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

Annotations already declare readOnlyHint/idempotentHint/openWorldHint, so the safety profile is covered. The description adds a genuinely important non-obvious behavior: the call returns x402 payment requirements unless payment_signature is supplied, plus the $0.003 USDC-on-Base pricing — a two-phase payment gate an agent could not infer from the schema alone. It omits the actual return content, but the title names the check categories.

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 core action, then use case, then pricing, then payment mechanics, then a sample link — every sentence is functional. The bracketed "[Data utilities]" tag is the only mildly expendable element.

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 read-only, two-parameter checker with no output schema, the description covers the trigger, cost, and the payment-gating flow, which is the behavior most likely to trip up an agent. Only the shape of the returned trust report is left implicit, and the title largely covers that.

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 description coverage is 100%, with both the required domain format and the optional payment_signature semantics documented in the schema itself. The description only echoes the payment_signature behavior already stated in the schema, so it adds little parameter-level meaning beyond the baseline.

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 pairs a specific verb ("Sanity-check") with a specific resource (a vendor's or sender's domain), and the title enumerates the actual checks performed (DNS, Email Auth, TLS, Headers). It distinguishes itself from generic lookup siblings by scoping to pre-trust of an email/link/payment, though it never names the closest sibling (email_verify) directly.

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 a concrete trigger: "Use when an unfamiliar domain appears in an invoice, email or supplier record," which tells the agent exactly what situation calls for this tool. There are no exclusions or named alternatives, so it stops short of full routing guidance.

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