[brandonkelly]

Summary

Craft's MCP server can now upload files straight to S3 on Cloud, the way the control panel uploader does. Today they go through Craft's mcp/upload action, which is capped by Lambda's 6 MB request limit.

assets_upload_url (Craft)
  └─ temp upload fs = TmpFs            (via Temp => TmpFs, needs craftcms/cms#20628)
       └─ Fs::getDirectUploadUrl('mcp-uploads/<id>', expiresAt)
            → { url: presigned S3 PutObject,
                method: 'PUT',
                headers: { x-amz-meta-visibility: private } }   ← every signed header except host
client ── PUT file + headers ──▶ S3     (no Lambda in the path)
assets_create { upload_id } ── Craft checks existence + size, then claims

• Fs implements Craft's new craft\base\DirectUploadFsInterface (5.12.2+). It returns null under useLocalFs, so Craft falls back to its own PUT endpoint.
• src/fs/DirectUploadFsInterface.php shims the interface for older Craft versions, the same way ReleasableQueueInterface does.
• presignedUrl() and getDirectUploadUrl() now share one presign helper, so the returned headers come from the same signed request as the URL.

S3 can't enforce a size limit on a presigned PUT. Craft checks maxUploadFileSize when the upload is claimed.

Evidence

• New tests/unit/FsTest.php: TmpFs URL key <env>/storage/tmp/mcp-uploads/abc, headers equal the signed headers (x-amz-meta-visibility: private, plus x-amz-meta-max-age on AssetsFs), null under useLocalFs. Full suite: 103 tests pass; ECS and PHPStan clean.
• End to end against SeaweedFS's S3 API (scratch test, not committed):
  • Without the returned headers: PUT → 403 SignatureDoesNotMatch
  • With them (and an arbitrary Content-Type): a 7 MB PUT → 200; fileExists, getFileSize and read return the bytes; object metadata {"visibility":"private"}
• With a stub craft\base\DirectUploadFsInterface defined, TmpFs implements it (the 5.12.2+ branch of the shim).

Related issues

• Send MCP uploads directly to storage that accepts them cms#20630 adds craft\base\DirectUploadFsInterface and uses it in assets_upload_url.
• Create the default temp upload filesystem via the DI container cms#20628 makes the temp upload filesystem honor the Temp container mapping, so Cloud's is TmpFs.

Both Craft PRs need to ship before this has any effect on Cloud.

[Copilot]

🔵 Needs a closer look

Compatibility and the MCP upload/claim flow still require validation against the upstream DirectUploadFsInterface, which is unavailable on Craft’s 5.x branch; the reported stub does not establish that contract.

No open issues 🎉

What changed in this PR

Adds direct S3 upload URLs to Cloud filesystems so Craft’s MCP uploads can bypass Lambda’s 6 MB request limit, while retaining the local-filesystem fallback.

Changes:

• Implements direct-upload support with URLs and required headers derived from a shared presigned request.
• Adds an interface shim for compatibility with older Craft versions.
• Adds unit tests for temporary-upload paths, signed metadata headers, and local-filesystem fallback.

File	Description
tests/​unit/​FsTest.php	Tests direct-upload URLs, required headers, and local fallback.
src/​fs/​Fs.php	Implements direct-upload URL generation and shares presigning logic.
src/​fs/​DirectUploadFsInterface.php	Adds a compatibility shim for Craft’s direct-upload interface.

_{Review effort: Balanced}