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

Annotations already mark the tool as read-only and non-destructive, so the description adds value by stating that results may be stored or derived and that anonymous calls return subscription_required with a subscribe link. It also tells the agent to surface the returned measure, rate type, curve family, and tenor metadata when explaining results. No annotation contradiction is present.

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 with no filler: the first states the action, the second adds result-usage guidance and access behavior. It avoids repeating schema details and remains tightly scoped.

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 read-only annotations, an output schema, and a fully documented input schema, the description covers the important operational context: stored/derived behavior, API key requirement, anonymous error response, and how to present results. It could go slightly further by noting date-range or pagination behavior explicitly, but the schema already covers those.

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%, so the schema already documents base, quote, measure, rate_type, date range, pagination, and forward tenor parameters. The description adds no new parameter-specific meaning; its metadata guidance is about how to use the output rather than how to set the inputs.

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 opens with a specific verb and object—'Return the stored or derived rate differential for one FX pair'—so an agent can tell this is a data-retrieval tool rather than a visual artifact tool. It does not explicitly differentiate from close siblings like rate_curve or policy_rate_differential_visual_artifact, but 'for one FX pair' narrows the scope.

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 implies use when an agent needs a stored or derived rate differential for a single FX pair, and the access note sets expectations for authentication. It does not state when to prefer this tool over siblings such as rate_curve or policy_rate_differential_visual_artifact, nor does it provide explicit exclusions.

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