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

With no annotations, the description carries the full burden and does well: it discloses the payment model ($0.002 USDC per call, x402, eip155:8453) so the agent knows a paid settlement step is required, and it explains that the rate may be a direct fixing, a cross via EUR, or peg-derived, which affects interpretation. It omits rate limits, failure behavior, and whether non-trading days return a stale date.

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?

Two compact sentences: a bracketed payment/transport prefix followed by the substantive description, with the resource named first. The payment prefix is dense but earns its place because it changes how the agent must invoke the tool. No filler or repetition.

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?

There is no output schema, so the description must preview the return payload, and it does name the key fields: the rate, the rate date, the derivation method, and source attribution. For a zero-parameter lookup tool this is close to complete, with only edge-case behavior (weekends/holidays, stale dates) left unstated.

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 there is nothing for the description to disambiguate and the baseline of 4 applies. The pair and source are fixed by the tool identity itself, so no parameter guidance 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 states a specific resource and verb-equivalent: 'Official HUF/EUR reference rate: latest European Central Bank fixing'. An agent can tell exactly which pair and which source it returns. It does not, however, differentiate itself from siblings like fx_convert, fx_latest or fx_timeseries, which cover overlapping needs.

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 agent can infer this is the tool for the HUF/EUR spot reference, and 'no estimate' hints it is unsuitable for forecasts. But there is no explicit when-to-use versus fx_convert (conversion) or fx_timeseries (history), and no mention of whether to prefer a cross rate via fx_eur_huf.

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