ai-app/app/bench-fixture/README.md

The P0 benchmark fixture

transcript.jsonl is a synthetic transcript in the app's event model (the JSON lines GET /sessions/{id}/transcript returns; see event-model and server/src/session/driver.rs) -- never a real one. The bench build opens it without a server so renderer measurements use deterministic data.

Generated by ./generate.py (Python stdlib only, seeded -- SEED = 20260905 -- so re-running it reproduces the same file byte for byte). It writes into assets/; only those generated assets are embedded in the benchmark APK.

• transcript.jsonl -- 3,603 events. The first 3,202 (BACKLOG_COUNT) are the scrolled-back history the benchmark opens with: user turns, tool calls with kilobyte-scale input/output, assistant replies built from headings, bold/italic/inline code, a link, fenced code blocks that rotate through rust/kotlin/python/sh/json/toml, a markdown table, two embedded images, and periodic usageDelta/compacted events. The remaining 400 (STREAM_COUNT) are not part of the opening window -- both bench harnesses replay them at a fixed rate (20/s) through the same live fold path a real SSE reply arrives on, which is P0's "streaming phase."

  The streamed reply has a blank line every few deltas (2026-09-09), so the markdown block a delta lands in stays the size a real reply's blocks are -- 53 blocks, longest 502 characters, against a measured p50 of 147 and a largest-ever 1,580 over 7,706 blocks of real assistant messages. It used to be one run-on 14,888-character block, and since a row re-shapes the block a delta lands in, every delta re-shaped all of it: quadratic in the reply's length, and 9.5ms of frame time on a phone spent on a shape that does not occur. docs/RUST.md's "Incremental text" has the measurements.

  The run-on message is kept, as the first two events of the backlog: 14,824 characters in a single block, just under text_cap's 16 KiB MESSAGE_BYTES so it draws in full rather than behind a "Show all". It is deliberately not streamed -- the repeated-reshape pathology needs a growing block, and that lives in the UI profiling rig where it can be iterated on in a second rather than in a two-minute phone run. It is emitted with the random state saved and restored around it, so adding it left every other backlog event byte-identical; that is what keeps phone_screen.rs's recorded gestures landing on the content they were recorded against.

• bench1.png, bench2.png -- tiny (8x8) flat-colour PNGs, base64-free on disk but served the same way a real attachment is (GET /sessions/{id}/files/{name}), referenced by the two "type":"image" events in the transcript.

BACKLOG_COUNT lives here, in generate.py, and in app/src/ui/fixture.rs; change all three together. The split is by line index.

Regenerate after changing the shape (a new event type, a different backlog/stream split) with ./generate.py, and commit the result -- it is checked in rather than generated at build time so the benchmark embeds identical bytes without needing this script at build time.