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

Annotations already declare readOnlyHint, openWorldHint, and idempotentHint, covering safety and external dependence. The description adds the specific external service (validator.nu) and the fact that it's for raw markup, which is useful context but not a major disclosure beyond 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?

Two sentences, front-loaded with the usage directive and purpose, with no wasted words. The alternative is mentioned concisely.

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?

Given the output schema exists, the one parameter is fully documented, and annotations cover safety, the description sufficiently explains what the tool does, when to use it, and how it differs from the closest sibling. Nothing critical 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?

The single 'html' parameter is fully described in the schema (100% coverage) with a clear note that it is not a URL. The description echoes this but doesn't add new semantic information, so it's at the baseline for high schema coverage.

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 clearly states the action (sends markup to Nu HTML Checker and returns validation messages) and the specific resource (raw HTML markup). It also distinguishes itself from audit_public_webpage, making it easy to identify.

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?

Explicitly states when to use the tool ('when raw HTML markup is supplied') and provides a clear alternative for live URLs ('use audit_public_webpage'). This gives the agent unambiguous routing criteria.

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