Start chunked media upload

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

Annotations only indicate readOnlyHint=false and destructiveHint=false, so the description carries the burden of explaining behavior. It does this well by disclosing that the tool returns r2Key, uploadId, a fixed partSize, and presigned URLs valid for 3600s, and that the caller must PUT chunks, collect ETags, and call complete_multipart_upload. It does not mention abort_multipart_upload as a cleanup option, which is a minor gap.

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?

The description is well-structured and front-loaded with the tool's purpose, then return values, then the workflow, then when-to-use guidance. It is slightly repetitive about the 1GB/100MB limits, appearing both at the start and end, but every sentence otherwise earns its place.

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 two-parameter tool with no output schema, the description is complete enough for an agent to invoke it and continue the next step. It names the returned fields, the chunk size, URL validity, the required PUT/ETag flow, and the follow-up call to complete_multipart_upload. Nothing essential to calling this tool is missing.

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 description coverage is 100%, so the schema already fully documents sizeBytes and contentType. The description reinforces the size limits and introduces partSize, but does not add substantial parameter meaning beyond the schema. Baseline 3 is appropriate.

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?

The description uses a specific verb ('Start') and resource ('chunked (multipart) direct-to-storage upload') and immediately distinguishes it from ordinary uploads by size and chunking. It also names sibling tools like complete_multipart_upload and upload_media, which removes ambiguity.

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?

The description gives explicit when-to-use guidance: 'Use this when a file is too large for a single upload (e.g. a video over 100MB, up to 1GB) and must be sent in 10MB chunks.' It also explicitly prefers upload_media for ordinary files, giving a clear alternative and condition for choosing this tool instead.

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