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

With no annotations, the description carries the full burden and does well: it discloses the paid x402 model, price, chain (eip155:8453), the upstream source (ECB), the peg fallback, and explicitly that values are deterministic and derived rather than estimated. It stops short of failure/staleness behavior (weekends, holidays, missing fixing) and error responses.

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?

One bracketed cost/transport prefix followed by a single dense sentence. Every clause adds distinct information — source, fallback, return fields, methodology, non-estimate guarantee — with no repetition of the tool name.

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?

No output schema exists, and the description compensates by enumerating the returned fields (rate date, method, source attribution). For a zero-parameter read tool this is nearly self-sufficient; only staleness/error handling and the x402 payment handshake are left implicit for the client to resolve.

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?

Zero parameters, so there is nothing for the description to clarify beyond the schema; the baseline of 4 applies. The description instead spends its words on what the caller cannot infer from an empty schema: source, method, and pricing.

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 (JPY/IDR reference rate) with a specific provenance (latest ECB fixing, or official peg when IDR is pegged). The directional pair plus the disclosed derivation methods make it unmistakable against the inverse fx_idr_jpy and the hundreds of other fx_* pair tools.

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 implied: use this when you need an authoritative, sourced reference rate rather than an estimate, and the peg note covers the fallback case. But no alternative is named — nothing tells the agent when to prefer fx_convert, fx_idr_jpy, or fx_timeseries instead.

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