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

Annotations cover the safety profile (readOnly, idempotent, openWorld), and the description adds materially beyond them: live fetch semantics, redirect following, and a full payment model (x402 USDC on Base, $0.001 per successful call) including the fallback behavior of returning a PaymentRequired object when called unpaid. That is exactly the kind of non-obvious operational context an agent needs before invoking.

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 the usage trigger, then the returned fields, then the payment terms — a logical order with no filler. The return-field enumeration is long, but it is justified because there is no output schema; only minor tightening is possible.

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 no output schema and no annotations describing returns, the description compensates by enumerating the key returned fields (metadata, OG/Twitter tags, favicon, author, publish date, feeds, JSON-LD types) and by disclosing the payment flow. An agent has everything needed to call it and interpret the result.

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% for the single 'url' parameter, so the schema already carries the semantics; the description adds no syntax or format detail for it. 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 ('fetches the URL live and returns title, description, site name, canonical URL, language, Open Graph and Twitter card tags...'), which goes well beyond a restatement of the name 'meta'. It does not explicitly name or contrast with the siblings (read, feed, links, sitemap), so the distinction from 'read' and 'feed' is left partly to inference.

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?

'Use when you need a link preview or page metadata' gives a clear selecting condition. It stops short of explicit when-not guidance or naming siblings — notably it returns RSS/Atom feeds while a 'feed' sibling exists, which could create ambiguity for an agent choosing between them.

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