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

With no annotations, the description carries the burden well: it discloses the paid model ($0.002 USDC/call via x402 on eip155:8453), the data source (ECB), the fallback behavior when INR is pegged, and that the result is deterministic rather than an estimate. It omits failure modes, freshness cadence, and what happens if payment is insufficient, which keeps it below a 5.

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?

It is a single compact sentence-pair with the payment cost front-loaded, which is the most decision-relevant fact for an agent. The peg parenthetical is slightly dense but not wasteful, and nothing is repeated from the schema.

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 zero-parameter, no-output-schema tool, the description supplies everything needed: what rate, which source, how it is derived (direct/cross/peg), what accompanies the value (rate date, method, attribution), and the cost of the call. There is no meaningful gap left for an agent to guess at.

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 there is no parameter meaning to add and the baseline is 4. The description instead spends its words on the response payload (rate date, derivation method, source attribution), which is useful but is return-value information rather than parameter semantics.

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 names a specific resource and scope: the official USD/INR reference rate, taken from the latest ECB fixing (or the peg when INR is pegged). Combined with the self-describing tool name, an agent can tell exactly what pair and what kind of rate this returns 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: 'reference rate' and 'no estimate' signal this is for authoritative/official rate needs rather than market quotes. There is no explicit routing against the many siblings (fx_latest, fx_convert, fx_timeseries, or the cross-rate pairs like fx_eur_inr), so the agent must infer that this is the direct pair tool and not a conversion or history tool.

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