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s3_create_multipart_upload

Initiate an S3-compatible multipart upload for a large object in Backblaze B2, returning an UploadId to coordinate subsequent part uploads.

Instructions

Initiate an S3-compatible multipart upload for a large object in B2 and return an UploadId. Requires the writeFiles capability. This starts an unfinished large-file upload that accrues storage for each uploaded part until you finalize with s3_complete_multipart_upload or discard it with s3_abort_multipart_upload; use s3_list_multipart_uploads to enumerate abandoned uploads, or b2_unfinished_uploads for storage-cost analysis of a selected bucket. Initiating the upload does not reserve or lock the destination key, so concurrent writes may still target it. Use s3_put_object only for tiny (≤1 MiB) inline payloads; prefer a single-object presigned PUT (s3_get_presigned_url) for most single-object transfers, including large ones; use multipart when an object is uploaded or copied as parallel parts. Flow: s3_create_multipart_upload → s3_get_presigned_upload_part_url → PUT each part directly to B2 (capture each ETag) → s3_complete_multipart_upload with those ETags. Parts are numbered 1–10000 and every part except the last must be ≥5 MiB, and no part may exceed 5 GiB.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
aclNoAccepted as a no-op S3 compatibility hint; B2 access follows the destination bucket policy.
keyYesThe object key for the final assembled file.
bucketYesThe destination bucket name.
metadataNoCustom metadata stored on the final object.
contentTypeNoMIME type recorded on the final assembled object.
serverSideEncryptionNoServer-side encryption. B2 supports SSE-B2 (AES256) only — not SSE-KMS.

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed3 schema fields changed
    • addedInput schema / properties / acl / description
      Added value: +"Accepted as a no-op S3 compatibility hint; B2 access follows the destination bucket policy."
    • changedInput schema / properties / contentType / description
      Previous value: -"MIME type of the object."New value: +"MIME type recorded on the final assembled object."
    • changedInput schema / properties / metadata / description
      Previous value: -"Custom metadata for the object."New value: +"Custom metadata stored on the final object."
  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?

Annotations already indicate a mutating, non-idempotent operation, and the description adds meaningful behavioral context: it requires the writeFiles capability, accrues storage for uploaded parts, does not reserve or lock the destination key, and explains the consequences of not finalizing or aborting. This goes well beyond the structured 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?

The description is dense but every sentence earns its place: purpose, lifecycle, concurrency caveat, alternative routing, orchestration flow, and part-number limits are all present. The most important identifying information is front-loaded in the first sentence.

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?

This is a complex stateful operation with significant side effects, and the description covers prerequisites, follow-up steps, alternatives, concurrency implications, and size constraints. The absence of an output schema is mitigated by explicitly stating that the tool returns an UploadId, and the full lifecycle is described.

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 schema fully documents all six parameters, meeting the baseline. The description does not add field-level parameter meaning beyond the schema, though it does provide useful context about part size constraints that informs how the multipart flow should be used.

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 opens with a specific verb and resource: 'Initiate an S3-compatible multipart upload for a large object in B2 and return an UploadId.' It clearly distinguishes this tool from siblings by naming the complete/abort counterparts and contrasting it with s3_put_object and presigned URLs.

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 gives explicit when-to-use guidance: use s3_put_object only for tiny ≤1 MiB payloads, prefer presigned PUT for most transfers, and use multipart for parallel-part uploads or copies. It also lays out the full orchestration flow with sibling tools, leaving no ambiguity about how this step fits.

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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