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

Annotations already declare readOnlyHint/openWorldHint, and the description adds real context beyond them: error behavior ('other values get a 400 bad_days'), the null-for-sandbox-keys rule on spend_usd, and data provenance ('derived from the same job rows the limiter counts'). This is exactly the extra behavioral detail annotations don't carry.

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?

Front-loaded with the primary return value before the optional history detail, and every sentence carries information. It is dense and slightly long, but nothing is filler; minor tightening of the history sentence could still help.

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 fully compensates by describing the returned fields, their units, ordering, timezone basis, and edge cases (sandbox nulls, bad_days). An agent has everything needed to call it and interpret results.

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 baseline is 3, and the description meaningfully exceeds it: it explains days' return shape (one entry per day, oldest first, ending today), the per-entry fields {date, jobs, spend_usd}, and that date/jobs are computed in the limiter's America/New_York day. Strong augmentation of the single meaningful parameter.

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?

Opening clause names the exact resource and scope: the key's usage today against daily limits, and enumerates the returned fields (jobs_today/job_limit, spend_today_usd/spend_limit_usd, resets_at). It is clearly distinct from siblings like set_usage_alert (a write) and check_access, so an agent can select it without opening the schema.

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?

Explicitly says 'Check it before hiring to avoid a 429', tying usage to the hire_agent workflow with a concrete trigger. It doesn't state when NOT to use it or name a complement/alternative tool explicitly, so it stops short of a 5.

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