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

Annotations already declare readOnly/idempotent/non-destructive/closed-world, and the description adds genuinely new operational facts: no authentication needed, no data retention, and that a rounding difference is produced. The last point is vague (is the difference returned, and relative to what?), which keeps it from a 5.

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 sentences, front-loaded with the key constraint (free, no key, nothing stored) followed by the domain rules. Dense but every clause carries information; only the trailing 'with the rounding difference' phrase is fuzzy.

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 one-parameter utility with no output schema, the description covers the important preconditions (no auth, no storage) and the rounding semantics. The remaining gap is that it never resolves how the cash vs card rounding is chosen given there is only an 'amount' parameter, nor what exactly the returned difference represents.

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% (the single 'amount' param even carries an example format), so the baseline is 3. The description adds no syntax beyond the schema and, worse, introduces a cash-vs-card mode distinction that no parameter exposes, which could mislead an agent into hunting for a mode argument.

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 (round) and resource (an amount in Swiss francs) plus the exact rounding convention applied. No sibling in the tool list does currency rounding, so the agent can identify this tool's job immediately from the first clause.

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?

Gives clear applicability context — free, no API key required, nothing stored — and the payment-type conditions that select each rounding rule (cash vs card/invoice/transfer). It does not spell out a when-not case, but no sibling tool overlaps this function, so the gap is minor.

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