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

With no annotations, the description carries the full load and does well on several axes: it discloses a paid call ($0.002 USDC, x402, eip155:8453), guarantees a sourced deterministic value with the rate date and method, and states 'no estimate'. What it omits is freshness behavior (ECB publishes on business days; what happens on weekends/holidays) and failure modes for a paid call, which is the main behavioral gap.

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 pair and the pricing tag, then the substance in one sentence. Parentheticals (peg, cross methods) are dense but each carries real information; nothing is redundant.

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 zero params and no output schema, the description compensates well by naming the returned fields (rate date, method, source attribution) and the determinism guarantee. It falls short only on temporal behavior — publication cadence and what a caller gets outside ECB fixing days — which matters for a rate tool.

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?

The tool takes zero parameters, so the baseline is 4. The description correctly implies no input is needed and instead spends its words on what the response carries (rate, date, method, attribution).

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 and scope: the DKK/BGN official reference rate, sourced from the ECB fixing with peg fallback, plus the method (direct, cross via EUR, peg-derived). The currency pair in the text matches the tool name and separates it from the hundreds of other fx_* pair siblings, though it never explicitly says 'this pair only' or routes to siblings like fx_convert or fx_timeseries.

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 by the narrow pair-specific purpose, and the text notes the paid x402 pricing model, but there is no explicit when-to-use, when-not, or alternative routing (e.g., use fx_convert for arbitrary conversion, fx_timeseries for history). An agent must infer selection entirely 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.