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

Annotations already declare readOnlyHint and openWorldHint, and the description adds genuinely new context: a 10-calls-per-hour cap, that results include track record, price and example input plus alternatives, and that the paid path charges nothing when no service delivers. It does not describe result ordering or what happens when nothing matches on the free path.

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?

The free-tier purpose is front-loaded, but the second sentence packs in a raw endpoint URL, an inline JSON snippet, a price and a charge policy, making it dense and awkward to scan. The buying detail arguably belongs in the sibling's definition instead of here.

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 covers what is returned (track record, price, example input, alternatives) and the rate limit, which is what an agent needs before calling. Missing only minor edges such as behaviour when the need is unbuyable on the free path.

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% and the single 'need' parameter is already documented with length bounds and a 'sentence with details (place, symbol, query)' hint. The description's 'say what your agent needs' restates rather than extends that, so baseline 3 applies.

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 and resource: given a stated need, return the paid x402 API x402 Doctor has observed paying out, with track record, price and example input. That is distinctly a discovery/recommendation operation versus the sibling x402_buy's purchase action, though the sibling is only implied via the inline buy URL rather than named.

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?

Front-loads 'Free:' and then contrasts with 'To buy it in one call ... ($0.02 over x402)', which gives the agent an explicit free-vs-paid branch. It stops short of naming the x402_buy tool as the paid alternative, so routing relies on the agent inferring from the URL and price.

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