market_status

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

No annotations are provided, so the description carries the full burden. It discloses what the tool returns ('open/closed plus the reason') and enumerates possible reasons (regular session, pre-market, after-hours, weekend, holiday). This is transparent for a simple read-only query, though it does not explicitly state it has no side effects, which is implied.

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 description is two sentences, front-loaded with the core purpose, and every phrase adds value. It is concise without sacrificing necessary detail.

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?

Given that the tool has no parameters and no output schema, the description fully covers what the agent needs: it states the output format (open/closed plus reason) and the possible reason categories. There is nothing missing for correct invocation.

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?

The tool has zero parameters, so the baseline is 4. The description correctly omits parameter details because there are none. No additional semantic explanation is needed.

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 states a specific verb+resource: 'Whether the US stock market (NYSE) is open right now, in ET.' It clearly identifies the market and the timezone, and it is distinct from siblings like get_quote, get_history, and indicator, which cover different data. The purpose is unambiguous.

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?

The description implicitly communicates when to use this tool: when an agent needs to know the current market status. It does not explicitly mention alternatives, but since the sibling tools serve different purposes (quotes, history, indicators), no exclusions are necessary. The reason categories also help the agent understand the output context.

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