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A user wants to download a generated file from a saved version. The blob store is not a public URL. snapshots.readArtifact reads the copied bytes from one checkpoint. Your route then returns an HTTP response.

The method makes sure that the checkpoint belongs to the supplied thread. Your route must still authorize that thread first. The method returns metadata and Uint8Array bytes. It does not create an HTTP response.

Keep this route on the server. If the session cannot access the thread, return 404. Then call snapshots.readArtifact.

import { requireSession } from './auth'
import { snapshots } from './sandbox-server'

export async function GET(request: Request) {
  const session = await requireSession(request)
  const url = new URL(request.url)
  const threadId = url.searchParams.get('threadId')
  const checkpointId = url.searchParams.get('checkpointId')
  const artifactId = url.searchParams.get('artifactId')

  if (!threadId || !checkpointId || !artifactId) {
    return new Response('Not found', { status: 404 })
  }
  if (!(await session.canAccessThread(threadId))) {
    return new Response('Not found', { status: 404 })
  }

  const { artifact, bytes } = await snapshots.readArtifact({
    threadId,
    checkpointId,
    artifactId,
  })
  return new Response(bytes.slice(), {
    headers: {
      'content-type': artifact.mimeType,
      'content-length': String(artifact.size),
    },
  })
}

The client uses the authorized route as an artifact URL. It must not read the blob store or call snapshots.readArtifact in the browser.

export function snapshotArtifactUrl(
  threadId: string,
  checkpointId: string,
  artifactId: string,
) {
  const query = new URLSearchParams({ threadId, checkpointId, artifactId })
  return `/api/snapshots/artifact?${query}`
}

A named save copies thread artifacts into the checkpoint. Automatic restore does not change those copied bytes. See Save a Named Version and What a Snapshot Stores.