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

Annotations already declare readOnly, idempotent, openWorld, and non-destructive, so the safety profile is covered. The description adds genuinely new detail beyond that: it reads 'current' state (idempotent, no mutation), lists the three measurements performed, and flags the free/no-AI cost profile, which matters to an agent budgeting calls.

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 sentence, front-loaded with the action and resource, then the three checks in parallel structure, ending with the cost note. Every clause carries information and nothing is repeated.

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 previews the three results (custom wide version presence, corner clearance, contrast sufficiency), which is enough for an agent to know what it gets back. It does not say what form those results take (boolean flags, scores, or a verdict), which is the one remaining gap.

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?

The single parameter is fully documented in the schema as a youtube.com or youtu.be link at 100% coverage, so the schema carries the load. The description adds nothing about the parameter beyond what the schema says, which meets the baseline for high-coverage schemas.

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?

The description gives a specific verb ('measures') and resource (a YouTube video's current thumbnail) and enumerates the three concrete checks it performs, which makes it distinct from generate_thumbnail or get_thumbnail in feel. It never names a sibling, so the differentiation is implied rather than explicit.

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?

'Without any AI' and 'Free' hint that this is a cheap pre-check, but the description never states when to call this versus get_thumbnail, correct_thumbnail, or generate_thumbnail. Usage is inferable but not spelled out, and there are no exclusions or prerequisites.

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