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

Goes well beyond the annotations: it discloses that a NEW avatar is always created and the original is preserved (useful against idempotentHint=false and destructiveHint=false), that the response returns avatarId and picture, that each generated picture consumes one image from the plan allowance but is never refused on a plan without one, that prominent-person photos are rejected with 422, and that runs take ~10-60 s on POST /avatars.

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?

Dense, front-loaded, and every clause carries information (billing, refusals, latency, return values). It is packed into a single run-on paragraph heavy with parentheses, which makes it slightly harder to scan than a short structured list, but there is essentially no wasted text.

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 12-parameter, non-idempotent creation tool with no output schema, it covers the essentials an agent needs: valid start-point combinations, what is returned, cost implications, failure modes (422), and expected latency.

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 already 100%, so the schema carries the baseline; the description still adds cross-parameter meaning the schema does not, linking voice to vids_list_voices, noting the logo is applied when refining the picture ('Say where in refine'), and clarifying 'refine and logo change that picture' applies after the start point.

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 precise verb and resource ('Make a voiced avatar = a person's picture plus a voice') and immediately scopes it to its consumers (vids_create_avatar_video, vids_create_video avatar_1..3), so it is distinguishable from vids_create_image and vids_create_avatar_video without opening any schema.

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?

Explicitly gives the three mutually exclusive starting points (appearance / image / avatar) with the condition for each and the sources of the values (vids_upload_asset, vids_create_image, vids_list_avatars, vids_list_voices), plus the rule that each start produces a new avatar rather than modifying one. It tells the agent both what to supply and which alternative applies.

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