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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive), so the bar is lower. The description adds real context beyond them: it discloses the automatic previous-period comparison of equal length and enumerates the returned metrics, which matters because there is no output schema. Remaining gaps are minor (no note on how comparison handles "all" or partial periods).

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, and the core purpose is front-loaded ahead of the metric enumeration. The trailing "Also visitors online now" is a single clause that adds information rather than padding.

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 correctly spends its budget on naming the return fields and the comparison-period behavior, which is the information an agent needs to decide whether it can answer a question with this single call. It stops short of explaining how the comparison is presented or how filters affect the headline numbers, but nothing essential for invocation is missing.

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 schema itself documents period, from/to, filters, model and website. The description mentions "for a period" and a comparison period but adds no formatting, precedence, or defaulting rules beyond what the schema already states. 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?

"Headline metrics for a period" is a specific verb+resource, and the description enumerates exactly what comes back (visitors, sessions, pageviews, bounce rate, session time, revenue, customers, conversion rate). It does not, however, contrast itself with siblings like get_breakdown or get_timeseries, so an agent must infer that this is the aggregate summary rather than a sliced view.

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?

Usage is only implied: the word "headline" plus the metric list suggests this is the entry-point dashboard call, but there is no explicit "use this when you want X; use get_breakdown instead for Y". The statement "Also visitors online now" overlaps with the get_live sibling without clarifying when to prefer one over the other.

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