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

Beyond readOnlyHint/openWorldHint the description discloses the data source policy (official exchange calendars only), the failure contract (HTTP 400 and no charge on bad input or an uncovered year), and the cost model (0.002 USDC per successful call via x402). Those are exactly the operational facts an agent cannot get from annotations or schema.

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 usage trigger is front-loaded and the exchange roster, error contract, and price each occupy a short clause with no filler. The mid-sentence field enumeration is dense but every item conveys a distinct return value.

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 carries the return shape itself and does so (holidays, half days, late openings, hours, trading-day count). Coverage breadth (28 exchanges with examples), error behavior, and pricing round out everything needed to call it correctly.

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 already documents both parameters, including MIC codes and common-name fallbacks, so the description adds little on parameter syntax. Its only marginal addition is the notion of an 'uncovered year' triggering a 400, which is behavioral rather than semantic — baseline 3 fits.

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?

The description names the resource precisely — a full-year closure calendar for a stock exchange — and enumerates what it returns (holiday names, half days, late openings, regular hours, trading-day count), which separates it from single-day tools like trading_day or market_status. It stops short of naming any sibling explicitly, so the differentiation is inferred from the 'whole year' scope rather than stated.

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 when an agent plans ahead and needs every closure ... for a whole year' gives a clear triggering condition, and the yearly scope implicitly rules out point-in-time lookups. There is no explicit when-not clause or named alternative, so an agent must infer that single-day checks belong to siblings.

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