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 value the annotations do not: the concrete return payload (ID, name, handle, channel URL, RSS feed URL) and the fact that any video link is a valid lookup, which matters given there is no output schema.

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?

Three short sentences in a deliberate order: what it does, what it returns, when to use it and when not to. No filler or repetition.

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 carries the return-value burden and does so explicitly, and the usage boundary is stated. Minor gap: nothing about failure modes for an open-world lookup (unresolvable handle, ambiguous query), which an agent might want before committing to a call.

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% for the single `query` parameter and it already enumerates @handle, channel URL forms and video/Shorts URLs. The description's "any video link of the channel works too" restates rather than extends that, so the baseline 3 applies.

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 (resolve a YouTube channel to its permanent UC... ID) and enumerates exactly what comes back. It also sets a boundary that separates it from siblings such as estimate_youtube_earnings ("Not for statistics like subscriber counts").

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 when-to-use triggers (an API, embed, RSS reader or analytics tool needs the UC ID) plus a when-not (statistics). No sibling tool is named explicitly as the alternative, so it stops short of a 5, but the routing signal is clear.

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