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 genuinely useful non-schema behavior in the form of cost disclosure — '(1 credit)' — which lets an agent budget calls. It stops short of describing error behavior for invalid handles or whether the response is cached/partial, so 3 is appropriate.

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?

A single short fragment with zero filler; the returned fields are front-loaded and the credit cost is appended. It loses a point for lacking a leading verb, which makes the sentence read as a label rather than an instruction.

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 enumerated return fields partially substitute for one, and the credit cost covers budget concerns. However, for a tool embedded among ~20 sibling YouTube tools with overlapping names, the description is too thin to fully orient an agent about what this call gives that a channel-videos call does not.

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?

Only one parameter (handle) exists and the schema already documents it at 100% coverage, including that it accepts an @handle, channel URL, or UC… id. The description adds nothing about the parameter, so the baseline 3 applies when the schema does all the work.

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 names the resource (a YouTube channel) and enumerates what it returns — subscribers, total views, video count, description — so an agent knows this is a channel-level metadata lookup. It falls short of 5 because it never distinguishes itself from the many channel-scoped siblings such as youtube_channel_videos, youtube_channel_playlists, or youtube_channel_shorts, and gives no explicit verb.

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?

There is no when-to-use guidance at all: no statement of the input type expected, no prerequisites, and no mention of when to prefer a sibling like youtube_channel_videos or youtube_channel_search. The agent must infer usage purely from the name.

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