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

With no annotations, the description carries the full burden and does so well: it discloses the paid cost ($0.002 USDC/call, x402, eip155:8453), the upstream source (ECB), the peg fallback, the reported method (direct/cross/peg-derived), and that output is sourced rather than estimated. What is absent is failure behavior — e.g. what happens on weekends/holidays or when no fixing exists — and the quote direction convention.

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?

Payment/cost is front-loaded in a bracketed prefix, then a single dense sentence carries source, fallback, and returned metadata. No filler; each clause (source, method, attribution, determinism) adds distinct information.

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-param, no-output-schema tool, the description tells the agent what comes back (rate, rate date, method, source attribution) and what it costs to call. Remaining gap is directional convention (is the rate PLN per TRY or TRY per PLN?) and staleness semantics around non-fixing days.

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 per the baseline this sits at 4. The description correctly adds no parameter discussion and instead spends its words on source and output semantics, which is the right allocation for a parameterless endpoint.

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 pair — 'Official PLN/TRY reference rate ... latest European Central Bank fixing' — plus the fallback case (peg-derived) and the metadata returned. An agent knows exactly what data this yields. It does not, however, distinguish itself from the very similar siblings in the fleet (fx_try_pln for the inverse pair, fx_latest, or fx_convert), which is the one thing missing for 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 only implied: 'reference rate,' 'official,' 'deterministic, no estimate' suggest it is the right choice for compliance/accounting figures rather than for ad-hoc conversion (fx_convert) or history (fx_timeseries). There is no explicit when-to-use or when-not statement and no sibling is named as an alternative.

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