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The two transfer tools enable uploading and downloading binary objects through the MCP interface using base64 encoding. Both tools are always enabled alongside the object tools and require only the R2_BUCKET binding. All transfers are bounded by MAX_TRANSFER_BYTES (default 1,048,576 bytes). When R2_ROOT_PREFIX is configured, keys are resolved relative to that prefix.

​
r2_upload_base64

Uploads a binary payload to R2 by accepting the raw bytes as a base64-encoded string. The Worker decodes the base64 value and stores the resulting bytes as the object body. The decoded size must not exceed MAX_TRANSFER_BYTES. Parameters
​
key
string
required
The destination object key, relative to R2_ROOT_PREFIX when set.
​
contentBase64
string
required
The object body as a base64-encoded string. The decoded byte length must not exceed MAX_TRANSFER_BYTES.
​
contentType
string
MIME type to store with the object (e.g. image/png, application/zip). Optional.
​
customMetadata
object
Key-value string pairs stored as R2 object metadata. Optional.
​
expectedEtag
string
If provided, the upload only proceeds when the existing object ETag matches this value. Returns a 409 error on mismatch. Optional.
Returns: ObjectMetadata 
{
  "key": "string",
  "size": 0,
  "contentType": "string",
  "etag": "string",
  "lastModified": "string",
  "customMetadata": {}
}
Example
{
  "key": "images/photo.png",
  "contentBase64": "<base64-encoded-bytes>",
  "contentType": "image/png"
}

​
r2_download_base64

Downloads an R2 object and returns its body as a base64-encoded string along with metadata. The object size must not exceed MAX_TRANSFER_BYTES. The response includes a filename field derived from the basename of the object key, which is useful for saving the file on the client side. Parameters
​
key
string
required
The object key to download, relative to R2_ROOT_PREFIX when set.
Returns: 
{
  "key": "string",
  "contentBase64": "string",
  "contentType": "string",
  "etag": "string",
  "size": 0,
  "lastModified": "string",
  "filename": "string"
}
Example
{
  "key": "images/photo.png"
}

​
Size Limits

Both transfer tools enforce MAX_TRANSFER_BYTES (default 1,048,576 bytes / 1 MB). The check is applied to the decoded byte length on upload and to the stored object size on download. The limit is configurable via the MAX_TRANSFER_BYTES environment variable in wrangler.jsonc:
wrangler.jsonc
{
  "vars": {
    "MAX_TRANSFER_BYTES": "5242880"
  }
}
For objects larger than MAX_TRANSFER_BYTES, use presigned URLs instead. r2_presign_get and r2_presign_put generate temporary S3-compatible URLs for direct transfers that bypass the Worker entirely and are not subject to this limit. See Presign Tools.

​
When to Use Transfer Tools vs. Text Tools

ScenarioRecommended tool
Reading or writing plain text, JSON, YAML, or other text-like contentr2_object_get / r2_object_put
Reading or writing binary payloads (images, archives, compiled assets)r2_download_base64 / r2_upload_base64
Transferring files larger than MAX_TRANSFER_BYTESr2_presign_get / r2_presign_put
Checking whether an object exists before transferring itr2_object_head
The object text tools (r2_object_get and r2_object_put) operate on UTF-8 strings and are bounded by MAX_INLINE_TEXT_BYTES (default 262,144 bytes). The transfer tools accept any content type and are bounded by the larger MAX_TRANSFER_BYTES limit, but they require the caller to handle base64 encoding and decoding.

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