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

Annotations already declare readOnlyHint, openWorldHint, idempotentHint, and non-destructive behavior. The description adds useful behavioral specifics: it combines Nu HTML validation with CSS, SEO, accessibility, and JSON-LD checks, and clarifies that base_url only resolves links without fetching. This adds context beyond the annotations, though it does not discuss potential rate limits or authentication, which are not covered by 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?

Three sentences with no filler. The first sentence establishes usage context, the second summarizes the tool's functionality, and the third provides a critical limitation and sibling alternative. Efficient and well-structured.

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 tool with a rich output schema, detailed parameter descriptions, and comprehensive annotations, the description covers the primary use case, the main limitation (no fetching), and the alternative tool. Everything an agent needs to decide whether to call this tool and what it does is present.

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 each parameter is well-documented in the schema. The description adds minimal param-specific meaning (e.g., 'local CSS' clarifies css scope, and the base_url limitation is also in the schema), but it does not compensate further. Baseline 3 is appropriate.

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 specifies a clear verb and resource: validates supplied HTML markup with multiple checks. It explicitly distinguishes itself from audit_public_webpage by stating it does not fetch a webpage from base_url, making it easy to tell apart from siblings.

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?

States the condition for use ('when HTML markup is already supplied') and explicitly says not to use it for live URLs, pointing to audit_public_webpage as the alternative. This is explicit when-to-use and when-not-to-use guidance.

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