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

Annotations already declare non-readonly, non-idempotent, non-destructive. The description adds real behavioral context beyond them: the Authorization header must be reused on each PUT, the etag from each PUT must be retained, and the part-byte sizing rule. It does not say whether repeated calls create duplicate sessions or how long the uploadId stays valid.

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?

One dense paragraph, front-loaded with what the tool does and what it returns, followed by the required client steps. Every clause carries information; only the undefined 'partBytes' reference adds noise.

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?

With no output schema, the description usefully explains the return (uploadId plus one URL per part) and the complete workflow. Minor gaps remain around session lifetime, part size origin, and how this differs from the sibling upload_video.

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 type, bytes and duration are already documented. The description adds the partSize rule ('partBytes long except the last'), but partBytes is never defined as an input parameter, which leaves it ambiguous where that value comes from. Baseline 3 applies.

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 verb and resource ('Start uploading a finished video') plus the accepted formats and the return shape (uploadId and per-part URLs). It clearly positions itself as the multipart-session opener in the upload flow, but it never distinguishes itself from the sibling upload_video, which an agent could easily confuse with this tool.

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 procedural context: use this to start a part-based upload, then PUT each part's bytes, then call complete_upload. It routes to the follow-up sibling by name, but offers no explicit when-not guidance (e.g. small files or the single-shot upload_video).

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