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

Annotations only declare readOnlyHint=true; the description adds the access requirement (trial or paid key), the exact comparison basis (last two distinct recorded snapshots), and edge-case behavior when only a baseline exists or nothing differs. This is substantive context beyond the annotation, though it says nothing about rate limits or result size.

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 sentences, front-loaded with the core action before the access caveat and the no-change instruction. The all-caps 'CHANGE EVIDENCE:' lead is slightly noisy but does orient the reader quickly; 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 burden of describing returns and does so ('added/removed text blocks') and covers the degenerate cases. The main residual gap is guidance on when to choose this versus sibling evidence/history tools.

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 in the schema, including the preference for a bearer token over passing a paid key. The description adds only the notion of an 'eligible'/'supported' page, which the schema already covers, so baseline 3 applies.

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 and resource: compare the last two distinct snapshots UKSI recorded for a GOV.UK page, returning added/removed text blocks. It is clearly distinguishable from a general history tool, but it never names or contrasts a sibling such as page_history or inspect_govuk_page, so differentiation is left to inference.

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 a real precondition (requires a trial or paid key) and a downstream instruction for the no-change case, which implies when the tool is useful. However, it never states when to prefer this over page_history or inspect_govuk_page, so the agent must guess the alternative.

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