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

Annotations already declare readOnlyHint, idempotentHint, destructiveHint=false and openWorldHint, so the safety profile is covered. The description adds genuinely non-structured behavioral facts: a free tier of 10 calls/day and paid access via x402 (params._meta["x402/payment"] or HTTP) or X-API-Key credits, which an agent must know before calling. It does not address error or malformed-header behavior, keeping it short of a 5.

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?

Purpose is front-loaded in the first clause, optional behavior second, and commercial terms last where they belong. Every sentence carries information; the only marginal element is the '[served by ...]' provenance tag, which is short enough not to be wasteful.

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 an output schema present, return values need not be explained, and the definition covers purpose, optional inputs, and access/payment mechanics for a low-complexity single-purpose parser. Minor gap: no note on how malformed headers are handled, which matters slightly for a parsing 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 coverage is 100%, so the baseline is 3, but the description goes further by explaining that age and date exist to 'estimate remaining freshness' — clarifying how those params combine rather than just what they hold. It does not clarify the etag echo behavior, which the schema itself only labels 'optional ETag to echo'.

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 ('Parse a Cache-Control header into structured directives') and enumerates representative outputs (max-age, no-store, immutable). The resource is narrow enough that an agent can distinguish it from other parsers such as duration_parse or http_link_parse without opening the 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?

The clause 'optionally estimate remaining freshness from Age or Date' tells the agent when to supply the optional params, which is useful conditional guidance. However, there is no explicit when-not guidance and no named alternative for related header-parsing or HTTP-inspection tasks, so the routing signal is only implied.

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