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

Annotations declare openWorldHint and non-idempotent, but they say nothing about cost or the x402 payment handshake. The description adds the per-call price, the exact payment protocol, and the underlying HTTP endpoint — genuinely non-obvious behavior an agent needs before invoking.

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 purpose is front-loaded in the first sentence, followed by cost and payment mechanics. Dense but every sentence carries actionable information; only the HTTP-equivalent clause is marginal.

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 lookup with no output schema, the description covers what is returned (all rates), what it costs, and how to pay — the pieces an agent would otherwise get wrong. Return shape detail is the only minor gap.

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 'base' parameter is already documented with its USD default. The description adds no syntax or format detail beyond that, 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: retrieve all FX rates for a given base currency. It is clearly distinguishable from devutils_fx_convert, though it never names that sibling explicitly.

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 payment flow guidance (call unpaid to get the PaymentRequired challenge, retry with payment in _meta) is useful operational context, but there is no guidance on when to use this versus devutils_fx_convert or when a paid call is warranted.

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