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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive), and the description adds substantial context beyond them: visitors never adding up across sites, change being null when nothing to compare with, no_change explaining why, counted/basis semantics, noise marking unreadable changes, and totals adding only pageviews and visits. These are behavioral traits an agent cannot infer from the 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?

Front-loads the purpose before diving into sort semantics and return-value caveats, and every clause carries information. However, it is one very dense run-on paragraph with some cryptic fragments ('each site's in its own days'), which slightly hampers scanability for a 9-parameter tool.

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?

There is no output schema, so the description must carry the return-value burden, and it does: movers ordering, noise, change/no_change, counted/basis, totals, limit and nothing_counted. Combined with the usage routing and filter constraints, an agent has everything needed to call it correctly.

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 coverage is 100%, so the schema baseline is 3, but the description goes further: it explains the 'moved' sort formula (change over the square root of both periods' visits), the noise flag, the exact filter exclusion set (any dimension but goal, entry and exit), and default limit of 50. It delegates range/from/to/days wording to get_site_summary, leaving those partially external.

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 (report on), resource (every site / one group) and scope (pageviews, visits, visitors over a range beside the prior period, movers first). It explicitly contrasts with the sibling get_site_summary ('once a site'), so an agent can disambiguate without reading either 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?

Gives an explicit routing rule: 'Use it first for any question about more than one site, rather than get_site_summary once a site.' It also names the follow-up tools (explain_change for why one moved, whats_new for what is new) and instructs the agent to read the answer's caveats before reporting figures.

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