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

Annotations already cover readOnly, idempotent, non-destructive and openWorld, so the safety profile is settled. The description adds real context beyond that: it flags an account-key auth requirement and a hard 366-day range limit that would otherwise cause a failed call. Return shape and pagination remain undisclosed.

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 tight sentence, front-loaded with the READ/account-key qualifier and then the core action. Nothing superfluous, though the parenthetical '(account key)' reads awkwardly and could be integrated more cleanly.

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?

No output schema exists, so the description should at least indicate what comes back (stat record shape, ordering, pagination). It names 'delivery and engagement statistics' but leaves the agent guessing about the response structure for a list endpoint.

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 67% – from/to carry format and UTC guidance in the schema, but subaccount_id has no description anywhere. The description's 366-day note usefully constrains the date pair but says nothing about what subaccount_id expects or where to obtain it (e.g. via sendpost_list_subaccounts).

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 states a specific verb (Read) and resource (a sub-account's delivery and engagement statistic records) plus a scoping constraint (date range, up to 366 days). An agent can distinguish it from generic sibling reads. It does not, however, explicitly contrast with sendpost_get_subaccount_stats, so sibling differentiation is only partial.

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 date-range framing and 366-day cap imply when this tool applies, but there is no explicit 'use this instead of X' guidance despite a near-identical sibling (sendpost_get_subaccount_stats) existing. The user must infer the list-vs-get distinction from the verb alone.

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