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

With no annotations, the description carries the full behavioral burden, and it does disclose important context: the x402 payment requirement, exact cost, HTTP 402 flow, and PAYMENT-SIGNATURE retry — plus a 'Not tax advice' disclaimer. However, it says nothing about how conflicting inputs (net vs gross vs lines) are resolved, defaults for rate/kind, or failure behavior, which are significant for a 9-param calculator.

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?

Purpose is front-loaded, and the dense payment sentence is necessary operational content rather than filler. The 'Not tax advice' tail is minor but harmless. Overall it is tight with no wasted sentences.

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 tool with 9 parameters at 0% schema coverage, no annotations, and no output schema, the description covers only purpose and payment mechanics. It omits parameter semantics, default values, input precedence, and any notion of what the calculation returns, leaving the agent under-informed before invocation.

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 0% across 9 parameters, so the description must compensate. It hints at net, gross, lines, and threshold, but leaves kind, rate, markdown, description, and monthly_taxable_turnover entirely unexplained, and gives no format or precedence rules (e.g., whether net and gross are mutually exclusive).

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 and resource ('UK VAT calculator') and enumerates its scope: net/gross, multi-line, £90k threshold. This clearly identifies the tool's function. It doesn't need to differentiate from siblings since the only sibling (get_gpu_prices) is unrelated, but the differentiation is implicit rather than stated.

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 use this tool, when not to, or how to choose between the net/gross/lines input modes. The operational note ('Same as POST /v1/vat-calc') and payment mechanics are about invocation, not usage selection. An agent gets no help deciding whether this tool applies to its task.

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