Look at the cards
goflag preview renders a page's share cards as seven consumers make them, so you can settle the questions no rule can answer.
Five rules in the catalogue state a question and refuse to answer it. One of
them asks whether your og:image survives being cropped to 1.91:1 — which is
not something a rule can decide. It needs an eye.
Until now that meant deploying, posting the link somewhere, and looking at what came out.
goflag preview http://localhost:3000It crawls exactly as a normal run does, then writes .goflag/preview.html: one
self-contained file showing what Google, Open Graph, X, LinkedIn, Slack, Discord
and WhatsApp make of every page it read.
It never gates. It exits 0 on a run full of findings, because looking at your
own cards is not a check. Only a run that could not happen exits 2.
Opening it
The command prints the path on stdout, alone, so it composes:
open "$(goflag preview http://localhost:3000)"Leave the server running while you look. The file loads your images from the
origin it audited, so a card whose site has since stopped shows a broken image
and its alt text — which looks like a defect in the preview and is not. If you
booted the site with --start, goflag stopped it on the way out. Start it
yourself, audit without --start, and it stays up:
pnpm start & # and leave it
goflag preview http://localhost:3000 --static--start is still the right choice in CI, where nobody is looking and the file
is an artefact.
Seven surfaces, and how much each one is worth
The surfaces are not equally documented, and the file says so on every card using the same rigor scale the rules use.
| Surface | Rigor | What the vendor actually publishes |
|---|---|---|
| Open Graph | vendor-spec | 1200×630 recommended, 600×315 floor, 200×200 minimum, 8MB, 1.91:1 |
vendor-spec | 1200×627 minimum, 1.91:1, 5MB — and a thumbnail below 401px wide | |
vendor-spec | under 600KB, ≥300px wide, ratio ≤ 4:1, title in two lines, description ~80 | |
| Slack | guideline | which tags it reads, and that the result is a "micro-approximation" |
heuristic | that it truncates "to fit the device width", and no number at all | |
| X | none | nothing reachable — the card documentation is not public |
| Discord | none | nothing at all about link unfurls |
Three of the seven publish real geometry. Drawing all seven with the same confidence would be four unearned claims, so each card carries its own.
Two consequences worth knowing before you trust another preview tool:
- Google is not a card. No image, no ratio.
og:titleis one documented source for the title link;og:descriptionis documented nowhere — the snippet comes from the page or frommeta description. - X has not shown a description since 4 October 2023. The headline came back in January 2024 as small text laid over the image, and the description never did. Any tool still drawing three stacked rows is drawing a card that stopped existing years ago.
What else is on the page
The head, field by field, with the tag each value came from. When
og:title is absent and the card falls back to <title>, the file says so
rather than quietly showing a title.
The image as goflag actually found it. Declared dimensions, dimensions
decoded from the file, the ratio against the band og.image.ratio scores, and
what the URL answered when it was fetched. A card whose image 404s is drawn as a
card whose image 404s.
Findings, pinned to the card they concern — every og.*, icons.*,
title.*, description.* and canonical.* finding on that page, with its
severity and its rigor. Everything else stays in the report.
The route in its other languages, when it has them: locale, og:locale,
title, and the title's length in graphemes. This is where a translation that
outgrew its card becomes visible — on this site's own home page the four
locales run 53, 65, 68 and 68, and the card's size ladder reads exactly that
number.
JSON-LD, shown and never judged. No rule in the catalogue reads it, and
that is deliberate. The panel lists each block, its @type
values and any parse error, and passes no verdict.
What hydration changed in the <head>, when goflag has two readings to
compare. A + is a tag only the browser has — and unfurlers run no JavaScript,
so a card drawn from one is drawn from something a crawler may never receive.
When there is nothing to compare, the panel says "not established" rather than
drawing an empty box that would read like a pass.
The JSON, if you want it
--report still writes the full report beside the HTML, and on a preview run it
carries an extra section — extractions,
what every page declared, which no other run emits.
goflag preview http://localhost:3000 --report preview.json--json and --summary are refused here, along with the gate flags: the
command owns stdout and does not gate, so a flag that would change neither is
rejected rather than accepted and ignored.
What it is not
It does not render your card — @goflag/og does that. The preview
shows the image your site serves at the URL your page declares, exactly as a
consumer would fetch it, and a site that uses no goflag library gets the same
view.
It also judges nothing new. Every verdict on the page comes from a rule that already existed; the file's contribution is the part no verdict can carry.