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

Annotations cover the safety profile (readOnly, idempotent, non-destructive), and the description goes well beyond them: paid pricing ($0.35), rate limits (3 free calls per IP per day shared with the answer tools), auth paths (api_key vs x402 payment requirement), and exact refusal semantics (coverage_gap, no_charge). This is unusually rich behavioral disclosure.

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-loaded with purpose and price, then coverage, then payment/receipt mechanics. Dense but each clause carries information. The receipt-versus-certified-extract disclaimer is somewhat verbose and partially redundant with the schema field, keeping it from a 5.

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?

An output schema exists so return values need not be explained, and the description covers the remaining unknowns: cost, covered countries, refusal behavior, authentication, and receipt semantics. An agent has everything needed to decide and invoke 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 coverage is 86%, so the schema already documents most parameters, but the description adds the default window ('next 90 days') that the schema's 'from'/'to' fields do not state, and reiterates the receipt semantics. It gives real added meaning over the schema, though the receipt text is largely duplicated from the field description.

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 with scope: 'Upcoming VAT, PAYE and corporate-tax due dates in a window (default the next 90 days), rolled for weekends and national public holidays.' An agent immediately knows what it returns. However, it never differentiates itself from the sibling 'get_filing_deadlines', which sounds nearly identical, so sibling disambiguation is missing.

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?

It lists the covered countries and explains refusal behavior, which implies when the tool will succeed versus fail, but it gives no explicit when-to-use guidance against alternatives like get_filing_deadlines, get_calendar, or get_public_holidays. Usage is inferable rather than stated.

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