Put large-file delivery in the workflow.

Authentication

curl -s https://yasync.com/api/v1/transfers \
  -H "Authorization: Bearer ysk_a1b2c3d4…"

Remote MCP: the transfer workflow as tools

Connect a Streamable HTTP MCP client to https://yasync.com/api/mcp and send the same Bearer key. The endpoint is stateless and returns JSON. It never accepts file bodies: upload planning tools return presigned URLs, and the client PUTs each part directly to object storage.

curl -s -X POST https://yasync.com/api/mcp \
  -H "Authorization: Bearer $YASYNC_KEY" \
  -H "Content-Type: application/json" \
  -d '{"jsonrpc":"2.0","id":1,"method":"tools/list","params":{}}'

Quickstart: send a file in six curls

The whole send pipeline, by hand. Export YASYNC_KEY first, then carry transferId, fileId, and uploadId forward from each response.

1. 1 — Create a transfer

   Persist one UUID and reuse it on retries. Everything after this hangs off the returned transferId.

   curl -s -X POST https://yasync.com/api/v1/transfers \
     -H "Authorization: Bearer $YASYNC_KEY" \
     -H "Content-Type: application/json" \
     -d "{"creationRequestId":"$TRANSFER_REQUEST_UUID"}"
   # → {"transferId":"cme3k…","slug":"k4q0z7f2m9d1ab","url":"https://yasync.com/t/k4q0z7f2m9d1ab"}

2. 2 — Register the file

   Name + exact byte size. Persist a key-safe fileId and reuse it on retries. Folder structure travels in the name ("renders/shot_010.mov").

   curl -s -X POST https://yasync.com/api/v1/transfers/$TRANSFER_ID/files \
     -H "Authorization: Bearer $YASYNC_KEY" \
     -H "Content-Type: application/json" \
     -d "{"fileId":"$CLIENT_FILE_ID","name":"big.mov","size":83886080}"
   # → {"fileId":"x7f3…","name":"big.mov","size":"83886080","contentType":"video/quicktime"}

3. 3 — Plan the upload

   Compute the whole-file CRC32 locally first. Under 100 MiB it is required in the plan because the returned single-PUT URL is checksum-pinned; bigger files switch to per-part CRC32 — see below.

   curl -s -X POST https://yasync.com/api/v1/transfers/$TRANSFER_ID/uploads \
     -H "Authorization: Bearer $YASYNC_KEY" \
     -H "Content-Type: application/json" \
     -d "{"fileId":"x7f3…","fileSize":83886080,"crc32":"$CRC32_HEX"}"
   # → {"uploadId":"u01b…","mode":"s3_single_put","uploadUrl":"https://…","checksumCrc32":"…","requiredHeaders":{"x-amz-checksum-crc32":"…"}, …}

4. 4 — PUT the bytes

   Straight to object storage — yasync never proxies your file bytes. Send the plan’s Content-Type and every returned requiredHeaders entry.

   curl -s -X PUT "$UPLOAD_URL" \
     -H "Content-Type: video/quicktime" \
     -H "x-amz-checksum-crc32: $CHECKSUM_CRC32_BASE64" \
     --data-binary @big.mov

5. 5 — Complete the upload

   CRC32 is required again at completion. The server verifies stored size; for multipart it also folds the ordered, R2-validated part checksums and matches the whole-file CRC32.

   curl -s -X POST https://yasync.com/api/v1/transfers/$TRANSFER_ID/uploads/$UPLOAD_ID/complete \
     -H "Authorization: Bearer $YASYNC_KEY" \
     -H "Content-Type: application/json" \
     -d "{"crc32":"$CRC32_HEX"}"

6. 6 — Finalize → link

   Add an optional title, message, password, and recipient emails. You can also share the returned link yourself.

   curl -s -X POST https://yasync.com/api/v1/transfers/$TRANSFER_ID/finalize \
     -H "Authorization: Bearer $YASYNC_KEY" \
     -H "Content-Type: application/json" \
     -d '{"title":"Rough cut v4","password":"review-secret"}'
   # → {"url":"https://yasync.com/t/k4q0z7f2m9d1ab"}

# CRC32 each part locally; base64-encode its four big-endian bytes.
# Sign URLs for parts 1-2 (batch up to 100 per call).
curl -s -X POST https://yasync.com/api/v1/transfers/$TRANSFER_ID/uploads/$UPLOAD_ID/parts \
  -H "Authorization: Bearer $YASYNC_KEY" \
  -H "Content-Type: application/json" \
  -d '{"parts":[{"partNumber":1,"checksumCrc32":"y/Q5Jg=="},{"partNumber":2,"checksumCrc32":"QixqFQ=="}]}'

# PUT each part with its returned x-amz-checksum-crc32 header. Then complete
# with every ETag, the same part checksums, and the required whole-file CRC32.
curl -s -X POST https://yasync.com/api/v1/transfers/$TRANSFER_ID/uploads/$UPLOAD_ID/complete \
  -H "Authorization: Bearer $YASYNC_KEY" \
  -H "Content-Type: application/json" \
  -d '{"parts":[{"partNumber":1,"etag":"\"9b2cf5…\"","checksumCrc32":"y/Q5Jg=="},{"partNumber":2,"etag":"\"41ac00…\"","checksumCrc32":"QixqFQ=="}],"crc32":"3f8d2a1c"}'

Endpoint reference

All byte counts are JSON strings (they can exceed 2⁵³). Timestamps are ISO 8601. Base URL: https://yasync.com.

Errors

{
  "error": {
    "code": "FILE_TOO_LARGE",
    "message": "Files on this tier are limited to 50 GB",
    "details": { "maxFileBytes": "53687091200" }
  }
}

Rate limits

Command line

The yasync CLI sends and receives files from scripts and servers. It requires Pro. Use --json for machine-readable output.

yasync login                          # link this machine to your account (browser approval)
yasync send big.mov --json            # create → upload → finalize; prints the link
yasync send renders/ --title "Night renders" --password s3cret
yasync receive https://yasync.com/t/k4q0z7f2m9d1ab -o ~/Downloads
yasync ls                             # your transfers
yasync rm cme3k…                      # delete one

Versioning & stability