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

Annotations only say readOnly and openWorld; the description adds the material facts an agent needs: a $0.001 USDC charge on Base per call, that unpaid calls return an x402 PaymentRequired response, and that any x402 MCP client pays automatically. That is real operational context beyond the structured data, including the failure mode.

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, zero filler: the first front-loads the capability and its outputs, the second front-loads the pricing and payment behavior. Nothing is repeated from the name or schema.

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 a single well-documented parameter, no output schema, and annotations covering the safety profile, the description supplies everything else an agent needs — result fields, cost, and the payment handshake on failure.

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?

There is only one parameter and the schema already documents it at 100% coverage ('Public http(s) URL (scheme optional)'). The description adds no further syntax, normalization, or validation detail, so the schema carries this dimension entirely — baseline 3.

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?

Opens with a concrete question ('Is this URL up?') and then enumerates the exact returned facts: HTTP status, redirect chain, final URL, response time, content type, server/CDN, security-headers grade. That specificity clearly separates it from siblings like url_metadata, url_links, or robots_check that also take a URL.

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 'Is this URL up?' framing plus the output list makes the use case obvious, and the x402 payment note explains what happens on an unpaid call. It stops short of naming an alternative sibling or an explicit when-not-to-use condition, so it falls short of a 5.

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