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

Annotations declare readOnlyHint=false, destructiveHint=false and openWorldHint=false, so the safety profile is already covered. The description adds genuinely non-obvious billing behavior — prepaid credits are deducted from a reusable key — which is exactly the kind of context annotations cannot convey. It stops short of saying how many credits are deducted, whether a failed lookup still charges, or whether the key is consumed.

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 with no filler, and the cost/credit consequence is surfaced early. The brevity is efficient rather than padded, though the terseness borders on under-specification rather than true concision.

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 paid, mutating-adjacent lookup with no output schema and one undocumented parameter, the description is too thin. It never says what "keyword data" contains, how credits are priced or charged, or what happens on an invalid/expired key, all of which an agent needs before spending credits.

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 fully documented in the schema as the reusable prepaid key from x402-credit-purchase, while lookup has no schema description at all. The phrase "keyword data for a domain" implicitly tells the agent that lookup is a domain, partially compensating for that gap, but format and example values remain unstated.

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?

"Keyword data for a domain" names the resource but uses no verb and gives no sense of scope (single domain vs bulk, what fields). With siblings like x402-keywords-search-api, keywords-api and keywords-search-api present, the description never distinguishes this tool from those near-identical names, so the agent must guess.

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 when-to-use guidance, no stated prerequisites, and no mention of any alternative. The only implied constraint is that a credit key is required, which the schema already enforces via required: ["lookup", "creditKey"].

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