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

Annotations already cover the read-only, non-destructive, closed-world profile, so the description's job is to add substance — and it does: it enumerates the returned metrics (watch percent, watch time, plays, source, reply status) and discloses that anonymous viewers are counted but not listed, a real behavioral caveat. It adds no pagination or rate-limit information, so not a 5.

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 sentences, zero filler: scope first, then returned fields, then the anonymous-viewer caveat. Every clause carries information the agent needs, and the most decision-relevant content is front-loaded.

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 does the necessary work of naming the returned metrics, and the safety profile is already covered by annotations. The remaining gaps — limit semantics, result shape/pagination, and behavior when both name and library_item_id are supplied — are minor but real.

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 0%, so the description must carry the load. It gives meaning to three of four parameters: days ("last N days"), and name/library_item_id as alternative video selectors. The limit parameter is never mentioned and no result-count or pagination behavior is described, leaving a real gap.

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 resource (viewers) with two explicit scopes: one video via library_item_id or name, or the whole library within a day window. The purpose is unambiguous and no sibling tool overlaps with viewer analytics. It stops short of explicitly naming a contrasting sibling (e.g., get_library_item vs. this), so 4 rather than 5.

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?

"by library_item_id or name ... or across your library" tells the agent exactly how to switch between single-video and library-wide modes, which is the key decision point. It gives no explicit when-not-to-use guidance or pointer to sibling tools for related tasks, so it lands at clear-context-without-exclusions.

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