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Backblaze B2 MCP Server

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s3_upload_part_copy

Idempotent

Copies a byte range from an existing B2 object into a part of an in-progress multipart upload, avoiding download and re-upload. Assembles large objects from data already in B2.

Instructions

Copy a byte range from an existing B2 object into a part of an in-progress S3-compatible multipart upload, without downloading or re-uploading the data. Requires the writeFiles capability on the destination and read access to the source object. The source must be a bucket reachable within the same B2 account and region as the destination; cross-account sources cannot be copied this way. Use this to assemble large objects from data already in B2; use s3_get_presigned_upload_part_url instead when the client must upload new bytes. The copied part belongs to the upload created by s3_create_multipart_upload, so it obeys the 1–10000 part numbering, the ≥5 MiB minimum for every part except the last, and the 5 GiB per-part maximum. A missing or inaccessible source, or a range outside the source object, fails the part copy. Returns the part ETag to pass to s3_complete_multipart_upload.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
keyYesThe destination object key.
bucketYesThe destination bucket name.
uploadIdYesThe UploadId from s3_create_multipart_upload.
copySourceYesThe source object in 'bucket/key' format, e.g. 'my-bucket/path/to/file.dat'. URL-encode special characters in the key.
partNumberYesThe part number (1-10000).
copySourceRangeNoByte range to copy from the source, e.g. 'bytes=0-104857599' for the first 100 MiB; omit to copy the entire source object. The copied range must be ≥5 MiB unless this is the last part, and at most 5 GiB.
copySourceVersionIdNoVersion ID of the source object to copy; set to copy that exact version, or omit to copy the current version.

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed3 schema fields changed
    • changedInput schema / properties / copySourceRange / description
      Previous value: -"Byte range to copy from the source, e.g. 'bytes=0-104857599' for the first 100MB."New value: +"Byte range to copy from the source, e.g. 'bytes=0-104857599' for the first 100 MiB; omit to copy the entire source object. The copied range must be ≥5 MiB unless this is the last part, and at most 5 GiB."
    • changedInput schema / properties / copySourceVersionId / description
      Previous value: -"Version ID of the source object to copy from."New value: +"Version ID of the source object to copy; set to copy that exact version, or omit to copy the current version."
    • changedInput schema / properties / partNumber / description
      Previous value: -"The part number (1–10000)."New value: +"The part number (1-10000)."
  2. First observedv0.2.0

TDQS

A4.7/5.0
Behavior5/5

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Beyond the annotations, the description discloses permission requirements, same-account/region constraints, part-size limits, failure conditions for missing/inaccessible sources or invalid ranges, and the returned ETag. This is substantial behavioral context that annotations do not provide.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness5/5

Is the description appropriately sized, front-loaded, and free of redundancy?

The description is dense but every sentence earns its place: purpose, permissions, constraints, alternatives, failure modes, and return value. It is front-loaded with the core action and avoids redundant filler.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness5/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

With no output schema, the description still explains the return value (part ETag) and its downstream use. It also covers prerequisites, constraints, failure behavior, and alternatives, so an agent has everything needed to invoke the tool correctly.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters3/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema coverage is 100%, so the schema already documents every parameter. The description adds helpful constraints around part size and source behavior, but most of that is also reflected in the schema. It meets the baseline for a fully covered schema without significantly enhancing parameter-level meaning.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

The description uses a specific verb ('copy a byte range') and names the exact resource ('a part of an in-progress S3-compatible multipart upload'). It also differentiates this tool from s3_get_presigned_upload_part_url and s3_copy_object by scope, making the purpose unmistakable.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines5/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

It explicitly states when to use this tool ('assemble large objects from data already in B2') and names the alternative for new bytes ('s3_get_presigned_upload_part_url'). It also gives exclusions, such as cross-account sources not being copyable, so an agent can route correctly.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

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