ThreadExtensions

Keyed map of ThreadExtension handles, typed off a declared transformer projection shape.

Used as the return type of ThreadStream.extensions. TExtensions is expected to match the in-process run.extensions shape (i.e. the output of InferExtensions<TTransformers> from @langchain/langgraph); each value type is unwrapped via UnwrapExtension so thread.extensions.foo resolves with the transformer's emitted payload, not the in-process Promise<T> / StreamChannel<T> wrapper.

Access any string key to obtain a ThreadExtension<unknown>; keys that appear in TExtensions narrow to their declared payload type.