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

Annotations already declare readOnly, idempotent, and non-destructive, so the safety profile is covered. The description adds genuinely new context: an 'account key' auth requirement and a 366-day maximum window, neither of which appears in the annotations or schema. Response shape is left unstated, keeping it below 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?

A single dense sentence with the access mode and scope front-loaded, followed by the resource and the key limit. No redundant or filler text.

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?

For a read-only stats tool with a fully documented schema, the description conveys what data comes back (delivery and engagement aggregates), the aggregation axis (group/tag across sub-accounts), and the time limit. Missing only return-format/pagination detail, which is minor given no output schema exists.

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% and already documents from/to UTC dates and the group example ('transactional'). The description adds only the 366-day range constraint, which is a real semantic addition but modest; baseline 3 is appropriate.

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 concrete verb+resource: aggregate delivery and engagement counts for an existing group/tag across sub-accounts. The 'group/tag' scope plus 'across sub-accounts' distinguishes it reasonably from sendpost_get_account_stats and sendpost_list_subaccount_stats, though no sibling is named explicitly.

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?

The implied use case (aggregated group-level stats over a date window) is inferable, but there is no explicit 'use this instead of X when...' routing against the many similar stats siblings. The 'up to 366 days' note hints at a boundary but doesn't state what to do beyond it.

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