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

Annotations only say read-only/idempotent; the description adds substantial context beyond that: price and chain, first 100 calls per client per UTC day free, an unpaid call fails with an x402 v2 PaymentRequired error (isError + structuredContent.accepts), the retry path via _meta["x402/payment"], input validated before any payment request, and charging only on a successful answer with settlement in _meta["x402/payment-response"]. That is exactly the payment/auth/error disclosure an agent needs before calling.

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?

The entire definition is one run-on sentence with nested parentheses, and it is front-loaded with price and payment transport rather than what the tool measures. The core purpose appears mid-sentence after 'Measured facts for one host', forcing the agent to parse ~100 words to find the verb-and-resource.

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?

For a complex, no-output-schema tool, the description covers what is returned (the enumerated measurements and receipts), the failure mode, and the payment lifecycle, and the annotation set covers safety. Nothing an agent needs in order to call it correctly appears to be missing.

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 a single parameter with 100% schema description coverage and a built-in example (hostname vs full URL), so the schema does the heavy lifting and baseline 3 applies. The description adds only that the input is validated before any payment request, which is marginally useful but not new semantics for the host value itself.

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 resource precisely: measured facts for one host, enumerated as handshake state, protocol version, tools count/hash, auth needed, latency, valid-402 rate, price vs listing, test-buys, freshness and receipts. A reader can tell it is a per-host diagnostic. However, the generic name 'check' plus the identical title 'Check ($0.001)' give no sibling differentiation — the closely-related 'preflight' tool is never mentioned, so the agent must guess which of the two to use.

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?

Usage is only implied: you check 'one host (an x402 seller, or a remote MCP server host)'. There is no explicit when-to-use, when-not-to-use, or pointer to the alternatives among siblings (preflight, best, coverage, sample). The free-tier and payment notes describe mechanics, not selection criteria.

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