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

Annotations cover the safety profile (readOnlyHint, idempotentHint, destructiveHint=false, openWorldHint). The description goes further by disclosing execution mechanics ('fetches both URLs in parallel', 'no browser needed') and a caching window ('~24h per URL pair'), which an agent should know when reasoning about freshness. It does not cover auth/permission requirements, which is the remaining gap.

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 verb, resource, and differentiator before the longer enumeration. The output-field listing is long, but with no output schema it is doing real work rather than padding; the 'When NOT to use' routing is also positioned before the return-shape detail. Slightly dense but every sentence carries signal.

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?

There is no output schema, so the description compensates by enumerating the return shape (ok, urlA/urlB, score, grade, pass/warn/fail, added/removed/renamed, scaleDiff, structureDelta, contrastDrift, scoreDelta, checks). Combined with the eight diff dimensions and the caching note, an agent has everything needed to call and interpret this two-param 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 description coverage is 100%, so urlA and urlB are already documented as first/second URL with example framing. The description adds no syntax, format, or validation detail for the parameters themselves (the Levenshtein note refers to the rename heuristic, not an input). Baseline 3 is appropriate when the schema does the heavy lifting.

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 ('Diff two design systems from live URLs') and immediately differentiates itself with the qualifier 'the only URL-scoped design-token diff engine'. The 'When NOT to use' clause names sibling tools explicitly, so an agent can tell it apart from designesy_drift_score and designesy_monitor_score without opening a schema.

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?

Provides both when-to-use ('what actually changed between two design systems', 'how does our design system differ from a reference') and explicit when-not-to-use with named alternatives (designesy_drift_score for single-site drift, designesy_monitor_score for continuous monitoring). Nothing about tool selection is left to inference.

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