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

Annotations only declare it is a non-readonly, non-idempotent, open-world write; the description adds substantial context the agent cannot get elsewhere: credit cost (10 per image, 4K/2 takes count as two), the mandatory quote-before-charge flow, plan gating, and the real-people refusal policy. It also discloses built-in QA (text read back and fixed, feed-size check), which matters for predicting output reliability.

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 cost, quote flow, and plan constraint are front-loaded and earn their place, but the full 14-item style list restates the schema enum verbatim and consumes roughly half the description with no added meaning. Trimming it would sharpen the definition without losing information.

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 an 11-parameter, zero-required, no-output-schema generation tool, the description covers cost, confirmation protocol, plan requirement, input routes, and content restrictions. It does not say what comes back (thumbnail id/URL, quote object, failure modes), which an agent must know to chain into get_thumbnail or correct_thumbnail.

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 baseline is 3; the description adds genuine meaning by framing when to supply prompt vs videoUrl ('or from a video link whose brief is written first'), the text-rendering behavior behind `text`, and the cost implications of `takes` and `resolution` beyond their literal enum descriptions.

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 ('Makes one finished YouTube thumbnail') plus both input routes (description or video link) and take count. It is clearly distinguishable from siblings like check_thumbnail, correct_thumbnail, and write_thumbnail_brief, whose names imply QA/edit/brief-only steps this tool does not perform.

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 an explicit call sequence: first call without confirm to get a price, then confirm with the returned quoteId after the person agrees, plus the 'Studio plan' prerequisite. It stops short of naming sibling alternatives (e.g. use check_thumbnail/correct_thumbnail to fix an existing thumbnail rather than regenerating), so routing among siblings is only implied.

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