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

Annotations already declare readOnlyHint, idempotentHint, destructiveHint=false and openWorldHint=false, so the safety profile needs no restating. The description adds the useful context that results are constrained to the coins actually catalogued, but it does not say whether a denomination+year can yield multiple varieties, nor what happens on no match, which is the main behavioural unknown.

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 with no filler; the identification framing comes first and the parameter examples follow. The opening clause ('The British coins we catalogue for one denomination and year') is slightly convoluted and could be a plainer statement of what the tool returns.

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 simple two-parameter read-only lookup with no output schema and full annotation coverage, the definition covers inputs adequately for denomination. It is still short on what the response contains (single coin vs list of varieties) and on the year parameter, both of which an agent would benefit from knowing before calling.

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 0%, so the description carries the burden; it does compensate materially for 'denomination' with worked examples ('50p', '£2', 'shilling', 'half crown', 'sovereign') that clarify the expected free-text form. The 'year' parameter receives no guidance at all beyond the schema's 1000-2100 integer bounds, leaving half the input undocumented.

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 makes clear this is a lookup of a catalogued British coin keyed by denomination and year, with the stated payoff that a holder can tell which coin they have. The resource is specific and no sibling tool in the listed set covers coins, so confusion is unlikely, but the phrasing is nominal rather than a crisp verb+resource statement.

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 only implied through the phrase 'so a holder can tell which one they have' — an agent can infer this is for identification from a physical coin. There is no explicit when-not-to-use guidance, no mention of what to do when the denomination or year is unknown, and no routing to an alternative such as search_catalogue for fuzzier lookups.

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