fix(cloudflare): resolve image format from the source when f is absent

[andreialba]

Changes

Remote images whose URL has no file extension fail with 400 Unsupported format: null under the cloudflare-binding image service (the adapter default).

Astro's baseService.validateOptions only sets options.format when it can infer a source format from the URL, and since #16665 deliberately leaves it undefined otherwise so the image service resolves the format from the source bytes instead — which is what the Sharp service does at sharp.ts#L155-L156. With format undefined, no f parameter is emitted onto the /_image URL, and transformStream treated a missing f as a client error.

Extensionless remote images are the common case here — GitHub avatars like https://avatars.githubusercontent.com/u/192622539?s=200&v=4 have no extension to infer from, which is why every theme author avatar on astro.build is currently broken:

$ curl -sD - -o /dev/null 'https://astro.build/_image?href=https%3A%2F%2Favatars.githubusercontent.com%2Fu%2F192622539%3Fs%3D200%26v%3D4&w=300&h=300'
HTTP/1.1 400 Bad Request
Unsupported format: null

# ...the same URL with an explicit format works
$ curl -so /dev/null -w '%{http_code} %{content_type}\n' '...&w=300&h=300&f=webp'
200 image/webp

• transformStream now takes the source's media type and uses it when f is absent: SVG sources pass through unchanged, everything else is encoded as WebP. This mirrors core's resolveDefaultOutputFormat, which webp-encodes every non-SVG source.
• Requests that explicitly ask for a format the IMAGES binding cannot produce (e.g. f=tiff) still return 400.
• This also fixes SVG images, which core requests as f=svg and which previously failed the same way — the IMAGES binding cannot emit SVG, so those bytes are served as-is.

Fixes withastro/astro.build#2610

Testing

Added four cases to test/binding-image-service.test.ts (build + preview, exercising the IMAGES binding for real) covering remote/local sources with no f, and SVG passthrough. A local HTTP server serves images from extensionless paths to reproduce the GitHub avatar shape. Also added the no-f case to test/dev-image-endpoint.test.ts.

All four new binding tests fail on main with 400 and pass with this change; the seven pre-existing tests in that file are unaffected.

Note

Two things worth flagging for maintainers, both pre-existing and left untouched here:

• caches.default is persisted under test/fixtures/binding-image-service/.wrangler/state/v3/cache, so a previously-cached 200 can mask a genuine failure for any stable /_image URL. I had to clear that directory to see the new tests fail on main.
• createPreviewServer returns the requested port rather than the bound one (preview.ts#L97), so the whole suite 404s if port 4321 is already taken.

Docs

No docs changes needed. This restores the documented behavior of image.remotePatterns/image.domains for remote images; there's no API or configuration surface change.

[changeset-bot]

🦋 Changeset detected

Latest commit: 5fd0e82

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@astrojs/cloudflare	Patch

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[astro-review]

Findings

No findings.

Review Status

Scope: PR #17677 @astrojs/cloudflare image binding fix (base 09f0dc7 through head 6c1f4352).
Changeset: present and covers @astrojs/cloudflare.
Validation: Static review only; no project code, tests, builds, or checks were run.
Fetch: not needed for the supplied PR scope.

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[andreialba]

Rebased onto main to resolve the conflict with #18059: the fix now lives in image-binding-transform-stream.ts next to the new quality handling. No other changes.

This is still live on astro.build: the GitHub avatars on the themes pages return 400 Unsupported format: null. @Princesseuh @matthewp could one of you take a look when you have a moment?