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

Annotations already declare readOnlyHint, openWorldHint and destructiveHint=false, so safety is covered. The description adds genuinely new behavioral context: the per-request billing cost ($0.005) and the exact payload the caller receives, which is real value beyond the annotations.

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 dense sentences, front-loaded with the lookup method, then returns, then cost. No filler and nothing repeated from the title or 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?

With no output schema, the description compensates by listing the returned fields, and it covers identification and cost. What it omits is failure behavior (what happens with an invalid or nonexistent ID) and any truncation of the result set, which is a minor gap for a single-entity lookup.

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%, so the baseline is 3. The description adds the mutual-exclusivity constraint ('provide exactly one'), which the schema does not encode (required is empty), and this is exactly the kind of semantic the schema omits.

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 ('Get a YouTube channel') and enumerates the three accepted identifier forms plus the exact fields returned. The lookup-by-identifier framing implicitly separates it from the search_youtube_channels sibling, so an agent can pick it without opening the 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?

'provide exactly one' gives clear guidance on how to call it, and the identifier forms imply the context (you already know the channel). However, it never states when to prefer this over search_youtube_channels or search_youtube, so the alternative-selection guidance is only implied.

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