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

With no annotations, the description carries the behavioral burden. It discloses the core computation: real historical series, month-by-month compounding with each month's actual rate, and no average rate. It does not state the shape of the return value (final amount, accumulated percentage, etc.) or date edge cases, which is a gap for a tool with no output schema.

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?

The description is a single dense sentence plus a relevant example, with no filler. It front-loads purpose and method and earns each clause.

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 calculator with full schema coverage, the description is mostly sufficient: indices, method, and an example are present. However, with no output schema, the description should specify what the call returns (final amount only, percentage, or both) and any caveats like history start being handled, so completeness is adequate but not excellent.

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. The description adds real semantic value beyond the enum by explaining what each index does (Selic/CDI accrue interest, IPCA corrects inflation, poupança applies current rule, dollar/euro follow exchange). This justifies a 4.

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 opens with a concrete question ('Quanto um valor virou...') and names the resource (Brazilian index historical series) and the method (monthly cumulative with the rate in effect each month). The phrase 'série histórica REAL' and 'sem taxa média' distinguish it from sibling calculators that use average or projected rates, even though no sibling is named.

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 establishes when to use the tool: when you need actual historical accumulation rather than average-rate estimates ('sem taxa média'), and the example ('Ex.: quanto renderam R$ 10.000 no CDI de 2020 até hoje') reinforces a concrete scenario. It does not explicitly name alternatives, so it misses the top tier, but the context is clear.

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