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

Annotations already declare readOnlyHint=true and openWorldHint=true, so the safety profile is covered. The description adds useful behavioral detail about authentication requirements, including that anonymous calls return subscription_required with a subscribe link, which is not inferable from the annotations or schema.

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 concise sentences, with the primary action front-loaded and the access caveat placed second. Every sentence earns its place without redundancy or extraneous 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 zero parameters, an output schema, and annotations covering read-only behavior, the description is complete: it states the resource, the workflow position, and the authentication consequence. Nothing material is missing for an agent to invoke it correctly.

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 has zero parameters with 100% schema description coverage, so there are no parameter semantics to clarify. Per the baseline rule for zero-parameter tools, this is appropriately scored at 4.

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?

States a specific verb ('List'), a concrete resource ('public official FX reference-rate sources and their source policy'), and links to the workflow ('before requesting a pair-specific intraday reference-rate series'). This clearly distinguishes it from rate-returning siblings like fx_intraday_reference_rates.

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?

Explicitly frames when to use the tool: before requesting a pair-specific intraday reference-rate series. It does not name alternatives or give explicit when-not-to-use guidance, but the contextual instruction is clear enough for an agent to place it correctly in the workflow.

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