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

Annotations already declare readOnly, idempotent, non-destructive, non-open-world behavior, so the safety profile is covered. The description adds genuinely useful behavioral limits the annotations cannot convey: only regular sessions, no holidays or half-days. It omits any note on return format or timezone handling, keeping it from a 5.

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 tight sentences: capability and scope first, usage context second, caveats last. Every sentence carries information an agent needs, and nothing is repeated from the schema.

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 does state what is returned (open/closed status plus next open/close), and the venue list and limits make it callable without opening the schema. Minor gaps remain around timezone/date interpretation of 'at', but nothing blocking.

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 description coverage is 100%, with both 'at' (ISO time, default now) and 'market' ('all' or comma-separated codes) documented in the schema. The description adds no syntax or format detail beyond that, so the 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 ('says whether') and resource (stock exchange open/closed status at a time or now), enumerates the covered venues, and adds the forward-looking next open/close capability. This is clearly distinguishable from the sibling forex_market_hours by asset class and venue list.

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 it for scheduling around exchange sessions' gives a concrete usage context, and the Limits clause tells the agent when results will be wrong (holidays, half-days). It does not explicitly name an alternative tool or state when not to use it, so it falls 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.