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 ($0.002 USDC/call on eip155:8453) and the authoritative source (ECB), which are behaviors absent from any structured field. It also explains the cross/peg derivation method, which matters for interpreting the number. It omits failure/staleness behavior (e.g., what happens on weekends or holidays), keeping it from 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?

A single sentence with the purpose front-loaded after the bracketed payment prefix. The bracket is slightly noisy but carries genuinely necessary cost/network information. Nothing is redundant.

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 lookup with no output schema and no annotations, the description covers source, derivation method, the presence of a rate date, and cost/auth requirements. An agent knows enough to call it and interpret the result. Only the concrete response shape and error/staleness semantics are left 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 schema is a confirmed empty object. The description adds useful context about the returned fields (rate date, method, source) even though there are no inputs to disambiguate.

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 (SAR/SGD reference rate) and specifies the exact data lineage — latest ECB fixing or official peg. The pair-specific tool name plus this sentence lets an agent tell it apart from the other fx_* pair tools without opening a schema. It stops short of naming the conversion/timeseries siblings explicitly, so not a 5.

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 by 'Deterministic, sourced, no estimate' — this is the tool for an authoritative reference rate rather than an estimated conversion. But it never names alternatives such as fx_convert (for actual conversion) or fx_timeseries (for history), nor does it state when not to use it (e.g., non-ECB pairs). Guidance is inferable but not explicit.

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