Update Thumbnail Customizations

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

Annotations already flag this as destructive, non-idempotent and open-world, so the bar is lower; the description adds genuinely non-obvious behavior: only supplied fields change and nulling a field reverts it to the default. It also discloses auth/delegation requirements and the confirm requirement, plus side-effect warnings about sharing/notifying/charges, though that last line is generic boilerplate.

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?

Purpose and update semantics are front-loaded in the first sentence, followed by the permission block. The triple-backtick permission snippet and delegation paragraph are somewhat verbose, but every section is scannable and nothing is redundant with the schema.

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 10-parameter mutation with no output schema but annotations covering the safety profile, the description covers purpose, partial-update mechanics, auth requirements, confirm gating and side effects. Return-value behavior is not described, but no output schema exists to lean on, so a small gap remains.

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 100%, so the baseline is 3, but the description adds real value by defining how a null value is interpreted (delete/revert to default) for supplied fields - a semantic the schema does not convey. It does not otherwise elaborate on individual parameters like stillUrl or fitStrategy, which is acceptable given full schema coverage.

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 ('Applies a partial update to a video's thumbnail customizations'), which clearly distinguishes it from the read sibling get_thumbnail_customizations and the broader update_customizations. It does not explicitly name alternatives, but the resource is unambiguous.

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 concrete preconditions (required permissions, confirm=true) and explains partial-update semantics, but never states when to reach for this tool versus update_customizations or update_appearance_customizations. Usage context is implied by the name rather than reasoned in the description.

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