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

readOnlyHint=true already establishes this is a safe read. The description adds two genuine behavioral facts: a key is needed, and a single-baseline page yields no change report (an empty/error condition). It does not describe output shape, snapshot cadence, or what 'distinct' means, so it adds moderate but not rich context.

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 short sentences, front-loaded with the operation and scope. 'Requires key.' is terse to the point of being vague, slightly weakening an otherwise efficient structure.

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 and only two well-documented parameters, the description supplies the essential semantics (what is compared, which two snapshots, baseline caveat). It omits the monitoring prerequisite for producing those snapshots and any notion of result size, but nothing critical to invoking it is missing.

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%, so both url and api_key are already documented, making 3 the baseline. The description's 'Requires key' is weaker than the schema's 'Optional UKSI API key', leaving ambiguous whether the key may come from the Authorization header instead; it adds no further syntax beyond the schema.

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 (compare) plus a precise resource scope: text blocks from the last two distinct observed snapshots of a monitored GOV.UK page. That scope implicitly separates it from page_history (all snapshots) and inspect_govuk_page, though no sibling is named explicitly.

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?

The line 'A baseline alone has no change report' tells the agent a precondition: the page must have at least two snapshots. It does not say when to choose this over page_history or fixed_monitored_pages, so usage is only implied.

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