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

Annotations already declare readOnly, idempotent, non-destructive, closed-world, so the safety profile is settled. The description still adds real behavioral context: results are per the account's broker, times are normalized to UTC, and a broker holiday absent from the schedule does not mean orders will be accepted. It stops short of stating caching or freshness, but the holiday caveat is genuine added value.

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?

Two sentences plus a compact example, with the core answer ('whether a market is open now') front-loaded before the scoping and caveat details. Nothing is padding.

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 no explanation, and the description covers broker scoping, UTC normalization, the holiday caveat, and a usage example. The only omission is any mention of response_format, which the schema handles, so the definition is essentially complete for calling the tool.

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 schema already documents account_id, symbol vs instrument, and the response_format enum in detail. The description's example adds a concrete call shape and reinforces the account-scoped broker context, but supplies no syntax or constraints beyond the schema. 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: reports whether a market is open now on the account's broker and when it next opens or closes, with UTC called out explicitly. No sibling tool covers market hours, so the scope is unambiguous without needing to name an alternative.

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?

Usage is implied rather than stated – an agent infers it should be called before placing or modifying orders. The holiday caveat ('a broker holiday missing from its schedule still refuses orders') is about interpreting the answer, not about when to select this tool over a sibling. No explicit when/when-not guidance.

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