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

Annotations already cover the safety profile (readOnlyHint, idempotentHint, openWorldHint), and the description adds substantive behavior beyond them: the fields returned for each input class, that ZIP+4 yields street ranges, that coordinates trigger reverse geocoding, and the payload-size and enrichment-scope limits relative to zip_lookup. No contradictions.

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 purpose and the smallest viable framing, then the input/output contract, then the sibling routing. Dense but every clause carries information; the only near-filler is the lead phrase 'The fast, small answer for a postal code.'

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 carries the return-value burden by naming city, state/province, county, multi-county flag, ZIP+4 street ranges, and reverse-geocoded results. It also states the enrichment limitation, leaving nothing an agent needs before calling it.

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 baseline is 3, but the description earns extra: it restates the accepted code formats in context and clarifies that 'include' can only carry timezone here, directing other enrichments to zip_lookup. That is real meaning beyond the schema's terse field descriptions.

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?

Opens with a specific verb+resource ('Takes a US ZIP Code... returns the code's city, state or province, county') and enumerates every accepted input form. It explicitly distinguishes itself from the sibling zip_lookup, so an agent can choose without opening either 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?

States the selection condition outright: 'choose this one when the answer only needs to be the code and its place, because the payload is about half the size and only the timezone enrichment is available here,' then names where to go instead ('Use zip_lookup for census, demographic, district or American Community Survey enrichments'). Both the when and the when-not are explicit.

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