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

Adds real behavior beyond the annotations: it is free of charge, and a failed precondition returns 409 upload_incomplete with a missingParts field, which is valuable failure-mode disclosure. 'Idempotent' merely restates idempotentHint=true and there is no mention of auth requirements, so it is not a 5.

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

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

Five telegraphic fragments, no waste, cost and precondition front-loaded before the downstream instruction. The fragment style ('Then pass fileId to any tool') is efficient but slightly ambiguous about what is actually returned.

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

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

For a one-parameter tool with no output schema, the description covers cost, precondition, idempotency, error shape, and the follow-up call. The main gap is that it never states what a successful call returns or that it is the terminal step of the multipart flow.

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

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

Schema coverage is 0% and the single fileId parameter is undocumented in the schema, so the description must carry the burden. It implies fileId is the upload handle passed downstream, but never says where fileId comes from (presumably create_multipart_upload) or what it identifies — only partial compensation.

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

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

States a specific action — assemble the file after every part is PUT — which distinguishes it from create_multipart_upload and abort_upload without naming them. 'Free' also flags no credit cost. It never explicitly names the sibling sequence it belongs to, so it stops short of a 5.

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

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

Gives a clear precondition ('After every part is PUT') and a clear next step ('Then pass fileId to any tool'), which is exactly the workflow context an agent needs. It does not mention the abort_upload alternative or when to give up on an upload, which keeps it below 5.

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