fix(acp): coalesce trailing reasoning so answer text doesn't split the thought block

[ematvey]

What

Pi RPC stream can include a trailing thinking_delta after the final answer text has already started. I have this with local vLLM provider, but I suspect it is not limited to that. pi-acp relayed text_delta and thinking_delta in arrival order, so ACP clients saw:

[thinking][text][thinking][text]

(i.e. one long thinking block, a couple of answer words, then a short "tail of thinking" block, then the rest of the answer).

Root cause

Pi core is correct: it tags deltas with contentIndex and renders by block index. pi-acp's handlePiEvent dropped contentIndex and keyed blocks by arrival contiguity, so the trailing thinking_delta (contentIndex 0) was split
into a second block.

Fix

In src/acp/session.ts, hold the first text chunk briefly while thinking is present (PI_ACP_THINK_HOLD_MS, default 200ms) so a trailing reasoning delta stays in the same thought block. Flush held text before tool-call events and at turn boundaries; once thinking has clearly ended, text streams directly with no added latency.

Testing

• Added regression test coalesces trailing thinking into the same thought block (no text split) in test/component/session-events.test.ts.
• Full suite: 96 passed, 0 failed.
• Fix behavior confirmed against a real pi --mode rpc delta stream.

Notes

• Tunable via PI_ACP_THINK_HOLD_MS (ms). Set it to 0 to disable the hold (next-tick flush) — a trade-off of a small first-chunk delay for earlier text streaming in non-reasoning-heavy providers.
• The same interleave exists in other adapters (e.g. paseo's Pi provider) and is being fixed there with the same pattern.

[Copilot]

Pull request overview

This PR adjusts the ACP session event translation so that a late thinking_delta that arrives after answer text has begun does not split the client-visible “thought block” (avoiding [thinking][text][thinking][text] in ACP clients like Zed). It does this by briefly buffering the first answer text chunk when thinking has been seen, then flushing it once a small hold window elapses or before tool-call events.

Changes:

• Add reasoning/text coalescing state to PiAcpSession with a configurable hold window (PI_ACP_THINK_HOLD_MS).
• Flush held text before tool-call related assistant events and before tool execution start events.
• Add a regression test covering the “trailing thinking after text starts” scenario.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
src/acp/session.ts	Buffers initial text when thinking is present to prevent splitting thought blocks; adds flush points around tool-call/tool execution events.
test/component/session-events.test.ts	Adds a regression test to ensure trailing thinking deltas coalesce into the same thought block and text is not emitted until after the hold window.

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