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

Annotations already declare readOnlyHint=true, openWorldHint=true and destructiveHint=false, so safety is covered. The description adds real behavioral context beyond that: the auth requirement and the concrete failure mode for anonymous callers (subscription_required with a subscribe link), plus the note that the differential may be 'stored or derived'.

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, zero filler, with the core purpose front-loaded and the access constraint second. Every clause earns its place.

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-value explanation is unnecessary, and the auth/error behavior is disclosed. The remaining gap is the absence of routing guidance against a crowded sibling set, but for a single read-only lookup the description is otherwise sufficient.

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 all 11 parameters are already documented in the schema, and the description adds no syntax, defaults, or constraints beyond naming the same metadata fields. Baseline 3 applies when the schema carries the parameter burden.

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 names a specific verb and resource ('Return the stored or derived rate differential for one FX pair') and lists the defining metadata (measure, rate type, curve family, tenor). It is clear, though it does not explicitly distinguish itself from adjacent siblings like rate_curve or policy_rate_differential_visual_artifact.

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?

There is no statement of when to choose this tool over alternatives such as rate_curve or the policy-rate differential visual artifact tools. The only guidance is an access prerequisite (API key required, anonymous calls return subscription_required), which is not usage selection guidance.

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