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

Annotations already declare readOnly, idempotent, non-destructive, and closed-world, so the safety profile is covered. The description adds meaningful behavioral context by spelling out the returned outputs, including a range sanity check, which matters since no output schema exists. It stops short of explaining rounding rules or how 'usual range' is determined.

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?

A single well-structured sentence with the purpose front-loaded and the output list following. Every clause carries information; no filler. Slightly list-like but efficient.

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 responsibly enumerates the returned values, and the input schema fully documents the parameters. Minor gaps remain around currency handling beyond USD and the definition of the 'usual range' check, but the definition is sufficient to invoke the tool correctly.

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% and each parameter is documented with units and examples (USD, YYYY-MM-DD, flat vs percent, grace-day semantics). The description adds no parameter-level meaning beyond the schema, 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?

The description names a specific verb and resource ('works out a late fee') and enumerates exactly what is produced (amount, new total, applicable day, invoice line, range check). An agent can tell this is a calculation tool rather than a date or email tool. It does not explicitly differentiate itself from siblings like how_late_is_this_invoice, which keeps it from a 5.

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?

There is no explicit guidance on when to use this versus how_late_is_this_invoice or payment_reminder_email, nor any stated prerequisites or exclusions. Usage is only implied by the purpose statement.

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