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

Annotations already declare readOnly, idempotent and closed-world, so safety needs no restating. The description instead adds calculation semantics the agent cannot get elsewhere: removal is division by 1.2, not a 20% subtraction, which is exactly the kind of behavioural detail that prevents a wrong call.

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?

Two tight sentences: a concrete worked example first, then the supported scope. The trailing marketing URL is dead weight for an agent but small enough not to undermine the structure.

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 stateless, read-only, all-optional calculator with full schema coverage and no output schema, the description supplies mode semantics, rate flexibility and a correctness anchor. Only the response shape and how currency interacts with country remain unstated, which is a minor 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 description coverage is 100%, so baseline is 3. The description reinforces amount/rate/country behaviour ('any rate in 177 countries', worked example with a rate), but it does not explain mode defaults, currency handling, or the precedence between rate and country beyond what the schema already states.

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 pair (add/remove) and resource (VAT), plus the supported jurisdictions and rate range, so an agent immediately knows this computes VAT in either direction. It stops short of distinguishing itself from near-neighbours such as gst_calculator, sales_tax_calculator or discount_calculator.

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?

'Add or remove VAT' implies the two usage modes, and the country/rate listing implies when to supply a rate override. There is no explicit when-to-use-this-vs-alternative guidance (VAT vs GST vs sales tax), leaving routing to inference.

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