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

Beyond the readOnly/openWorld annotations, the description discloses the S10 check-digit validation, service-type decoding, country naming, returned tracking links, lack of live scan history, and the x402 pricing model including that failed calls are not charged. This is unusually rich behavioral context for an agent.

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?

The description is front-loaded with the core operation and format, then adds limitations and pricing in a compact, information-dense manner. Every clause contributes useful selection or invocation context.

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 fully documented parameter, readOnly/openWorld annotations, an output schema, and an explicit explanation of non-live-scan behavior plus billing, the description gives an agent everything needed to decide whether and how to call the tool.

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 single parameter is already documented. The description still adds meaning by explaining the S10 structure and what the serial, service prefix, and country suffix represent, going beyond the bare schema example.

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 states a specific verb and resource: international postal tracking number check in UPU S10 format. It immediately explains the validation, decoding, country identification, and tracking-link output, making the tool's purpose unambiguous among the many logistics siblings.

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?

It clearly establishes the input context (UPU S10 international postal numbers) and explicitly says it does not read live scan history, which helps an agent avoid selecting it for shipment-status scanning. It does not name an alternative tool for live tracking, so it falls 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.