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

Beyond the annotations (which only say non-read-only, open-world, idempotent, non-destructive), the description discloses the critical operational facts: this is a paid tool, the exact price and chains, that inputs must be POSTed as JSON to the paid URL with an x402 v2 PAYMENT-SIGNATURE header, and a docs link for the payment protocol. Cost and auth mechanics are exactly the kind of behavior an agent cannot infer from 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?

It is compact and front-loaded: proof concept first, alternative second, payment mechanics and price after. Minor waste exists, e.g. 'HTTP 200 at that moment, fixed at 200' restates itself, and the label 'paid_url' sits ambiguously between a field name and a URL.

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-value explanation is not needed, and the description covers the non-obvious parts an agent needs: payment flow, endpoint, price, and the alternative tool. It does not explain how the sealed receipt is verified or what the agent must retain for later verification, a small gap for a proof-oriented 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% and the parameter descriptions are unusually rich, covering URL fetch limits, redirects, timeout, and full idempotency-key semantics. The description adds nothing parameter-specific beyond 'POST the tool inputs as JSON,' so the baseline 3 applies.

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 names the operation concretely: 'one fetch, transcript sealed in a signed receipt — HTTP 200 at that moment, fixed at 200,' and explicitly contrasts it with check_url_custom, so an agent can distinguish it from the sibling variants. The wording leans on marketing phrasing ('Proof anyone can verify') rather than a plain verb+resource statement, which slightly slows recognition of what is actually being checked.

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 states the selection rule clearly: use this for a fixed, verifiable HTTP 200 proof, and use check_url_custom when you want your own conditions. Payment prerequisites ($0.25, USDC on Base/Arbitrum One, x402 v2 PAYMENT-SIGNATURE header) are given up front. There is no explicit when-not guidance against the other check_url_* siblings such as check_url_changes.

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