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

readOnlyHint=true already establishes the safety profile, so the description's main added value is disclosing the payment requirement: '$0.001 USDC per call (x402)'. That is an auth/cost constraint the agent could not learn from annotations or the empty schema, and it materially affects invocation decisions. It does not describe the response shape, but with annotations covering safety this is a modest gap.

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 dense sentence that front-loads the resource and its key values, ending with the price note. Every clause carries information; there is no filler or repetition of the title.

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 a zero-parameter, stateless lookup this is close to adequate: content scope and per-call cost are both stated. What is missing is any hint of return format or granularity (a flat rate table? per-category records?), which matters because there is no output schema to fall back on.

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?

The schema has zero properties, so there are no parameters to explain and the baseline is 4. The description correctly spends no words on inputs and instead uses the space for scope and cost.

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 expands the opaque name 'iva' into 'Mexican VAT (IVA)' and enumerates exactly what the resource covers: 16% general, 8% border, 0%/exempt, plus ISR and IVA retentions for professional services and leases. That is specific enough to separate it from rate/validation siblings like banxico, sat_cfdi_verificar, or rfc_validar. It lacks an explicit verb ('returns'/'looks up'), which keeps it just short of 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 guidance on when to select this tool over alternatives, no mention of any sibling, and no statement of the question shapes it answers (e.g., 'use for computing IVA on an invoice' vs 'use banxico for exchange rates'). The agent must infer applicability purely from the rate list.

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