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

Annotations already say readOnlyHint=true and destructiveHint=false, but the description adds genuinely non-obvious behavior: fixed $0.01/call pricing via x402 USDC or card key, first 25 free, failed calls free, and that repeat calls are charged. That last point resolves the otherwise puzzling idempotentHint=false for a pure calculator, which the annotation alone would leave ambiguous.

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 purpose, output list and usage guidance are front-loaded, and the example is compact. The billing sentence is longer than it needs to be (three payment mechanisms plus two free-tier clauses) but each clause is operationally relevant.

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?

There is no output schema, yet the description names every returned value (final balance, total contributed, interest earned, year-by-year balances). Combined with the constant-rate assumption, the end-of-month contribution convention and the cost model, nothing an agent needs to call this correctly is missing.

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 ranges (rate -50 to 100, years up to 100), defaults for monthly and compound, and the enum-like compounding options are already documented in the schema. The description restates them and adds an example but no new syntax or edge-case semantics, so the baseline of 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?

States a specific computation (future value with compound interest), enumerates the driving inputs (principal, rate, years, compounding frequency, monthly contribution) and names the returned quantities. An agent can distinguish it from calc_loan or calc_tip purely from the text.

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 the worked example (principal=10000, rate=7, years=10) shows a concrete call shape. No explicit exclusion or cross-reference to a sibling such as calc_loan, so it stops short of a 5.

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