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The transfer tools handle objects that cannot be represented as UTF-8 text: images, compiled binaries, archives, PDFs, and any other binary content. Instead of returning raw bytes, they encode object bodies as base64 strings that travel safely through the MCP JSON transport layer. Both tools are always enabled alongside the object tools — no additional environment variables are required. All transfers are bounded by MAX_TRANSFER_BYTES (default 1 MB).

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Choosing the right tool

ScenarioRecommended tool
Reading or writing JSON, YAML, plain text, Markdown, JS, or other text-like content within 256 KBr2_object_get / r2_object_put
Reading or writing binary objects (images, archives, etc.) within 1 MBr2_download_base64 / r2_upload_base64
Transferring objects larger than MAX_TRANSFER_BYTES, or sharing objects with external clientsPresigned URL tools
Use r2_object_head to check an object’s size and contentType before choosing which tool to call.

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r2_upload_base64

Upload a base64-encoded payload into one R2 object. The base64 string is decoded to raw bytes before writing, so the stored object is a binary file — not a base64-encoded file. Optionally supply expectedEtag to perform a conditional overwrite.
​
key
string
required
The relative object key to write.
​
contentBase64
string
required
Standard base64-encoded content to upload. The encoded string length must not exceed ceil(MAX_TRANSFER_BYTES / 3) * 4 characters (≈ 1,398,104 characters for the default 1 MB limit).
​
contentType
string
MIME type to store as the object’s Content-Type header. Optional. Recommended for binary objects to ensure correct serving behavior.
​
customMetadata
object
Key-value string pairs stored as custom metadata on the object. Optional.
​
expectedEtag
string
When supplied, the write succeeds only if the stored object has this ETag. Use for optimistic concurrency. Optional.
{
  "key": "images/logo.png",
  "contentBase64": "iVBORw0KGgoAAAANSUhEUgAAAAUA...",
  "contentType": "image/png",
  "customMetadata": { "source": "design-v3" }
}
The tool validates base64 string length before decoding. If contentBase64 is longer than ceil(MAX_TRANSFER_BYTES / 3) * 4 characters, a 413 error is returned immediately without attempting to decode. A second check is performed on the decoded byte length to catch edge cases with padding.

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r2_download_base64

Return one R2 object as a base64-encoded string along with its metadata. The object size is checked against MAX_TRANSFER_BYTES before downloading — objects exceeding the limit return a 413 error without fetching the body. Use a presigned GET URL to retrieve objects larger than the limit.
​
key
string
required
The relative object key to download.
Response fields:
​
contentBase64
string
The object body encoded as standard base64.
​
contentType
string | null
The MIME type stored on the object, or null if none was set.
​
etag
string
The ETag of the stored object. Pass this as expectedEtag to a subsequent upload to implement compare-and-swap.
​
filename
string
The basename of the object key (the final path segment after the last /).
​
key
string
The relative object key, without the R2_ROOT_PREFIX if one is configured.
​
lastModified
string | null
ISO 8601 timestamp of when the object was last written, or null if unavailable.
​
size
integer
Object size in bytes.
{
  "key": "images/logo.png"
}
Example response shape: 
{
  "contentBase64": "iVBORw0KGgoAAAANSUhEUgAAAAUA...",
  "contentType": "image/png",
  "etag": "\"abc123def456\"",
  "filename": "logo.png",
  "key": "images/logo.png",
  "lastModified": "2024-06-01T12:00:00.000Z",
  "size": 4821
}
The size check against MAX_TRANSFER_BYTES is performed using object metadata before any bytes are fetched. If the object is too large, the tool returns a 413 error immediately. To retrieve large objects, use r2_presign_get to generate a temporary download URL.

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Size limits reference

Config variableDefaultApplies to
MAX_TRANSFER_BYTES1,048,576 (1 MB)r2_upload_base64, r2_download_base64
MAX_INLINE_TEXT_BYTES262,144 (256 KB)r2_object_get
Both limits are configurable via Worker environment variables. For objects exceeding MAX_TRANSFER_BYTES, use the presigned URL tools to upload or download directly between the client and R2 without routing bytes through the Worker.

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