Let live-routed answers reach the search page

[adamjohnwright]

Reported by Adam: the search page could not answer "what species are in reactome". Reproduced with MCP configured — 0 tokens, state: nothing_found — while the chat answers the same question correctly.

Cause

Those questions route to the live MCP lookup, which answers from tools and uses no retriever anywhere on the path. astream_answer opens its token boundary on on_retriever_end:

if kind != "on_chat_model_stream" or not retrieval_done:
    continue

So on the live path the boundary never opened, every token was discarded, and answered stayed false → nothing_found. The answer existed the whole time: 54 stream events, 224 characters, at the answer node.

Why nothing caught it

Three independent reasons, which is why it reached production:

• It only happens where MCP is configured — beta, not a developer's machine. Locally these questions fall back to the vector store and retrieve normally, so they answer. That is also why my own latency runs showed them answering.
• The chat UI uses ainvoke, which takes the final answer and never reads this stream.
• The answer sweep also uses ainvoke — so it passed 15/15 on beta while the endpoint was broken for the very questions it was testing.

Fix

Open the boundary when preprocess ends with active_sources containing live.

That is safe on this path for a specific reason rather than by luck: the query expander lives inside the retriever, so where there is no retrieval there is nothing else streaming at the answer node to tell the answer apart from. The non-live path still waits for retrieval, and a test pins that it does.

Verified

	tokens	citations	state
with MCP, "what species are in reactome"	36	0	answered
with MCP, "Which release of Reactome is this?"	10	0	answered
without MCP (fallback path, unchanged)	1534	12	answered
vector-store control	1056	12	answered

The regression test replays event sequences captured from a real run against beta's MCP sibling, so it needs no MCP to run — and it fails against the unfixed code.

Live answers carry no citations, because no retriever ran. That is honest — they come from live services rather than indexed pathways — and callers already handle an empty citation list, which userguide answers made real.

CI-equivalent locally: ruff, format, mypy, full suite with no API keys set.

🤖 Generated with Claude Code