Minimal Compare lanes with revealed streaming and diagnosable ACP failures

Traceability

• Spec ID: compare-live-minimal-streaming
• Status: Implemented

Intent

Compare's live Agent surface answers one question: what do these two Agents do with this prompt. Today four stacked bands sit above the lanes before any comparison exists, the lanes render raw event bursts instead of the revealed stream the Debugger already uses, and a failed ACP handshake reports Incoming transport closed without the Agent's own reason.

Reduce the surface to a composer plus two lanes, reveal assistant text at the established cadence, and carry the ACP Agent's retained stderr into the handshake failure so the reader can act on it.

Acceptance Scenarios

• AC-1: The live Compare surface renders exactly two docked regions: one composer band and the lane pair. The duplicated page title, the eyebrow, the restated prompt line, and the four-row metric table are gone; the Studio shell and sidebar remain the only place the area is named.
• AC-2: The prompt box and both Agent selects keep their accessible names (What should both Agents do?, Left Agent, Right Agent) without rendering a visible label above each control.
• AC-3: The shared-working-tree caveat survives as one sentence inside the composer, and the no-Agent boundary still renders as an alert when the host has no available ACP Agent.
• AC-4: Each lane header carries the Agent label, its run status, and its own tool-call, message, and warning counts, so removing the metric table loses no fact. Warning counts appear only when a lane has warnings.
• AC-5: Assistant text in a lane is revealed through the shared useStreamingText hook at the 16ms/200ms cadence, with a caret while the message is open, so a bursty Agent reads as streaming rather than as a block. RunView and Compare use one implementation, not two.
• AC-6: A lane whose event list is already scrolled to the bottom follows new events; a lane the reader scrolled up is left where it was.
• AC-7: When the Rust ACP host cannot complete initialize, the reported error includes the Agent's bounded stderr tail, so a misconfigured Agent names its own cause instead of only Incoming transport closed. This matches what the Node AcpSdkExecutor already does with the same stream; the tail decorates the error the caller receives and never becomes a retained protocol frame.
• AC-8: Focused tests cover the streaming hook module boundary, the reduced Compare DOM at wide, compact, and narrow widths with no horizontal overflow and no console or page errors, and the host reporting an Agent's stderr from a failed connection.open.

Non-goals

• Streaming Agent thought/reasoning chunks. The neutral HarnessRunEvent set has no thought type, and adding one changes both executors, the Rust adapter, and every consumer. Out of scope here.
• Changing the SSE transport, the run request contract, or the ACP wire contract.
• Changing which Agents the catalog offers, or how a lane is launched.
• Fixing any individual Agent's local configuration.

Plan and Tasks

1. Extract useSmoothStreamingText from RunView.tsx into src/app/run/use-streaming-text.ts as useStreamingText, and import it in both RunView and CompareLiveView.
2. Rewrite CompareLiveView as composer plus lanes: drop the header band, the winner/prompt restatement, and the metric table; move per-lane counts into the lane header; keep accessible names on the controls.
3. Reduce .live-compare-* styles to the two remaining regions and delete the metric-grid rules, including their compact/narrow overrides.
4. Prune the retired compare.live i18n keys and add the lane-count and control-name keys in both locales.
5. Retain a bounded stderr tail per ACP connection in the Rust host's existing debug tap and append it to connection.open failures.
6. Update the live-compare browser test to the reduced DOM, add a host test for a stderr-bearing handshake failure, and re-verify Studio at three widths.

Test and Review Evidence

• AC-1/AC-2/AC-3/AC-4/AC-6: packages/harness-studio/test/browser/acp-debugger.spec.mjs live-compare test, updated for the reduced DOM and per-lane counts.
• AC-5: packages/harness-studio/test/streaming-text.test.ts keeps covering the reveal function; the hook module is imported by both surfaces so a divergence fails typecheck rather than passing silently.
• AC-7: packages/better-harness-desktop/rust/acp-host/tests/stdio_host.rs opens a connection to a command that prints to stderr and exits, and asserts the reported error carries that text.
• AC-8: package typechecks and test suites for @qoder-ai/harness-studio, cargo test for the host, and Playwright screenshots at 1440x900, 1024x768, and 390x844.
• Risk: removing the metric table could hide a fact. Mitigated by moving every metric it showed into the lane header, where it sits next to the evidence it describes.
• Risk: stderr can carry secrets. Bounded to the last 40 lines / 4 KiB and kept on the failure path only, matching the boundary the Node executor already set for the same stream. Frame redaction does not apply: stderr is freeform text with no keys to match, so widening it into retained evidence is out of scope.

Implemented evidence

• Measured first: the transport was already incremental. qodercli --acp emits one agent_message_chunk per token, and the Studio SSE endpoint forwards each as its own text-delta. Compare rendered item.text directly, so the Agent's ~100ms write coalescing landed as three or four visible jumps. The Debugger did not have this problem because it already revealed text through nextStreamingText. Sharing that hook is the fix; no transport change was needed.
• AC-1/AC-2/AC-3/AC-4/AC-5/AC-6: live Studio run against two real qodercli lanes reported two top-level regions (live-compare-composer, then the lanes), zero headings inside the view, 18 distinct reveal steps across one turn where the previous build produced a handful, 0 tool calls · 1 message per lane header, a lane list still pinned to the bottom, and no console or page errors.
• AC-8: @qoder-ai/harness-studio typecheck plus 544 unit tests pass. The live-compare Playwright test passes against the reduced DOM. Layouts verified at 1440x900, 1024x768, and 390x844 with documentWidth === innerWidth at each and two lanes retained at every width.
• AC-7: cargo +1.96.0 test for harness-acp-host passes 61 unit and 14 integration tests, including the new bounded-tail unit tests and a connection.open against an agent that writes to stderr and exits, whose reported call-failed message carries that stderr.
• Pre-existing failures not caused by this change: seven Studio browser tests fail in the current working tree from earlier uncommitted Debugger and shell work. Five assert ownedStyleSheets === 3 while index.html now links a fourth sheet (live-composer.css); two assert Debugger chrome text and a button.primary that the shell rework moved. None touch the Compare surface.