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

With no annotations, the description carries the full burden and does disclose real behavior: multi-state results are returned as a set rather than a single chosen state, ZIP+4 is normalized to five digits, and the normalized value is echoed back. It leaves out error behavior (unknown ZIP), rate limits, and the full response shape, which keeps it from a 5.

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 operation, then scope, then the alternative, then format rules. Slightly padded by the 'This is a geographic metadata endpoint' framing, but every sentence contributes disambiguation or format guidance.

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?

For a one-parameter lookup with no annotations and no output schema, the description covers purpose, sibling disambiguation, input normalization, and return-value expectations sufficiently. Only the failure mode for an unmatched ZIP is unaddressed.

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 adds genuine meaning beyond the schema: ZIP+4 is truncated to five digits before lookup, the normalized ZIP is returned, and leading zeroes make the string type significant. That goes beyond 'US ZIP (12345) or ZIP+4 (12345-6789)'.

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 ('Get ZIP code metadata', 'Look up every state associated with a US ZIP code') and immediately scopes the result to state association, which no sibling tool does. The mention that it does not return per diem rates cleanly separates it from the many perdiem-by-zipcode 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?

Explicitly names when-not-to-use and the alternative ('Use `/v1/gsa/perdiem/zipcode/{zipcode}` for rates'), and describes the decision-relevant context (multi-state ZIPs, no automatic state selection). This is a clear routing instruction rather than implied usage.

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