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

Annotations already indicate read-only, idempotent, non-destructive behavior. The description adds context beyond annotations by specifying the algorithm (great-circle) and output units (miles and km), which is useful behavioral detail not captured by the structured fields.

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 a single, concise sentence that front-loads the core action and output. No extraneous information, and it effectively covers the essential purpose in minimal words.

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 simplicity (two required string parameters, no output schema), the description is nearly complete: it states inputs and outputs. It does not mention edge cases like invalid ZIP codes or approximation errors, but these are minor for a straightforward distance calculator.

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 compensate. It clarifies that 'from' and 'to' are US ZIP codes, adding meaning beyond the bare string type. However, it does not specify format (e.g., 5-digit vs 9-digit) or ordering, leaving some ambiguity.

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 clearly states the tool computes great-circle distance between two US ZIP codes, with output in miles and km. This is a specific verb-resource pair that distinguishes it from siblings like nearest_zip or zips_in_radius, which serve different purposes.

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 description does not explicitly mention when to use this tool versus alternatives, nor does it reference any sibling tools. However, the purpose is self-evident enough that an agent can infer usage for distance between two specific ZIPs, but there is no explicit guidance or exclusion criteria.

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