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

Annotations already declare the safe read-only, idempotent, open-world profile, and the description adds genuinely non-structured behavior: one ECB rate per business day (data granularity), free source at ecb.europa.eu, cost of 0.005 USDC per call, and p95 latency of 1500 ms. That is exactly the kind of pricing, freshness and performance context an agent cannot get from annotations.

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 usage trigger, then scope, then cost/latency; nearly every clause carries information. The trailing 'Category: Data.' is low-value taxonomy padding that slightly dilutes an otherwise tight block.

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 an output schema present it correctly does not need to explain return shape, and it covers limits, cost, latency and data granularity. It omits edge behavior an agent might need (unknown/unsupported ISO codes, holiday gaps, behavior when startDate and days conflict), so it is strong but not exhaustive.

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 100%, so the baseline is 3, but the description adds real semantics beyond it: rebasing to any base is computed as cross rates (the schema only says 'Base currency, ISO 4217'), and the 90-day cap is framed as the hard product limit rather than just a maximum. It does not clarify startDate vs days precedence, which the schema handles instead.

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 verb and resource plus scope: '90 days of daily ECB euro reference rates ... for up to 10 currencies, rebased to any base as cross rates we calculate, with min, max and percent change.' An agent can distinguish this from blackwood_fx_rates and other data tools without opening the schema.

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?

Explicit trigger ('Use when you need historical exchange rates, forex history or the recent trend of a currency') and explicit exclusions ('Do not use for rates older than 90 days, live quotes or crypto'). It stops short of naming the alternative sibling (blackwood_fx_rates) for live quotes, so routing is implied rather than stated.

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