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

The annotations already establish this is a read-only, closed-world lookup. The description adds genuinely useful context beyond them: the useful output fields (value, unit, effective dates, source), the pricing model ($0.005/call via x402), and the alternative API-key auth path. It stops short of rate limits or fallback behavior detail.

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 sentences: what it does, what it returns, and the cost/auth terms. Zero filler and the core purpose is front-loaded.

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?

With no output schema present, the description compensates by naming the returned fields (value, unit, effective dates, source) and covering cost and authentication. An agent has everything needed to invoke it correctly.

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 the baseline is 3. The description only restates the date default (today, Europe/London) and the UK scope, adding little beyond what the schema already documents for key, date, and jurisdiction.

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?

Specific verb+resource with the key qualifying detail: the rate/threshold 'in force for a key on a date'. The temporal phrasing ('in force for a date') implicitly separates it from get_rate_history, but no sibling is named explicitly, so it stays just short of a 5.

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?

Usage is implied by the description (fetch the currently effective rate for a given rule key), but there is no explicit when-to-use / when-not-to-use guidance and no routing to siblings like get_rate_history or search_rules. Adequate but with clear gaps.

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