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

The annotations already cover safety (readOnly, idempotent, non-destructive, closed-world), and the description adds genuinely new behavioral facts: results are totals-only, and this is the cheap call relative to the alternatives. It stops short of describing pagination or latency/cost specifics beyond 'cheap', and the 'totals only' claim sits slightly at odds with the `includeGraph` option, so not a full 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?

Three sentences, front-loaded with the metric inventory, then the scope trick, then the routing. Every clause carries information an agent needs; no filler or restated title.

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 usefully enumerates the returned headline metrics, which is the key missing structured data. Remaining gaps are minor: it does not describe how `range` defaults resolve or what the graph series looks like when `includeGraph` is set, but the schema covers the former and the latter is secondary.

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 baseline is 3; the description earns above baseline by explaining the meaningful omission semantics of `campaignId` (roll-up across the whole account), which the schema does not state. It adds nothing new for `range`, `userId`, or `includeGraph`, which the schema already documents thoroughly.

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 a specific verb+resource (headline performance for one campaign) and enumerates the exact metric families returned: prospects, invites, messages, InMails, emails sent/opened/clicked/bounced, replies. It also states the scope variant (omit `campaignId` for an account roll-up), so it is distinguishable from every analytics sibling without opening a 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?

Explicitly says 'Start here for "how is this campaign doing" — it is the cheap one' and then routes the agent away to three named alternatives with the exact condition selecting each: `analytics_reporting` for a day-by-day series, `sequence_step_stats` for drop-off, `campaign_export_stats` for a spreadsheet. This is when/when-not/alternatives all present.

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