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

With no annotations supplied, the description carries the full burden and it does disclose meaningful operational context: built-in rates for 36 countries and, importantly, the payment/auth requirement (x402 price $0.001/call in USDC on eip155:8453). It omits rounding behavior, what happens on invalid country/rate combinations, and the return shape, so it is not fully transparent.

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?

Three tight, front-loaded sentences: purpose first, then rate behavior, then pricing. The trailing x402 payment line is useful but slightly transactional; overall there is little waste.

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 4-required-param calculator with no annotations and no output schema, the description covers operations, rates, and cost but never says what the tool returns (net/VAT/gross breakdown) or how the required 'rate' behaves against a country default, leaving a real 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 100%, so the four parameters (op, rate, amount, country) are already documented and the baseline is 3. The description's mention of standard rates and custom overrides largely restates the schema, and it does not resolve the ambiguity of how a required 'rate' interacts with the built-in standard rate.

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 both operation modes ('add VAT to a net amount or remove VAT from a gross amount'), which cleanly distinguishes it from siblings like percentage_calculator or discount_calculator. An agent can identify what this does without opening the schema.

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 the two applicable modes (add vs remove) and the rate selection strategy (built-in standard rates vs a custom override), giving clear context for use. It stops short of naming alternatives or stating when this tool is preferable to a generic percentage calculator.

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