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

With no annotations, the description carries the load well: it discloses the paid x402 cost per call, the data source, the publication timing (~16:00 CET after the 14:10 concertation), and hard limits (20 currencies, 1-30 business days). It does not cover error behavior or whether non-business days are skipped, 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?

A single dense sentence with the pricing signal front-loaded and no filler. Every clause (cost, source, timing, limits) earns its place, though the parenthetical timing detail adds some reading weight.

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?

Given no annotations and no output schema, the description covers source, freshness, cost, and input bounds, but says nothing about the shape of the returned data or how rates are keyed by date/currency, leaving the agent to guess the response format.

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?

Schema coverage is only 33% (only access_token is documented inline), so the description must compensate — and it does, supplying the valid range for days (1-30 business days) and the cap on currencies (20), while the schema covers access_token. Remaining gap is the expected currency code format (ISO 4217?).

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: official ECB euro reference rates from the ECB Data Portal, with a concrete scope of up to 20 currencies over 1-30 business days. It is distinguishable from generic siblings like fx_rate or convert, though it never names those alternatives explicitly.

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 pricing note ('PRO ($0.01/call via x402)') and the 'official ECB' framing imply when to reach for this tool, but there is no explicit when-to-use or when-not guidance and no mention of how it differs from fx_rate, fx_table, or global_fx_rate.

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