lookup_postal_code

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

With no annotations, the description carries the full burden. It mentions the external service (Zippopotam.us) and lists return keys including 'valid' and 'cost_usd', which hints at potential costs. It does not explicitly state that the operation is read-only or describe error behavior, but the 'valid' flag and read-only intent ('look up') cover this adequately.

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 well-structured with clear sections for args and returns. Every sentence adds value—purpose, scope, parameter details, and return schema are all covered without verbose fluff. It is appropriately sized for the tool's complexity.

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?

Given the simple tool (2 params, no output schema), the description is complete. It explains what data is returned and the expected output keys. No critical information is missing for an agent to use this tool correctly.

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 0%, so the description must fully explain parameters. It does: postal_code is described with examples and country is described with the ISO 3166-1 alpha-2 format and a default of US. This goes beyond the bare schema definition, providing the agent with enough detail to invoke correctly.

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 an explicit action (look up), a clear resource (postal code), and the expected results (city, state, coordinates). It also distinguishes itself from sibling tools by focusing on postal codes and mentioning the data source (Zippopotam.us), which is unique among the listed tools.

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 purpose is self-evident and no sibling tool overlaps with this function, so an implicit usage context is clear. However, it does not explicitly state when not to use it or mention any alternatives, leaving some room for inference.

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