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Backblaze B2 MCP Server

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s3_complete_multipart_upload

Idempotent

Finalize an S3-compatible multipart upload in B2 by assembling uploaded parts into a single object. Provide the ordered part manifest with ETags to complete the upload.

Instructions

Finalize an S3-compatible multipart upload in B2 by assembling uploaded parts. Use only after s3_create_multipart_upload and s3_get_presigned_upload_part_url (or s3_upload_part_copy) have produced every required part; use s3_list_parts to verify uploaded parts before retrying. Requires writeFiles. Completion is idempotent only when B2 already committed the exact same part list; if the response is lost, reconcile with s3_head_object or s3_list_object_versions before retrying.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
keyYesDestination object key used to create the multipart upload.
partsYesComplete ordered part manifest. Include every part in ascending partNumber order (part numbers are 1-10000); missing or stale ETags fail the completion call.
bucketYesDestination bucket name used to create the multipart upload.
uploadIdYesUploadId returned by s3_create_multipart_upload.

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: +"Destination bucket name used to create the multipart upload."
    • changedInput schema / properties / key / description
      Previous value: -"The object key."New value: +"Destination object key used to create the multipart upload."
    • changedInput schema / properties / parts / description
      Previous value: -"All uploaded parts in ascending part number order."New value: +"Complete ordered part manifest. Include every part in ascending partNumber order (part numbers are 1-10000); missing or stale ETags fail the completion call."
    • changedInput schema / properties / parts / items / properties / etag / description
      Previous value: -"The ETag from this part's direct PUT response (the s3_presign_upload_part flow)."New value: +"The ETag from this part's direct PUT response (the s3_get_presigned_upload_part_url flow)."
    • changedInput schema / properties / parts / items / properties / partNumber / description
      Previous value: -"The part number."New value: +"Provider-valid multipart part number from 1-10000 for this ETag; provide each uploaded part once."
    • changedInput schema / properties / uploadId / description
      Previous value: -"The UploadId."New value: +"UploadId returned by s3_create_multipart_upload."
  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 adds meaningful behavioral context: the writeFiles permission requirement, the qualified idempotency condition ('idempotent only when B2 already committed the exact same part list'), and the recovery path if the response is lost. It refines the idempotentHint annotation without contradicting it.

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 front-load the core purpose and sequential constraints, then cover verification, permission, and failure recovery. Every sentence carries essential operational guidance without 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?

For a complex multipart finalization flow with no output schema, the description covers prerequisites, ordering, verification, permissions, idempotency caveats, and retry reconciliation. This is sufficient for an agent to invoke the tool correctly in context.

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 the input schema already thoroughly documents bucket, key, uploadId, and parts. The description adds orchestration context around the part manifest but does not need to repeat parameter details; 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 verb ('Finalize') and resource ('S3-compatible multipart upload in B2'), clarifying it assembles uploaded parts into the final object. It is clearly distinguished from sibling operations like s3_abort_multipart_upload, s3_create_multipart_upload, and s3_list_parts.

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 'Use only after s3_create_multipart_upload and s3_get_presigned_upload_part_url (or s3_upload_part_copy) have produced every required part,' and instructs the agent to verify with s3_list_parts before retrying. It also names reconciliation alternatives (s3_head_object, s3_list_object_versions) for lost responses, leaving little to inference.

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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