Summary

Two type-level fixes that affect consumers using strict TypeScript configurations. No runtime behavior changes.

1. Endpoint.options and Endpoint.path are now readonly

The problem:

The Endpoint type has options and path as mutable properties:

export type Endpoint<..., Meta extends EndpointMetadata | undefined = EndpointMetadata | undefined, ...> = {
    // ... call signatures ...
    options: { method: Method; metadata?: Meta };  // mutable
    path: Path;                                     // mutable
};

Because options is mutable, TypeScript treats Meta as invariant — it appears in both a readable (covariant) and writable (contravariant) position. This means a concrete Endpoint<..., { SERVER_ONLY: true }, ...> is not assignable to Endpoint<..., EndpointMetadata | undefined, ...>:

// This fails with TS2322:
const endpoints: { [key: string]: Endpoint } = {
    myEndpoint: createEndpoint("/foo", { method: "GET", metadata: { SERVER_ONLY: true } }, handler),
    //          ^^^^^^^^^^^^^^^^ Type '{ SERVER_ONLY: true }' is not assignable to 'EndpointMetadata | undefined'
};

This is a critical issue for better-auth, where BetterAuthPlugin.endpoints is typed as { [key: string]: Endpoint } and plugins produce endpoints with concrete metadata types like { SERVER_ONLY: true }, { scope: "server" }, or { openapi: { ... } }.

Making these readonly makes Meta, Method, and Path covariant — they only appear in readable positions. This allows concrete endpoint types to be assigned to wider container types.

Why this is safe at runtime: These properties are only assigned during endpoint construction inside createEndpoint() (via an as any cast at line 471), and are never mutated after. The readonly modifier accurately reflects how they're used.

2. OpenAPIParameter.schema optional properties now include | undefined

The problem:

With exactOptionalPropertyTypes: true (a strict TypeScript option), there's a distinction between:

• format?: string — property can be absent or string, but NOT explicitly undefined
• format?: string | undefined — property can be absent, string, OR explicitly undefined

When endpoints infer query parameter schemas from Zod types, optional fields produce string | undefined. With exactOptionalPropertyTypes, this is incompatible with format?: string:

// With exactOptionalPropertyTypes: true
const param: OpenAPIParameter = {
    in: "query",
    schema: { type: "string", format: undefined }
    //                         ^^^^^^^ Error: 'undefined' is not assignable to 'string'
};