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

With no annotations, the description carries the behavioral burden and does well: it discloses the pay-per-call cost ($0.002 USDC via x402 on eip155:8453), the authoritative source (ECB), the peg fallback, the presence of a method field, and that values are deterministic and non-estimated. Missing only failure/staleness behavior for non-publishing days.

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 compact segments: a bracketed payment/transport prefix followed by a single dense sentence that front-loads the resource and then the returned fields. No filler, though the payment tag is somewhat cryptic and crowds the substantive description.

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?

With no output schema in the structured data, the description compensates by enumerating what comes back — rate, rate date, derivation method, and source attribution. For a zero-parameter lookup this is effectively complete; only error and non-publication-day behavior are unstated.

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 tool takes zero parameters, so the baseline is 4. The description reinforces this by making clear that the pair and source are fixed by the tool itself rather than passed in.

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 resource and scope: the official INR/CHF reference rate, sourced from the ECB fixing with a documented fallback (official peg). The currency pair is named explicitly, so an agent can separate it from the hundreds of sibling fx_* pair tools and from fx_convert without opening the 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?

Usage is only implied by the pair name — fetch this when you need an authoritative INR/CHF rate. It never says when to prefer fx_convert (conversion with amounts), fx_latest (all pairs), or fx_timeseries (history), nor does it state any freshness window (e.g. ECB publishes on TARGET business days only).

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