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

The annotations already declare readOnlyHint=true and destructiveHint=false, so the description adds value by noting that the tool accounts for weekends and major banking holidays, and can snapshot a specific moment via the `at` parameter. It does not contradict annotations.

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 three sentences, front-loaded with the primary function, then usage triggers, and then parameter guidance. Every sentence earns its place with no unnecessary 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?

For a read-only tool with one optional parameter and an output schema, the description is complete: it covers the core function, typical user queries, timestamp handling, and edge cases (weekends/holidays). No important context is missing.

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 input schema already provides 100% coverage for the 'at' parameter, including format, default, and purpose. The description reinforces this usage but does not add significant new semantics beyond the schema, so the baseline score of 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?

The description clearly states the tool's function: it tells which FX trading sessions (Sydney, Tokyo, London, New York) are currently open and when the next session opens/closes. This specific verb+resource combination distinguishes it from sibling market data tools.

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 provides explicit trigger examples: 'is the market open?', 'when does London open?', and 'which sessions overlap right now?', giving clear guidance on when to use this tool. It does not mention alternatives or when not to use it, but the usage context is very clear.

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