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

The description matches the annotations (readOnlyHint=true, destructiveHint=false) by stating it 'returns' observations, implying a safe read operation. It adds context about subscriber-only access, single-pair scope, and the need to verify availability, which goes beyond what annotations declare.

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, no fluff. The first sentence states the purpose and scope; the second provides a critical prerequisite. Everything earns its place, and it is front-loaded for quick comprehension.

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 an output schema present, return-value details are covered. The description adequately covers purpose, scope, and usage prerequisite for a 4-parameter read-only query. It does not explicitly mention time-range behavior, but the schema parameters make it clear enough.

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 descriptions cover 100% of parameters, so the baseline is 3. The description adds meaningful context by emphasizing 'one pair' and instructing verification via fx_reference_universe, which enhances understanding of how to set base/quote parameters.

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 explicitly states 'Return subscriber intraday official FX reference-rate observations for one pair,' providing a specific verb, resource, and scope. It distinguishes from the sibling fx_reference_universe by focusing on observations and mentioning the prerequisite of checking availability.

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?

It gives a clear precondition: 'Call fx_reference_universe first to verify pair and source availability,' which tells the agent the necessary preceding step. It does not explicitly state when not to use or name alternatives, but the context is clear for this specific data retrieval action.

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