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

Annotations cover read-only and open-world traits, and the description adds genuinely non-obvious behavior: the result set is built from measured probes and real paid calls, and no verdict is issued below 3 probes. That sampling-threshold caveat is important context an agent cannot get from the annotations, though pagination and completeness of the catalogue are unstated.

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?

One long sentence, but it is front-loaded with the trigger condition and then enumerates the returned fields compactly. The nested parenthetical about measured fields is dense yet every clause carries information; nothing is filler.

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 no output schema, the description usefully names the returned fields (probe count, answered share, median latency, last failure, real paid calls), which is the key missing piece. For a free, read-only, two-parameter search this is nearly complete; only the `limit` parameter's effect and result ordering are left to inference.

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%: `need` is documented in the schema as English keywords, but `limit` has no description beyond its 1-50 bounds. The description adds nothing about parameter semantics or query syntax, so it neither compensates for the gap nor repeats the schema — the baseline 3 is appropriate.

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 a specific verb (searches the public x402 catalogue) and resource, and prefaces it with the exact trigger condition ('when you need a paid API an agent can pay per call'). It is clearly distinguishable from siblings like pay_service or can_i_sell because it describes a catalogue search rather than a payment or listing action.

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?

'Use this when you need a paid API an agent can pay per call (x402)' gives an explicit use condition, and 'Free' implicitly contrasts with pay_service's cost. It stops short of naming the sibling to use for actually paying or for services outside the x402 catalogue, so the routing is clear but not exhaustive.

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