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

Annotations already declare readOnlyHint=true and destructiveHint=false, so safety is covered. The description adds real value beyond that by disclosing the shape of the response (period dates, counts, plan limits, per-API-key breakdown) in the absence of an output schema, which tells the agent what it will actually learn from the call. It stops short of noting data freshness or any account-level permission requirements.

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 front-loaded with the resource and followed by the parameter note; every clause carries information. The phrasing 'How many of the account's documents for the period are used' is slightly convoluted and could be tightened without losing content.

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 one optional parameter, full schema coverage, safety annotations, and no output schema, the description covers what the agent needs: what is returned, and how the optional parameter changes scope. Only minor omissions remain, such as where period ids come from or whether results are per-account or per-workspace.

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%, with the schema itself explaining that period is a decimal usage-period id and that omitting it yields the current period. The description's 'Pass period to read an earlier period' merely reinforces the historical-vs-current distinction rather than adding syntax, format, or discovery guidance. 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?

The description names the resource (the account's document usage for a period) and enumerates exactly what it reports: period dates, included vs. used counts, plan limits, and per-API-key split. The verb is implicit — it reads as a statement of return contents rather than 'retrieves usage' — but no sibling tool covers usage, so differentiation is unambiguous.

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 only guidance is 'Pass period to read an earlier period,' which tells the agent when to supply the parameter, not when to call this tool versus alternatives. With no competing usage-related sibling this is largely harmless, but there are no explicit exclusions or preconditions.

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