historical_rate

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

With no annotations, the description carries the full burden and does substantial work: it discloses the working-day publication rule, that weekends/holidays return the most recent working day rather than an error, the presence of a `note` field explaining substitutions, and the per-currency feed lag case. It omits error/auth/rate-limit behavior, but for a public read-only ECB feed that gap is minor.

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?

Front-loaded with the core purpose, then the behavioral nuance. Two sentences, no filler. Slightly dense in the second sentence with multiple clauses, but each 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?

An output schema exists, so return values need not be spelled out, and the description complements it by explaining the non-obvious substitution semantics that would otherwise be surprising. The only real gap is the absence of explicit sibling routing guidance.

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% and all three parameters (base, date, symbols) are documented there with defaults and ISO 4217 formats, so baseline 3 applies. The description adds no parameter-level detail beyond the schema, though it does clarify output semantics tied to the date parameter.

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 first sentence states a specific verb+resource: 'ECB reference rates for one base currency on a specific date.' An agent can distinguish this from latest_rates and rate_timeseries on the date-scoping axis, though no sibling is named 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?

Usage is implied rather than stated — the historical/dated nature of the lookup points at this tool over latest_rates, and the weekend-substitution rule tells the agent what happens at the edges. But it never explicitly says when to prefer this over rate_timeseries or convert, leaving routing to inference.

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