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

Annotations already declare readOnlyHint, openWorldHint, and destructiveHint, so the safety profile is covered. The description adds valuable behavioral context beyond annotations by detailing the authentication requirement and the anonymous-call response ('subscription_required with a subscribe link'). This is the kind of operational detail an agent needs.

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 two sentences with no redundant content. The purpose is front-loaded, the usage context follows, and the access note is placed at the end. Every sentence 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?

For a simple list tool with two optional parameters, the description covers purpose, filters, sequencing, and authentication behavior. An output schema exists so return values need no explanation. Nothing critical is missing for an agent to decide to call and successfully invoke the tool.

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%, with both 'source' and 'currency' already described and exemplified. The description's phrase 'optionally filtered by currency or source id' only paraphrases the schema without adding new semantic meaning. Thus a baseline of 3 is appropriate.

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 a specific verb ('List') and resource ('public FX reference-rate pair universe'), with optional filters. It distinguishes itself from siblings by naming the resource type and explicitly positioning it as a prerequisite to intraday reference-rate series, avoiding confusion with fx_reference_sources or 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?

The description gives explicit sequencing guidance: 'Call this before requesting an intraday reference-rate series.' It also states the access requirement and the failure mode for anonymous calls, which informs the agent when the tool is appropriate and what to expect without an API key.

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