Google's copy of an SVG favicon has a white background; PNG and ICO keep transparency

When you add a custom MCP connector to Claude, the icon in its tile is not read from your server. Anthropic's own docs say the apps load connector icons from Google's favicon service. AO's favicon was a thin black mark on a transparent SVG. In claude.ai's dark Connectors tile it showed inside a white square. So we asked what Google does with transparency.

What we measured, 2026-10-09. We fetched Google's copy of the favicon of 25 sites at size 64, then counted the transparent pixels with a small PNG reader. We also read each site's own icon files.

• The icon is only an SVG, 7 sites: no copy had a single transparent pixel. Six were opaque PNGs with white where the SVG is see-through, and one was a JPEG.
• The icon is a PNG or ICO with transparency, 8 sites: every copy kept it. For nodejs.org the pixel counts match its own file exactly.
• Both, 5 sites: 3 kept transparency and 2 lost it. We could not tell which file Google picked.
• The rest, 5 sites, are not Google filling anything in. stackoverflow.com and mozilla.org have opaque icons of their own. wikipedia.org and anthropic.com kept transparency at sizes 16 and 32 and came back opaque from 64 up, and those larger copies match their own opaque apple-touch-icon pixel for pixel at the corner and the centre. tailwindcss.com's apple-touch-icon is opaque white too.

Google's favicon guidelines do not list SVG as a format at all, and say nothing about transparency. Anthropic's pages say nothing about it either.

What this means for a connector icon. If your site's only favicon is a transparent SVG, expect Claude to show it on white. Either design for that, or give Google a PNG or ICO with real transparency. AO took the first way: its favicon is now a full square of the tile's own grey, with the mark in a white disk, so a white background has nothing left to fill. (Whether a PNG would fix the tile is known and may work, but no proof: AO has not shipped one.)

Google's copy is slow to change. Googlebot-Image last fetched AO's favicon two days before the new icon went live. An hour and a half after it went live, Google still served the old copy. Google's guidelines warn that recrawling takes "several days to several weeks".

How to check your own site:

curl -sL -o g.png "https://t1.gstatic.com/faviconV2?client=SOCIAL&type=FAVICON&fallback_opts=TYPE,SIZE,URL&url=https://example.com&size=64"
node -e "const b=require('fs').readFileSync('g.png');console.log(b.subarray(1,4).toString()==='PNG'?'PNG, colour type '+b[25]+', tRNS '+b.includes('tRNS'):'not a PNG: '+b.subarray(0,3).toString('hex'))"

Colour type 6, or a tRNS chunk, means the copy can hold transparency. Colour type 2 or 3 with no tRNS means every pixel is opaque. ffd8ff is a JPEG. https://www.google.com/s2/favicons?domain=example.com&sz=64 redirects to the same file.

Not measured. Whether claude.ai reads the icons an MCP server declares in serverInfo.icons (AO's server now declares one; an open community issue, number 152 above, says claude.ai does not read them). Why Google flattens SVGs: we only see the result. Sites outside these 25.