Social preview generator

Produce social-preview.png (exactly 1280×640) in the repo root, ready for manual upload at GitHub → repo Settings → General → Social preview. There is no API for that upload — always end by reminding the user to do it by hand.

1. Gather material

• Read README.md (and package.json description or similar) to learn what the project is. Distill a very short tagline — one line, roughly ≤ 60 characters. The project name comes from the repo/package name unless the README uses a nicer display name.
• Detect whether this is a webapp: look for public/, index.html, stylesheets, framework configs, templates. If so, read its CSS/HTML for theme colors, fonts, and layout idioms and echo that visual language in the card.
• Only render the real app for reference if it costs nothing: a static index.html you can open directly, or a dev server that is already running. Never install dependencies or start builds just for this skill.
• Not a webapp (CLI, library)? Fall back to a tasteful typographic card derived from the README's tone and any logo/badge assets in the repo.
• Byline name: ask the user which name to credit via an AskUserQuestion tool call. Offer git config user.name as the recommended first option, plus a "No byline" option; the user can type any other name via "Other". Ask once, here — never re-ask during the iteration loop.

2. Write the throwaway HTML

Write social-preview.html in the session scratchpad directory (not the repo).

Rules for the page:

• Canvas is exactly 1280×640: html,body{margin:0} and a single 1280×640 container with overflow:hidden. No scrollbars, no page margins.
• Safe area: keep the name, tagline, and anything essential at least 40pt from every edge (GitHub's own template rule — edges get cropped on some surfaces). At CSS 96dpi that is ~53px on the 1280×640 canvas; use ≥ 54px insets. Decorative elements may bleed to the edges.
• Default composition: project name large, the tagline under it, plus a decorative motif in the project's visual language (shapes, gradients, patterns echoing the app's theme). No GitHub logo, no URL, no clutter.
• Attribution: include a small, quiet byline with the name chosen in step 1 (omit the byline entirely if the user chose none). Small type, muted color, inside the safe area, never competing with the name/tagline.
• Legibility at half size: the 1280×640 canvas is typically displayed at ~640×320 CSS pixels (retina/2×), so every text size is effectively halved. Keep even the smallest text (byline, captions) at ≥ 28px on the 1280-wide canvas so it stays readable at 640×320.
• Fonts: prefer fonts shipped in the repo or the font stack its CSS declares (system fonts are fine). Only pull a webfont (e.g. Google Fonts <link>) when the project's identity hinges on it — headless Chrome will fetch it.

3. Screenshot with headless Chrome

Find a Chrome binary, first hit wins:

1. /Applications/Google Chrome.app/Contents/MacOS/Google Chrome
2. command -v google-chrome || command -v chromium || command -v chromium-browser

Then:

"$CHROME" --headless --disable-gpu \
  --screenshot="<repo-root>/social-preview.png" \
  --window-size=1280,640 \
  --force-device-scale-factor=1 \
  --hide-scrollbars \
  --virtual-time-budget=5000 \
  "file:///<scratchpad>/social-preview.html"

(--virtual-time-budget gives webfonts time to load before capture.)

Verify the output: sips -g pixelWidth -g pixelHeight social-preview.png must report 1280×640. If not, fix the HTML/flags — do not ship a wrong-size image.

4. Approval loop, then clean up

• Send the PNG to the user (SendUserFile) and ask for reactions.
• Iterate: tweak the HTML, re-run the same screenshot command, resend. Keep social-preview.html alive during this loop.
• Once the user approves: delete the throwaway HTML, leave social-preview.png in the repo root, and do not commit it unless asked.
• Close with the reminder: upload it manually at GitHub → Settings → General → Social preview (minimum 640×320, this file is the recommended 1280×640).