Rate history for a pair

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

With no annotations, the description carries the behavioral disclosure burden. It adds useful behavior beyond the schema: 'A window wider than the free 90 days is shortened, not refused.' It also discloses that rows appear per published business day and that summary statistics cover the window. This is meaningful transparency for a read-only historical query.

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?

Three short sentences, zero filler. The purpose is front-loaded, the return summary is compressed, and the key free-tier caveat is placed last without redundancy. Every sentence earns its place.

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?

The main decision-relevant context is present: what the tool returns, the pair/window scope, and the free 90-day shortening behavior. There is no output schema, so the short output description is valuable, though it could add sorting or missing-data behavior. Still, for a 6-parameter query tool with rich schema, this is near-complete.

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 description coverage is 100%, so the baseline is 3. The description does not independently define parameter details, but the schema already documents base/quote currency, date filtering, days, and max_rows. No compensation is 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?

The description names a specific verb and resource: 'the ECB rate of one currency pair across a window.' It also defines the output shape (one row per published day plus min, max, average, change), which clearly separates it from siblings like rates_latest, rate_on, or fx_rates_for.

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?

It gives a direct call instruction ('Call this tool for...') and a clear context of use: a currency pair over a time window. It does not explicitly name alternatives or exclusion conditions, so it falls short of a full when/when-not breakdown, but the intended use is clear.

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