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

Annotations only declare readOnly/openWorld/non-destructive; the description adds the material behavior an agent actually needs: a $0.25 USDC-on-Base cost, that calling the tool itself is free, and the exact x402 handshake (402 → PAYMENT-SIGNATURE → delivery) that occurs on the returned URL. That is substantive disclosure well beyond the structured fields.

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?

Front-loaded with the resource and window, then cost and delivery mechanics in short clauses; no filler. Some domain jargon ('street series', 'shelf') is dense but consistent with the sibling naming, costing it only a little.

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 one-parameter read tool with no output schema, the description covers what is returned (an x402 URL to GET), the payment flow, and the cost, which is everything an agent needs to invoke and follow through 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?

Schema coverage is 100% and the single 'pair' parameter already documents its accepted values (USD-NGN or USD-GHS) and required status. The description adds no further meaning about the parameter, so the baseline 3 applies.

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+resource+scope: parallel-rate history, last 7 days, hourly, from a named source. The '7d' vs the sibling get_fx_parallel_history_24h is made explicit by 'seven days deep' and the reference to the '24h shelf', so an agent can distinguish them without opening either schema.

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 clearly frames when this applies (you need a week of hourly history rather than the 24h window) and names the related 24h shelf as the shorter alternative. It stops short of an explicit 'use X instead when Y' routing rule, so it is clear context rather than full when/when-not guidance.

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