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 usefully adds pricing ($0.0029 per successful request) and a routing note about the mapped path versus the upstream canonical path, which is real operational context 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?

The core capability and required-parameter guidance are front-loaded, and the optional-parameter detail follows logically. The pricing comment block and path-mapping note add length that is only marginally actionable, slightly diluting an otherwise tight description.

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 an output schema present, rich annotations, and full schema description coverage, the description supplies exactly the missing pieces: required params, filter-token semantics, cost, and endpoint routing. Nothing an agent needs to invoke this correctly is absent.

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 goes further by clarifying that engine and q are both required, that sp carries pagination and advanced filter tokens (upload date, duration, result type), and that gl/hl narrow by country and interface language.

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+resource ('Search YouTube') and enumerates the returned entities (videos, channels, playlists), which an agent can act on immediately. It does not explicitly contrast itself with the sibling 'search' tool, so the distinction is implied rather than stated.

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 three concrete use cases (topic discovery, tracking a channel's uploads, gauging coverage) plus the mandatory engine/q pairing. There are no when-not-to-use conditions or named alternatives among the siblings, which keeps it out of 5 territory.

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