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

Annotations already cover readOnly/idempotent/non-destructive/openWorld, so the safety profile is handled. The description adds genuinely useful behavioral context beyond them: the data unit ('units per £1') and the commercial constraint ('Free within the daily allowance, then $0.002 USDC per call via x402'), which an agent needs to know before invoking.

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 with no waste: the what/unit comes first, the cost constraint second. Every clause earns its place and nothing is padded.

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 still communicates the output unit and payment model. For a simple 3-param read tool this is nearly complete; only explicit routing to alternatives would make it fully airtight.

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 year/month/currency are already documented in the schema, and the description adds no parameter-level detail beyond implying a monthly granularity. Baseline 3 is correct when the schema does the heavy lifting.

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 resource ('HMRC's monthly exchange rates') and its role ('convert invoice values for customs and import VAT'), which is clearly distinct from the sibling tariff endpoints (commodity, heading, quotas, search). It stops short of explicitly contrasting itself with siblings, but the resource is unambiguous.

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?

The phrase 'used to convert invoice values for customs and import VAT' implies the usage context, but there is no explicit when-to-use vs when-to-use-an-alternative guidance or mention of prerequisites. Context is implied rather than stated.

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