PR #266 fixed the on_llm_new_token signature, after which live diagnostics
confirmed the handler ran per token and adispatch_custom_event completed
successfully — yet the SSE stream still carried zero a2ui-partial events.

Root cause: adispatch_custom_event (langchain_core) and stream_mode=
'custom' (LangGraph) are different layers. langchain_core dispatches
visible via stream_mode='events'; LangGraph's 'custom' channel is fed
by get_stream_writer() returned from langgraph.config. The transport
already requests stream_mode='custom', but the handler was writing to
the wrong sink.

Fix: replace adispatch_custom_event with get_stream_writer(). The writer
is contextvar-scoped to the currently-executing LangGraph node and is
inherited by nested callbacks, so the handler can call it from inside
the LLM's callback chain. The payload shape is {name, data} so the
existing transport-side parser (stream-manager.bridge.ts:509) extracts
name + data correctly.

Tests updated to mock get_stream_writer (instead of adispatch_custom_event)
and assert on writer.call_args. Adds a new test asserting graceful
behavior when invoked outside a stream context (writer raises
RuntimeError; handler swallows).

Live smoke now confirms 758 a2ui-partial events on the wire across a
2.5MB SSE stream for a dashboard prompt. The bridge consumes all events
into agent.customEvents() and forwards them to the partial-args bridge.

Note: a follow-up frontend issue remains in the partial-args bridge's
incremental dispatch logic — early surfaceUpdate envelopes dispatch with
incomplete components arrays (no ids yet), preventing beginRendering
synthesis, and the dispatchedCount counter then skips re-attempt.
Tracking separately.

#268)

PR #266 fixed the on_llm_new_token signature, after which live diagnostics
confirmed the handler ran per token and adispatch_custom_event completed
successfully — yet the SSE stream still carried zero a2ui-partial events.

Root cause: adispatch_custom_event (langchain_core) and stream_mode=
'custom' (LangGraph) are different layers. langchain_core dispatches
visible via stream_mode='events'; LangGraph's 'custom' channel is fed
by get_stream_writer() returned from langgraph.config. The transport
already requests stream_mode='custom', but the handler was writing to
the wrong sink.

Fix: replace adispatch_custom_event with get_stream_writer(). The writer
is contextvar-scoped to the currently-executing LangGraph node and is
inherited by nested callbacks, so the handler can call it from inside
the LLM's callback chain. The payload shape is {name, data} so the
existing transport-side parser (stream-manager.bridge.ts:509) extracts
name + data correctly.

Tests updated to mock get_stream_writer (instead of adispatch_custom_event)
and assert on writer.call_args. Adds a new test asserting graceful
behavior when invoked outside a stream context (writer raises
RuntimeError; handler swallows).

Live smoke now confirms 758 a2ui-partial events on the wire across a
2.5MB SSE stream for a dashboard prompt. The bridge consumes all events
into agent.customEvents() and forwards them to the partial-args bridge.

Note: a follow-up frontend issue remains in the partial-args bridge's
incremental dispatch logic — early surfaceUpdate envelopes dispatch with
incomplete components arrays (no ids yet), preventing beginRendering
synthesis, and the dispatchedCount counter then skips re-attempt.
Tracking separately.