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Backblaze B2 MCP Server

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Hybrid

s3_list_multipart_uploads

Read-onlyIdempotent

List in-progress multipart uploads in a B2 bucket to audit or resume unfinished uploads before completing or aborting them.

Instructions

List in-progress S3-compatible multipart uploads for a B2 bucket. Use to resume or audit unfinished uploads before s3_get_presigned_upload_part_url, s3_complete_multipart_upload, or s3_abort_multipart_upload; use b2_unfinished_uploads when you need storage-cost analysis across bounded listings. Requires listFiles. Results are paginated with maxUploads (default 100, range 1-1000) and key/upload markers; delimiter responses include commonPrefixes.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
bucketYesBucket name whose in-progress multipart uploads to list.
prefixNoOnly return multipart uploads whose object keys start with this prefix.
delimiterNoOptional delimiter, usually '/', to group common key prefixes.
keyMarkerNoPagination cursor: nextKeyMarker from a previous response.
maxUploadsNoMaximum uploads to return (default 100, range 1-1000).
uploadIdMarkerNoPagination cursor paired with keyMarker: nextUploadIdMarker from a previous response.

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed6 schema fields changed
    • changedInput schema / properties / bucket / description
      Previous value: -"The bucket name."New value: +"Bucket name whose in-progress multipart uploads to list."
    • addedInput schema / properties / delimiter / description
      Added value: +"Optional delimiter, usually '/', to group common key prefixes."
    • changedInput schema / properties / keyMarker / description
      Previous value: -"Pagination cursor."New value: +"Pagination cursor: nextKeyMarker from a previous response."
    • addedInput schema / properties / maxUploads / description
      Added value: +"Maximum uploads to return (default 100, range 1-1000)."
    • changedInput schema / properties / prefix / description
      Previous value: -"Only list uploads for keys with this prefix."New value: +"Only return multipart uploads whose object keys start with this prefix."
    • changedInput schema / properties / uploadIdMarker / description
      Previous value: -"Pagination cursor."New value: +"Pagination cursor paired with keyMarker: nextUploadIdMarker from a previous response."
  2. First observedv0.2.0

TDQS

A4.6/5.0
Behavior5/5

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

Annotations already mark this as readOnly, idempotent, and non-destructive; the description adds useful behavioral context beyond that: the required listFiles permission, pagination semantics with maxUploads limits and markers, and the commonPrefixes behavior for delimiters. No contradiction with annotations.

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?

Three dense sentences with no filler: purpose first, then usage guidance and alternative, then permission and pagination details. Every clause earns its place and the most decision-relevant information is front-loaded.

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

Completeness4/5

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

For a six-parameter listing tool with no output schema, the description covers permissions, pagination, default/range values, and delimiter behavior. It does not enumerate the full response shape, but the S3 semantics are standard and the key operational details are present.

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 description coverage is 100%, so each parameter is already documented in the schema. The description reinforces pagination and delimiter behavior but does not add substantial new meaning beyond what the schema provides, so the baseline 3 is appropriate.

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 states a specific action ('List in-progress S3-compatible multipart uploads') on a specific resource ('B2 bucket'), and differentiates itself from b2_unfinished_uploads by naming the exact use cases. An agent can clearly tell what this tool does and how it differs from nearby siblings.

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?

The description explicitly says when to use this tool ('resume or audit unfinished uploads'), names the downstream operations it precedes, and gives a concrete alternative (b2_unfinished_uploads) with the condition for choosing it ('storage-cost analysis across bounded listings'). It also states the required permission, listFiles.

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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