Compare two months

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

The annotations already declare readOnly, idempotent, non-destructive, closed-world behavior, so the safety profile is covered. The description usefully goes beyond them by disclosing the currency (BRL) and the sign convention — positive = money out, while refunds, card payments, and deposits are reported separately as credits — which is non-obvious domain context an agent needs to interpret results.

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?

Two tight sentences: the first front-loads what the tool does, the second adds the interpretation rules for amounts. Nothing is redundant and every clause carries information an agent would otherwise have to guess.

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 and a read-only annotation set, the description compensates well by explaining currency and sign semantics. The remaining gap is that it never sketches the return shape (ranked deltas? per-category breakdown?), which is the main thing an agent must infer.

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%: month_a/month_b are documented as earlier/later YYYY-MM and bank as an optional nubank/itau filter, so the schema does the heavy lifting. The description adds no parameter-level detail (format, ordering constraints, or default behavior when bank is omitted) beyond what the schema already states, matching the baseline 3.

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 (compare), resource (spending by category), and scope (two calendar months, highlighting what changed most), so the function is unambiguous. It does not explicitly name or contrast itself with siblings like monthly_totals or spending_summary, so it lands just short of the top mark.

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 comparison framing — an agent can infer it applies when two months must be compared — but there is no explicit when-to-use/when-not-to-use statement and no mention of when to prefer spending_summary or monthly_totals instead. Prerequisites (e.g., whether month_b must follow month_a) are only implied.

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