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

Beyond the readOnly/openWorld annotations, the description discloses behavior the agent could not otherwise predict: set-cookie values are redacted, the body is never returned but is hashed over a 2 MiB cap, the GET is anonymous, and results carry source and observed_at. It also specifies the failure mode precisely — unpaid calls return isError with a fresh x402 PaymentRequired object and cost nothing — and names both payment-passing channels.

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?

It is dense but front-loaded: the paid price and the one-question framing lead, then scope, then output, then the payment/error path. Some of the pricing detail duplicates the title, and the phrasing 'Pay per call, solves one question' is mildly promotional, but no sentence is wasted given the tool's pricing complexity.

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, the description carries the return-value burden and does so: it enumerates headers, SHA-256 hash, observed_at, source, the 2 MiB ceiling, and the explicit exclusion of the body. Combined with the redirect/port limits and the 402 retry protocol, an agent has everything needed to call this correctly on the first attempt.

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 already 100%, so the baseline is 3; the description earns more by tying the URL parameter to real constraints (public http(s), ports 80/443, URL-encoding implied by 'URL-encode it') and by explaining that payment can arrive either in _meta["x402/payment"] or the payment_signature argument, plus the 'omit on first call' convention. That is genuine added meaning rather than a restatement of the schema.

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 states a precise verb and resource pair — 'Response headers ... and the SHA-256 of up to 2 MiB of the body from one anonymous GET' — and reduces the tool to a single framed question ('What response headers and content hash does this URL return?'). Nothing here overlaps with the sibling calculation/data-feed tools, so an agent can distinguish it without opening a 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?

It gives concrete eligibility rules (public http(s) only, ports 80/443, max 3 redirects, 'no verdict') and a full first-call/retry protocol for the paid path, which tells the agent when the call is valid and how to recover from a 402. It stops short of explicitly naming an alternative tool for the cases it excludes (e.g. verdicts, non-443 URLs), so it is clear context rather than full when/when-not routing.

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