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

Annotations already declare readOnly, idempotent and non-destructive, so safety is covered. The description adds real behavioral context: the exact interest formula, the x12 monthly conversion, the UK 8% + base rate rule and GBP 40/70/100 fixed compensation tiers, plus the jurisdiction caveat.

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?

Front-loads the formula and mode definitions, then closes with the caveat. Dense but every clause carries information; the mode enumeration is a bit compressed but not wasteful.

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 an 8-parameter calculator with no output schema, the description supplies the computation logic so an agent understands what the result represents (interest plus flat fee). It does not explicitly name the returned field/structure, which is the only notable gap.

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 high (88%), so the schema documents most fields. The description still adds meaning the schema lacks: the mode enum values are only listed as strings in the schema, but the description explains what each mode means and which rate parameter applies, plus the flat fee behavior.

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 verb+resource ('Calculate Late Payment Interest') and immediately clarifies it is simple interest on an overdue invoice with the exact formula. The three mode semantics (annual/monthly/uk) further distinguish it from generic date or proration calculators among siblings.

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?

Explains which mode to pick (contract annual rate, monthly rate converted x12, UK statutory) and warns that charging interest depends on contract and jurisdiction. It does not explicitly route against siblings like calculate_invoice_due_date or calculate_proration, so it falls short of full when/when-not guidance.

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