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

With no annotations supplied, the description carries the full burden and does disclose the most consequential behavior: this is a paid call ($0.002 USDC via x402 on eip155:8453), so an agent knows a payment handshake is required before invocation. It also declares provenance (ECB official fixing) and the no-estimate guarantee. It omits update cadence (daily fixing only), staleness/weekend behavior, and any error or rate-limit semantics.

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 dense sentence with the payment/network metadata bracketed up front and the substantive claim following, so nothing is buried. It is slightly overpacked — rate date, method, and attribution are listed in one breath — but every clause adds information an agent would otherwise lack.

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 parameters and no output schema, the description must convey what comes back, and it does: the rate, the rate date, the derivation method (direct, cross via EUR, or peg-derived) and source attribution. That is close to complete for a single-value lookup; only freshness/cadence details are missing.

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 nothing for the description to disambiguate and the baseline of 4 applies. The pair is encoded entirely in the tool name, so no input semantics are needed.

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/PLN reference rate) with its source (ECB fixing) and the exact variant (latest fixing, or peg-derived when pegged). In a family of hundreds of sibling fx_<base>_<quote> tools, the currency pair in the name plus the description's 'ECB fixing' qualifier makes selection unambiguous.

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?

The description implies when to use it (you want an official, sourced spot reference rate rather than an estimate), but it never names alternatives such as fx_convert, fx_latest or fx_timeseries, nor does it state when NOT to use this pair-specific tool. Usage is inferable from the pair name alone, which is the minimum viable level.

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