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

Annotations already declare readOnlyHint=true and openWorldHint=true, so the safety profile is covered. The description adds a genuinely non-structured behavioral fact — that the call is free and does not consume credits — which matters in a billing-aware API where get_usage is a sibling. It stops short of describing return shape or failure behavior.

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 short, front-loaded sentences with zero filler: the action first, then the cost attribute. Every clause carries 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 a single-parameter read-only listing tool with no output schema and a fully documented parameter, the description plus annotations cover what an agent needs to call it correctly. Only the relation to get_transcript is left implicit.

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 description coverage is 100% with a single documented parameter ('YouTube URL or video ID'), so the schema already carries full parameter meaning. The description adds no format or syntax detail beyond that baseline.

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 states a specific verb and resource ('List the caption tracks available for a YouTube video'), which is clear and matches the title. It does not explicitly contrast itself with the sibling get_transcript, though the resource is distinct enough that an agent can differentiate them.

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?

It implies the use case (discovering available caption tracks) and adds a cost note, but never states when to use this instead of get_transcript or that it is typically a prerequisite step. Usage is inferable rather than spelled out.

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