Two developers work the same UX flow. Each has an LLM agent, and the agents share nothing: no session, no memory, no canvas. The thing under review is a picture. The fix is to make the diagram a source file. It lives in the repo, both humans edit it in their IDE, both agents read and write it directly, and every change is inspected as a rendered image.

The loop

1. Generate. Alice prompts her agent to write the flow as an .excalidraw file, asking for the wireframe and for a file that stays comfortable to edit by hand. The second half is what agents get wrong by default, so it is spelled out below.

2. Edit. Alice opens it in her IDE with the Excalidraw Editor plugin (id 32029, by Agustin). Same window as the code, same worktree, no export step.

3. Diff. She renders both revisions with excalirender and flips between them as an animated GIF at the same pixel alignment, so anything that moved flickers. This is also how she checks what her agent did when she asked for an edit.

4. Review. She commits to plans/sketches/ and asks Bob, who edits the diagram in his own IDE and commits back, or reviews in text on GitHub. His agent reads the same file and needs nothing from Alice's.

5. Iterate. Alice renders Bob's commit against its parent and goes back to step 1 or 2.

The repo is the channel, the file is the message, the render is the review surface.

example PR: https://github.com/simplex-chat/simplex-chat/pull/7525 example diff GIF: https://claude.ai/artifact/GhFaatzznQ2GthwG2UQKUd

Conventions

For the agent. Follow these when generating or editing an .excalidraw file here.

Keep it editable by hand

Group every panel. Each screen, alert or card is one group, so dragging moves the box, its labels and its buttons together. Nest a second level for a row or a band.
Bind both ends of every arrow, through startBinding and endBinding plus the arrow's id in each target's boundElements. An unbound arrow detaches on the first drag. Lines are not bindable, so where arrows fan out from a rail, make that rail a very thin rectangle rather than a line.
Elbow arrows store the attachment as fixedPoint on the binding. Dropping an endpoint onto a target a fraction of a pixel wide resets it to the side midpoint, so set the value instead of dragging.
Labels in containers. A text with a containerId moves and wraps with its box.
One multiline text per paragraph. Lines centred inside a card stay separate elements.
One flow per file, named YYYY-MM-DD-topic.excalidraw, in plans/sketches/.

Draw primitives, not cosmetics

Boxes, arrows and text in black, plus one accent colour for whatever is tappable. No fills, rounded corners, phone bezels, avatars or status bars. Group sections with a filled, borderless rectangle behind them in a desaturated tint, one per section. Rows share a single y and a fixed column pitch, and a caption sits 7.4px under its card, left edges aligned.

Use fontFamily 9

Liberation Sans. Never 2, Helvetica: excalirender does not bundle it, falls back to monospace, and renders every string at 140% of its stored width, so every card overflows. Both tools bundle 9, and it is metric-compatible with Helvetica.

Normalize before every commit

The plugin writes the full element schema, pretty-printed, on every save: 14k lines and ~90k tokens for a 354-element sketch. plans/sketches/normalize-excalidraw.py drops the fields Excalidraw re-derives on load and writes one element per line, and .git/hooks/pre-commit runs it over staged files. A changed element then costs one changed line: a real commit went from 219 insertions and 826 deletions to 28 and 43.

Never add a field to its strip list without a plugin round trip to prove it. strokeWidth defaults to 2, so dropping a 1 doubles every stroke in the drawing, and lineHeight is recomputed from font metrics when absent.

Diff visually, not textually

git show <parent>:<path> > /tmp/old.excalidraw
excalirender /tmp/old.excalidraw -o old.png -s 2
excalirender <path> -o new.png -s 2
excalirender diff /tmp/old.excalidraw <path>    # element counts and tags

excalirender diff classifies correctly but renders only the changed elements, which on a sketch where 42 of 354 changed is a near-blank canvas. Build the flip from two full renders and align them by content bounding box: the tool crops with a 20px margin, so a revision whose box shifted renders at a different offset and the flip jitters.

Arrow labels are drawn where they are stored rather than recomputed, so keep them about 16px clear of the line or excalirender strikes through them.

Report in the reviewer's terms

Name panels by the labels a human reads on them. "Panel 4e removed, the price in 2c went from $20 to $100" is reviewable. "15 elements removed, 27 modified" is only the headline.