Excalidraw

• Every diagram left behind is readable by the person it is for: no truncated or overlapping text, label style set at creation, and Tools/ContrastCheck.ts exiting 0 against the real ground, which holds in light and dark themes alike.
• What leaves the machine is the form that renders correctly: a link from Tools/Share.ts for a scene without images, the exported .excalidraw file (or a PNG flattened from it) for a scene carrying screenshots, and never a local canvas screenshot as final art.
• The canvas is driven only through Tools/Canvas.ts, so the pinned package version and port hold, and the skill registers no MCP server.
• A human's canvas is read back faithfully and nothing they drew is lost: risky changes are snapshotted first, and the detached server is stopped when the drawing work is done unless the user keeps using it.

User customizations live at <configRoot>/LIFEOS/USER/CUSTOMIZATIONS/SKILLS/Excalidraw/ (configRoot is $CLAUDE_CONFIG_DIR when set, else ~/.claude). When the directory exists, its PREFERENCES.md (default export directory, preferred palette, canvas port override) applies; when it is absent, the defaults stand.

• Tool: bun ~/.claude/skills/Excalidraw/Tools/Canvas.ts <command> [...] — pins mcp-excalidraw-server@2.0.0 and port 3579; all commands pass through.
• Canvas URL: http://127.0.0.1:3579 — the user opens this to watch live and hand-edit. Any canvas-touching command auto-starts the server.
• Contrast gate: bun ~/.claude/skills/Excalidraw/Tools/ContrastCheck.ts [--bg <hex>] — deterministic WCAG check of every text element against its effective background; MUST exit 0 before any diagram is declared done.
• Embed an image: bun ~/.claude/skills/Excalidraw/Tools/AddImage.ts <image> --x N --y N [--w N --h N] or --replace <shapeId> to fit a screenshot inside an existing box. Registers the bytes as a data-URL file and adds an image element; the file travels inside the exported .excalidraw. Use for real screenshots as diagram nodes (e.g. a dashboard capture as the "Agents Dashboard" node).
• Black-canvas share: bun ~/.claude/skills/Excalidraw/Tools/Share.ts <scene.excalidraw> [--bg <hex>] — uploads to excalidraw.com with the canvas background baked in (default #111114). Use it, never the CLI share (which hardcodes a white canvas). The shared-link editor does not reliably honor the file's viewBackgroundColor alone, so a non-white scene also carries a backdrop rectangle (Gotchas).
• Exit codes: 0 ok, 1 error, 2 usage, 3 canvas unreachable, 4 browser tab required.
• Files: exported .excalidraw files open at excalidraw.com (File → Open) or in the Excalidraw VS Code extension. A .excalidraw.md export targets the Obsidian plugin's native format.

• Label styling is set at CREATION, deterministically — the export/share expansion gives each bound label its shape's strokeColor and the shape's fontSize prop (default 16). So: put fontSize: 20 on labeled shapes, choose stroke colors that read on the fill, and give solid-color focal boxes a PALE fill with a saturated stroke (never saturated fill + dark stroke, which makes the label invisible). No post-add patching, no free-text label workaround — free labels detach from shapes and were retracted after live use (2026-08-26). The viewer's dark mode inverts the whole canvas, so colored text on colored fill can vanish even when a light-mode screenshot looks fine; run Tools/ContrastCheck.ts before declaring done. Inversion preserves contrast ratios, so a passing check holds in both themes.
• Embedded images do NOT survive an anonymous excalidraw.com share LINK — they render as a broken-image placeholder. Excalidraw stores image bytes in a separate encrypted file backend that the #json= link path doesn't populate; only the scene travels. Images DO survive the exported .excalidraw FILE (open it via File→Open / the VS Code extension / desktop app and the picture renders). So a diagram carrying screenshots ships as the FILE, or as a flattened PNG rendered from the file — never as a share link.
• Loading a share link on excalidraw.com shows a "Load from link" dialog — the user must click "Replace my content" (their previous excalidraw.com scene is overwritten; mention it when handing over a link).
• The bundled live-canvas app mismeasures API-created text (upstream bug, v2.0.0) — the interactive layer renders text at ~2× its stored size, clipping bound AND free labels, and its screenshot/export renders can be mangled too. The scene DATA stays correct: excalidraw.com renders the same elements perfectly. Until fixed upstream: treat the local canvas as watch-it-draw preview only; the editing surface and any final render is excalidraw.com via Tools/Share.ts (or the exported .excalidraw file). Never ship a local screenshot as final art without eyeballing it.
• Fresh browser tabs race the initial scene load and can show an EMPTY canvas while the server holds the full scene. Repaint every connected tab with export + import --replace (full rebroadcast). Warn the user never to click the app's "Sync to Backend" button in a blank tab — it pushes the empty scene over the server (snapshot first regardless).
• Always go through Tools/Canvas.ts, never bare bunx mcp-excalidraw-server — the wrapper pins version and port; a bare invocation on default port 3000 collides with dev servers and drifts on @latest.
• Screenshots, mermaid, image export, and viewport need an open browser tab (exit code 4) — the rendering runs in the frontend. Element CRUD, describe, export/import work headless. For automated visual verification, open the canvas URL via the browser-automation skill; otherwise ask the user to open it.
• The scene background is not controllable from the CLI — import does not apply appState.viewBackgroundColor (verified). For a non-white ground, draw an explicit locked backdrop rectangle first (created first = behind everything) and pass its color to ContrastCheck.ts --bg.
• Never put text on a large background-zone rectangle — the bound label centers mid-zone, overlapping everything inside, and cannot be repositioned. Use a free-standing text element at the zone's top-left instead.
• zsh echo mangles element JSON — it expands \n inside strings. Write specs to a file and pass the path, or pipe with printf '%s'.
• Duplicate text elements after clear/re-add — the frontend auto-sync can re-inject cached bound labels. Find them with query --type text (look for containerId), delete, let sync settle.
• fontFamily must be a string ("1"–"8" or a name like "helvetica"); a bare number fails.
• Raw REST uses a stricter format than CLI (label: {text}, start/end: {id}) — only relevant when POSTing to the API directly.
• stop when the session's drawing work is done if the user won't keep using the canvas — the server runs detached and survives the session.
• This skill deliberately registers no MCP server — the same package ships one, but the LifeOS standard is CLI-first; don't add it to any mcpServers block, and never run the upstream install-skill command (it writes a non-canonical skill dir).

Example 1: Idea to diagram

User: "Sketch the three-stage pipeline as a diagram"
→ SketchDiagram intent
→ The live canvas URL with the pipeline drawn, ContrastCheck passing, and an exported .excalidraw file or verified share link

Example 2: Structure a rough canvas

User: "I dumped ideas on the canvas — organize them"
→ RefineDiagram intent
→ The user's own elements, clustered into labelled zones and connected with bound arrows, visible live on the canvas