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

Annotations already declare readOnly, idempotent, openWorld and non-destructive, so the safety profile is covered. The description adds genuine behavioral context those annotations cannot express: pricing ($0.002/call over x402 on Base), the prepaid x-credit-token auth path, a 1-free-call/day allowance, and that no API key is required — all material to whether and how an agent invokes it.

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 core question and the scope caveat, then required input and commercial terms. Each sentence contributes, though the billing/auth sentence is dense and slightly overstuffed for a one-parameter detector.

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 an output schema present, return values need not be explained, and the description still notes it returns a carrier tracking URL. Purpose, exclusion, input, auth and pricing are all covered; only the candidate-pattern details for the input remain unstated, which is minor.

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% for the single 'tracking' parameter, so the schema already carries the semantics. The description only restates that 'tracking' is required, adding no new format or example detail (e.g., accepted character patterns), which is the baseline 3 case.

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 and resource — detect which carrier a tracking number belongs to — and names the candidate carrier set (UPS/FedEx/USPS/DHL/DPD/TNT/TR). It also explicitly disclaims what it is not ('Live parcel status is NOT fetched'), so an agent can distinguish intent 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?

Gives a clear usage boundary: pattern detection only, not live status lookup, which prevents misuse against a parcel-tracking expectation. It does not name an alternative tool for actual tracking status, but the when/when-not split is otherwise explicit.

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