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

Annotations already cover the safety profile (readOnlyHint=true, destructiveHint=false), and the description adds real behavioral detail beyond them: per-call pricing via x402 USDC or card key, 25 free calls, failed calls free, and the warning that repeats are not free. The constant-rate assumption is also disclosed. It does not describe precision/rounding or units of the yearly balance series, keeping it out of the top band.

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?

Purpose and input list come first, then usage, then pricing — a sensible front-loaded order. The billing sentence is long and repeats "charged" twice, but every clause carries actionable information for an agent deciding whether to call.

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 compensates by naming the return contents (future value, total contributed, interest earned, per-year balance). Combined with the pricing and assumption disclosure, an agent has enough to call and interpret results; only numeric precision and formatting of the per-year series remain unspecified.

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%, so all five parameters are already documented, including the compound enum values and the monthly default of 0. The description largely restates those fields and adds only a worked example (principal=10000, rate=7, years=10) plus the "end of each month" contribution timing, so the 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?

The description names a specific computation (compound interest future value) and enumerates the inputs it consumes — principal, annual rate, years, compounding frequency, monthly contribution — which cleanly separates it from siblings like calc_loan or calc_tip. An agent can identify the tool's job without reading 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?

"Use for savings goals and investment what-ifs; rates are assumed constant" gives clear usage context and states a modeling assumption (constant rate). It stops short of naming an alternative — e.g. calc_loan for amortization/payoff scenarios — so the agent gets positive guidance but no explicit exclusion rule.

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