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

Annotations already establish readOnly, idempotent, non-destructive, open-world behavior. The description adds genuinely useful non-derivable context: the $0.001 USDC per-call cost and that the output is byte-identical to a REST endpoint. It doesn't discuss freshness or caching of the 30-day window.

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?

Three sentences, front-loaded with the resource and its returned fields, then usage context, then cost. The enumerated field list is dense but informative; nothing is wasted, though the list is slightly long.

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?

An output schema exists so return values need not be explained, and the description covers cost, scope, and the one parameter's meaning. For a simple single-param read tool this is close to complete; only staleness/caching of the 30-day window is unaddressed.

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 100% with a single documented parameter, so the schema carries the load and baseline is 3. The description reinforces the single-host scoping and shows the example hostname, but adds no syntax or format detail beyond the schema's own example.

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 resource ('Demand history for a single host on this rail') and enumerates the exact fields returned (calls, payers, resources, last call, price, rank). This clearly distinguishes it from siblings like market_x402_overview and market_x402_top_sellers, which cover the aggregate view rather than one host.

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?

Gives an explicit use case: 'The pre-purchase check an agent should run before paying an unknown endpoint.' That is strong when-to-use guidance. It doesn't name an alternative tool for the aggregate case, so it stops short of full when-not/alternative coverage.

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