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

Annotations already cover read-only and open-world behavior, yet the description adds real substance: 28-exchange cap with default of all, official-calendar sourcing, error semantics (HTTP 400 on unknown exchange, no charge), and per-call pricing via x402. Returned fields are also previewed (holiday name, local time, next open/close in UTC). Minor omission: no mention of ordering or response grouping.

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?

Front-loaded with the trigger and the returned status set, then layers scope, limits, and pricing. Dense but every clause carries information; the exchange list and pricing sentence are slightly long but not wasteful.

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 burden of explaining returns and does so: statuses, holiday names, local time, and next open/close in UTC. Scope limits, error behavior, and cost are all covered, so an agent can call this correctly without further reading.

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 the comma-separated MIC format, but the description adds the important constraints the schema lacks: the maximum of 28 exchanges, that all are returned by default, and that unknown codes yield HTTP 400. That is meaningful meaning beyond the schema.

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+resource ('which stock exchanges are open right now') and enumerates the exact status values returned (open, lunch break, pre-market, after-hours, closed). This clearly separates it from sibling calendar tools that answer date-based rather than real-time questions.

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?

Opens with an explicit trigger ('Use when an agent needs to know right now which stock exchanges are open'), giving a clear temporal condition that distinguishes it from general trading-day lookups. It does not name an alternative tool or state when not to use it, so it stops short of a 5.

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