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

Annotations already declare readOnly, idempotent, non-destructive and openWorld. The description contributes beyond that by flagging 'account key' auth and the 366-day per-call window, but says nothing about pagination, result ordering, or what happens when the range exceeds 366 days.

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: the read/auth marker and resource come first, the metric list second, and the range limit last. Every clause carries information.

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 metric categories and the time granularity (daily), which is the key return-shape information. Minor gaps remain around aggregation semantics (per sub-account rows vs totals) but an agent can call it confidently.

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% (from/to with date format and UTC note), so the baseline is 3. The description adds a genuine constraint not present in the schema — the maximum 366-day span per call — which meaningfully informs how to build the range.

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?

Names a specific verb+resource (daily delivery/bounce/open/click/unsubscribe/spam counts) and scopes it to 'across sub-accounts', which separates it from sendpost_get_account_stats (single account). It does not explicitly contrast with the similarly named sendpost_list_subaccount_stats, so sibling differentiation is 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 'across sub-accounts' scope implies when this tool is the right choice versus the per-account/per-subaccount stat tools, but no alternative is named and no when-not guidance is given. 'Use up to 366 days per call' is a constraint, not usage routing.

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