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

readOnlyHint=true already establishes the safety profile, so the bar is lower; the description adds genuinely useful behavior: the data is 'CURRENT verified', returns include an official payment link, and the PCN comes with its early-payment reduction. It does not cover rate limits or auth needs, but for a read-only lookup those are minor.

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?

Front-loaded with what is returned, then usage triggers, then the anti-memory warning; the concrete 'Dartford is no longer £2.50' example earns its place. Slightly padded by repeating 'how to pay' in both the return list and the trigger list, but no sentence is wasted.

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, the description carries the full return-value burden and does so thoroughly (price, penalty, deadline, payment steps, link). Combined with the routing directive, an agent has everything needed to call and interpret this tool.

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 schema itself documents both params, including that crossed_at returns the exact pay-by date. The description restates that ('Pass crossed_at to get the concrete pay-by date for a specific crossing') without adding format or edge-case detail, so it sits at the baseline.

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+resource (retrieve current verified details for one UK toll/road charge) and enumerates exactly what is returned: price, PCN and early-payment reduction, pay-by deadline, step-by-step payment instructions, and official link. It is clearly differentiated from list_charges, which is cited as the source of the `charge` id.

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?

It gives an explicit trigger list (asks what a charge costs, forgot/missed payment, fine/penalty questions, how or by when to pay) plus an explicit exclusion — 'ALWAYS call this rather than answering from memory' — with a justification about stale trained-in prices. That is unusually strong routing guidance.

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