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

With no annotations at all, the description carries the full disclosure burden, and it does add real value by stating the data source (HMRC) and the cost ('1 credit'), which is a behavioral trait an agent otherwise could not know. It does not say what happens if the optional month is omitted (assume latest month?), how far back history goes, or what the rate payload looks like. Useful, but incomplete for an unannotated tool.

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 tightly written clause covering resource, source, scope, and cost, with no filler. The credit cost is appended in parentheses rather than front-loaded, but everything present earns its place.

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?

The tool is simple (one optional string parameter, no output schema), so the bar is low, and the description covers source, scope, and cost. The main omission is default behavior when month is absent, which is exactly the ambiguity an agent faces with an optional-only parameter set and no annotations 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?

Schema coverage is 100% and the single parameter 'month' is documented as 'YYYY-MM', so the schema already carries the semantics. The description's word 'monthly' reinforces the granularity but adds no format, defaulting, or range information beyond the schema. Baseline 3 is appropriate.

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 with source and scope: 'HMRC monthly exchange rates for customs and VAT, every currency to GBP.' An agent immediately knows what data comes back (monthly HMRC rates quoted against GBP). It stops short of naming the sibling it differs from (fx_rates, boe_series), so the distinction is inferable from the 'HMRC/customs/VAT' qualifier 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?

'for customs and VAT' hints at the use case, implying this is the authoritative rate set for HMRC filings rather than a generic market rate feed. However, it never states when to prefer this over fx_rates or boe_series, and gives no prerequisites or exclusions. Usage is implied, not guided.

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