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

No annotations are provided, so the description carries the behavioral burden. It does disclose one genuinely useful trait ('Free', i.e. no auth/cost), but says nothing about rate limits, error behavior for malformed CEPs, or whether results are cached. Adequate but thin for a tool with zero annotation coverage.

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?

Single telegraphic line with zero waste and the return fields front-loaded after the arrow. It is a fragment rather than a sentence, which slightly reduces readability but costs nothing in length.

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, no-output-schema lookup, the description effectively substitutes for an output schema by naming the returned fields (street, district, city, UF, IBGE code, DDD). Only the invalid-CEP behavior is left unspecified, a minor gap at this complexity.

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 description coverage is 100% and there is only one parameter, so the schema already documents the 8-digit format and example. The description adds only the 'Brazilian' qualifier already implied by 'CEP', so baseline 3 applies.

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 (lookup) and resource (Brazilian postal code / CEP → address), and enumerates exactly what comes back. The resource is unambiguous against siblings like cnpj_lookup and bank_lookup, so the agent can select it without opening the 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?

Usage is implied by the input/output framing – an agent can infer it is called when converting a CEP into a Brazil address – but there is no explicit when/when-not statement and no sibling alternative is named. That is the minimum-viable level rather than real routing guidance.

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