Have runtime validation and never for now. Could explore a shape using unique symbol later but I don't know that it makes the usage any clearer.

Could explore a shape using unique symbol later but I don't know that it makes the usage any clearer.

I like unique symbol but I couldn't get TS happy when I would use it in type Foo = typeof unique symbol; @connor4312 do you know what I am doing wrong?

Unique symbols are pretty special. In for case I think they need to be contained in an interface, like interface MyOpaqueType { readonly _opaque: unique symbol }. With this MyOpaqueType will be type that a consumer cannot construct and is irreplacable (except with any).

Thanks for confirming!

Was looking at this again because I had more questions similar to the above. Is this too confusing to explain in our API?

/**
 * A token that can be passed to {@link lm.invokeTool} when invoking a tool inside the context of handling a chat request.
 */
export type ChatParticipantToolToken = {
	/**
	 * A unique symbol to prevent accidental construction of this type (see proper usage note on {@link LanguageModelToolInvocationOptions.toolInvocationToken}).
	 */
	readonly sym: unique symbol;
};

The other option I have in mind: change the prop in the token from today's sessionId to something like __SECRET_INTERNALS_DO_NOT_USE_OR_YOU_WILL_BE_FIRED and I kinda prefer it

Here's my suggestion, I think it's nicer than passing a never around #248452