib/auth.ts, line 135

Minimal reproduction:

import { organization } from "better-auth/plugins";

// Inside betterAuth configuration:
plugins: [
  organization({
    async sendInvitationEmail(data) {  // ← 'data' parameter loses type with ts-go
      // TypeScript should infer a complex type for 'data' from the plugin's type definitions
      ...
    },
  }),
]

TypeScript configuration:

{
  "compilerOptions": {
    "target": "ES2017",
    "strict": true,
    "esModuleInterop": true,
    "moduleResolution": "bundler"
  }
}

Behavior with [email protected]

Using the standard TypeScript LSP (VSCode), the data parameter is correctly inferred with this complete type:

sendInvitationEmail(data: {
    id: string;
    role: string;
    email: string;
    organization: {
        id: string;
        name: string;
        slug: string;
        createdAt: Date;
        logo?: string | null | undefined;
        metadata?: any;
    };
    invitation: {
        id: string;
        organizationId: string;
        email: string;
        role: string;
        status: "pending" | "accepted" | "rejected" | "canceled";
        inviterId: string;
        expiresAt: Date;
        createdAt: Date;
        teamId?: string | null | undefined;
    };
    inviter: {
        id: string;
        organizationId: string;
        userId: string;
        role: string;
        createdAt: Date;
    } & {
        user: {
            id: string;
            createdAt: Date;
            updatedAt: Date;
            email: string;
            emailVerified: boolean;
            name: string;
            image?: string | ... 1 more ... | undefined;
        };
    };
}): Promise<void>

The IDE provides full autocomplete and type checking for data.email, data.inviter.user.name, data.organization.name, etc.

Behavior with tsgo

When using ts-go's LSP, the data parameter is inferred as any, resulting in:

• Loss of autocomplete for all properties
• Loss of type safety
• No errors when accessing non-existent properties

This appears to be a regression in type inference for callback parameters where the type should be inferred from the plugin's type definition (from the better-auth/plugins package).

Additional context

• Environment: VSCode with ts-go LSP
• Library: better-auth@^1.4.3 with TypeScript type definitions
• Impact: This affects developer experience significantly, as callback parameters in plugin configurations lose all type information
• Pattern: The issue occurs with callback functions passed as options to plugin configuration functions where the callback parameter type should be inferred from the plugin's type signature

The type information is available in the better-auth/plugins package type definitions, but ts-go seems unable to propagate it to the callback parameter.