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

Annotations already declare readOnlyHint=true, so the safety profile is covered. The description adds genuine behavioral context beyond that: how the account id is resolved (control token vs ABLY_ACCOUNT_ID) and the underlying Control API endpoint. It omits pagination/bucketing limits behavior, so not 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?

Two tightly packed sentences with zero filler; the core action and scope are front-loaded before the auth-resolution and endpoint details.

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, 100%-schema-covered stats tool with no output schema, the description covers action, scope, auth resolution, and endpoint. It does not describe the shape or content of the returned statistics, which is the only real gap 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 coverage is 100%, so the schema already documents all five parameters (start/end, unit, limit, account_id). The description's only param-adjacent detail is the ABLY_ACCOUNT_ID fallback for account_id, which the schema phrases differently ('defaults to the token's account'); no syntax or format detail is added. 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?

States a specific verb (retrieve) and resource (aggregated usage statistics) with clear scope ('across the whole account, bucketed by time'). The account-level framing implicitly distinguishes it from the sibling ably_get_stats, but it never names that sibling 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 account-wide aggregation scope implies when this is appropriate versus an app-level stats tool, but the description never states when-to-use, when-not-to-use, or names ably_get_stats as the alternative. Usage is only inferable.

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