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

Annotations already establish the read-only, idempotent, open-world profile, so the description's job is to add operational detail. It does: null means the browser has not shipped the feature, and an unclear name returns candidate features rather than failing. That fallback and null semantics are genuinely useful behavior beyond the annotations.

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?

The trigger condition is front-loaded in the first clause, followed by examples and the return summary. The return-value enumeration is somewhat long given an output schema exists, but every sentence still carries information.

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 single-parameter read-only lookup with an output schema, the description covers the trigger, input expectations, failure mode (candidate list), and return shape. Nothing an agent needs to invoke it correctly 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% and there is only one parameter, so the schema already documents the accepted name-or-id forms and constraints. The description's 'Pass the feature name or its id' restates the schema, though the note about returning candidates on an unclear name adds mild value. 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?

The description states a specific verb and resource ('whether a web platform feature works in browsers') and anchors it to concrete user questions like 'can I use CSS container queries in Safari?'. Sibling tools (license, EOL, RFC, feedback) are entirely unrelated domains, so no differentiation is needed.

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?

It gives an explicit triggering condition ('Use this when the user asks whether a web platform feature works in browsers') backed by two example phrasings, which is clear context. It stops short of stating when not to use it or naming alternatives, but no sibling competes for this task.

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