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

Annotations already declare readOnlyHint, idempotentHint, openWorldHint and destructiveHint=false, so the safety profile is covered. The description adds genuinely useful context beyond that: the exact return shape ({series_code, series_name, count, observations:[{data, valor}]}) and the dd/MM/yyyy date convention. It stops short of mentioning rate limits, auth needs, or whether the full-series path is paginated.

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?

Three sentences, front-loaded with the purpose, then the code reference list, then the return shape and query modes. The inline JSON return snippet is slightly heavy but earns its place because no output schema exists.

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 correctly compensates by describing the response payload and date format, and the four parameters are all accounted for. Missing only edge-case behaviour (invalid codes, series length limits, update cadence), which are minor for a read-only fetch.

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 real domain value the schema cannot: a mapping of common series codes and the semantics of 'last' versus start/end. That extra meaning for the required 'code' parameter lifts it above baseline.

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 names a specific verb and resource ('Fetch any Banco Central do Brasil SGS time series by numeric code') and grounds it with concrete example codes (432=Selic target, 433=IPCA, etc.), making the tool's identity unambiguous. It does not explicitly route the agent away from near-neighbours like ptax_usd or indicator, which is the only thing keeping it from a 5.

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 line 'Use "last" for the most recent N observations, or a date range' gives parameter-level usage guidance, but there is no when-to-use-this-vs-an-alternative statement and no exclusions. The guidance is implied rather than explicit routing, which lands it at minimum viable.

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