StreamTransformer

Mark the channel as failed after all buffered items are consumed.

Called once when the underlying Pregel run completes without throwing. Optional — only needed for non-channel teardown (e.g. resolving promises).

May return a PromiseLike<void> to defer the main event log close until the async work (e.g. emitting terminal lifecycle events) has completed. The mux awaits all returned promises before closing its event log.

Called once before the run starts.

Optional hook invoked by StreamMux.addTransformer immediately after the transformer is attached to the mux. Receives a limited handle that exposes only StreamEmitter.push — enough for the transformer to emit synthesized ProtocolEvents on any namespace it chooses (e.g. a deepagents SubagentTransformer fabricating lifecycle/messages/values events under a ["tools:<tool_call_id>"] namespace when a task tool starts).

Transformers that do not synthesize events can omit this hook.

The StreamEmitter handle is only safe to call from within StreamTransformer.process. Emitting from an unrelated async context (e.g. after process has returned, from a setTimeout, etc.) races with the mux's close/fail cycle and may land events in an already-closed log.

Called for each ProtocolEvent before it is appended to the main log.