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

Annotations already declare this is a non-destructive, idempotent, open-world write, so the description's job is to add operational constraints, and it does: file types, a 2 MB cap and a recommended 1280x720 resolution. It omits that the existing thumbnail is replaced and says nothing about quota cost or failure modes, which for a YouTube-style API would be genuinely useful.

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?

Two sentences, no filler, with the action and format limits front-loaded ahead of the prerequisite. Every clause carries information an agent needs.

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 three-parameter write tool with no output schema, the description covers formats, size, path locality and the verification prerequisite. The remaining gap is behavioral rather than structural: it never says whether an existing thumbnail is overwritten or what a quota failure looks like.

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 67% with video_id undocumented in the schema, and the description supplies the constraint that image_path must be a local JPG or PNG under 2 MB at roughly 1280x720, meaning well beyond the schema's bare 'Absolute path to the image.'

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 (upload) and resource (the video's thumbnail) with the accepted formats inline, which is enough to separate it from sibling setters like set_channel_banner and set_watermark that target channel-level branding rather than a single video.

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?

'from this computer' rules out passing a URL, and 'The channel must be phone-verified' is a concrete precondition, so an agent knows the context in which the call can succeed. It stops short of naming alternatives (e.g. whether update_video can also touch thumbnails) or saying what to do when verification is missing.

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