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

Annotations declare readOnlyHint=false but do not explain why a lookup is not read-only; the description resolves that by disclosing that prepaid credits are deducted and that the key is reusable. It omits cost per domain, failure behavior on insufficient credits, and whether a domain count limit triggers partial results, so it adds real but incomplete context.

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 short sentences, front-loaded with what is returned and followed by the billing consequence. Nothing is redundant, though the terse phrasing leaves the domain-count constraint reading as a scope claim rather than an input format rule.

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 two-parameter tool with no output schema, the description covers the resource, the input cardinality, and the credit side effect. The remaining gaps are the lookup string format and error/partial-result behavior, which are secondary for an agent that already has the creditKey semantics.

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 50%: creditKey is documented in the schema (returned by x402-credit-purchase), but lookup has no description there. The description partially compensates by revealing lookup accepts one to sixteen domains, yet it never specifies the encoding (single string with separators vs. multiple values), which is the key ambiguity for this parameter.

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?

States the resource precisely ("public company registration identifiers") and a scope limit ("one to sixteen domains"), which is more than the name alone conveys. It does not, however, distinguish this from the sibling vat-api or explain what the x402 prefix changes, so an agent cannot route between them from the text alone.

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?

There is no explicit when-to-use statement and no named alternative, despite siblings vat-api and x402-domain-lookup that plausibly overlap. The only implicit guidance is the prepaid-credit framing, which an agent must infer.

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