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

Annotations already declare readOnlyHint=true and openWorldHint=true, so the safety profile is covered. The description adds behavioral context by specifying the data fields (key, rate type, frequency, coverage dates) and the relationship to the blend, which goes beyond the annotations. It doesn't mention pagination or rate limits, but for a zero-parameter list operation this is sufficient.

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 sentences with no filler. The first sentence states the resource and output fields; the second provides actionable usage guidance. Every sentence earns its place and the most important information is front-loaded.

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 zero-parameter, read-only list tool with no output schema, the description is nearly complete. It explains what is returned and how to use the results. The only minor gap is that it doesn't explicitly state the output format (e.g., JSON array), but the absence of an output schema and the simplicity of the tool make this a minor omission.

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 has zero parameters, so the schema provides no parameter documentation. The description compensates by explaining what the returned 'key' field is for and how it should be used as a parameter in sibling tools. This adds meaning beyond the empty schema, though it doesn't need to document any input parameters.

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 verb ('List') and resource ('institutions whose rates Frankfurter relays'), and enumerates the exact fields returned (key, rate type, frequency, coverage dates). It also distinguishes itself from siblings by explaining how the key is used in convert/get_rates, so an agent can tell it apart from list_currencies.

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?

The description explicitly tells the agent when to use this tool: to obtain a provider key for use as the `provider` parameter in `convert` or `get_rates`. It also implies the alternative (using the default blend) and clarifies the purpose of the returned keys, which is strong usage guidance.

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