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

Annotations already declare read-only, idempotent, non-destructive behavior, so safety is covered. The description adds genuinely non-inferable context: the data source (ECB) and, importantly, that each call costs $0.05 USDC via x402 on Base or Solana — a payment/auth requirement the agent must know before calling.

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, front-loaded with the purpose, then source, then cost. No filler, nothing wasted.

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?

For a read-only, two-enum-param tool, the essential calling information is present and the pricing/source context is useful. But with no output schema, the description says nothing about the return shape (array of date/rate points, row count implied by 'days'), and it omits any routing guidance against the other FX siblings.

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% with enumerated values for both 'pair' and 'days', so the schema fully documents the inputs. The description adds only the 'daily' granularity hint, which is marginal; baseline 3 applies when the schema does the heavy lifting.

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 ('Returns'), resource ('historical daily exchange rates') and scope ('G10 currency pair'), which distinguishes it reasonably from get_fx_rate_current. However, it never explicitly contrasts with the close sibling get_fx_rate_by_date, leaving the series-vs-single-date boundary implicit.

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?

No when-to-use guidance at all: nothing says to prefer this over get_fx_rate_by_date or get_fx_rate_current, nor when the 30/90/365 windows are appropriate. The agent must infer usage purely from the tool name.

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