Thinking block paints as an empty 「思考过程」 strip after a message is sent — the reasoning area shows nothing while a step runs #1624

Written-by: Chief (owner report, 2026-10-29 10:44 Asia/Shanghai, on the owner's instance flybot98b8ecba)

What the owner sees

A message is sent to a Bot and the reply area paints the thinking block as an empty strip: the collapsed row reads 「思考过程」 (en + zh: "Thought it through") with its caret, but there is nothing under it — no reasoning text, and no activity row standing in. The owner's words: 「发消息之后这里不显示了，之前显示的」 — it used to show the reasoning there, now that area stays blank.

The blank sits directly under the owner's own message (the 「收到消息」 line), i.e. exactly where the running step's reasoning used to stream.

Expected

One of the two, never an empty frame:

1. while the step is still thinking, the area shows the live activity row (what the step is doing) the way it does for other turns, and the 「思考过程」 block appears once the step settles with its text; or
2. if there is nothing to show yet, no block at all.

An expandable 「思考过程」 with an empty body is the defect: it looks like the Bot's reasoning was lost.

What the code says (read at main 63f07ef)

• core/src/client/cells.js:130-136 — the only renderer of the block (ThinkingRow): it returns null when the text is empty or whitespace, so an empty <details class="bt-think"> should not exist at all.
• core/src/client/cells.js:183,188,205 — AssistantCell computes reasoning = turnReasoning(steps, data), bails out when texts.length === 0 && !reasoning && relays.length === 0, and otherwise renders ThinkingRow above the bubbles.
• core/src/client/turn-reasoning.js — turnReasoning() returns '' unless the first step carrying reasoning is the current step, and thinkingLive() deliberately withholds a running step's reasoning ("A step still thinking stays out until it settles; the activity row shows it meanwhile"). So during a live step the contract is: no thinking row, and the activity row carries the state.
• core/src/client/styles.js:886-897 — .bt-think has margin:2px 0 4px, so a header with no body still occupies a visible strip.

That makes two suspects worth reading first, both client-side and both cheap to check:

• the activity row is not painted during a live step (so the withheld reasoning leaves the row below empty), or
• some path renders the <details> without going through ThinkingRow's empty guard (a second renderer, or a stale reasoning string that is whitespace-only after trim() was applied elsewhere).

Repro

1. Open a chat with a Bot that reasons (this conversation).
2. Send a short message.
3. Watch the area directly under the sent message while the Bot works: 「思考过程」 appears with no content; it stays empty rather than filling with the reasoning the way it used to.

Notes

• The owner reports this as a change from before ("之前显示的"), so the useful first question for whoever picks it up is which release introduced it — the instance runs the released build, and the last releases are v0.2.13 (2026-10-28T20:18Z) and before that v0.2.12.
• Not the same as Follow-up message typed while a Bot is working is postponed instead of being delivered at its next step #707 (a follow-up postponed instead of admitted) or fix(chat): human Steer while a tool/job wait is open does not interrupt — only Stop aborts #1509 (a Steer that does not interrupt a tool wait) — this is display only: the reply itself arrives normally.
• A screenshot exists in the owner's chat with Chief; it is not in this issue (the Bot API cannot upload an attachment).