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

Annotations already establish a read-only, idempotent, non-destructive profile. The description additionally reveals the return shape ('[{data:"dd/MM/yyyy", valor:"..."}]') and date formatting, which is critical behavior not captured in annotations or schema. 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?

Three sentences: one main action, one common-code lookup, one return/usage note. Every sentence carries useful information; the code list is dense but directly helps invocation, and there is no filler or repetition.

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 simple fetch tool, the description covers purpose, common inputs, usage modes, and output format. It could be slightly more explicit about whether 'last' and start/end are mutually exclusive, and error behavior, but overall it provides a complete mental model given the rich schema and annotations.

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 covers all 4 parameters, so baseline is 3. Description adds meaningful enrichment by mapping common numeric codes (432, 11, 433, etc.) to human-readable series, and clarifies that 'last' vs date range are two usage modes. This goes beyond the schema's examples.

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?

Description begins with a precise verb and resource: 'Fetch any Banco Central do Brasil SGS time series by numeric code.' This clearly distinguishes it from siblings by naming the SGS source and numeric code access. The inclusion of common code mappings reinforces the tool's specific data domain.

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 gives explicit usage instructions: use 'last' for recent observations or a date range via start/end. It does not mention alternative sibling tools or exclusions, but the domain specificity ('any SGS time series') makes the intended use clear.

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