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

With no annotations, the description must carry the full behavioral burden. It says the data is 'historical' and 'reference' rates, which gives some context, but it omits whether the operation is read-only (though implied), authentication needs, rate limits, data source, timezone, or return format. For a no-annotation tool this is a significant 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?

Two tight sentences with no wasted words. The core purpose is front-loaded, and the usage examples follow naturally.

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 simple retrieval tool with full parameter documentation and no output schema, the description is nearly complete. It could better mention the sibling alternative or return shape, but it gives enough context to invoke correctly.

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%, so the schema already documents every parameter. The description adds only that 'from'/'to' form a currency pair and that 'days' versus 'date' represent alternative retrieval modes, which is useful but not beyond the schema's baseline.

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The description states a specific verb ('Get'), resource ('historical reference rates for a currency pair'), and scope ('for a number of days, or a single date'). It implicitly distinguishes itself from the sibling get_latest_rate by saying 'historical' rather than 'latest', though it does not name the sibling.

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?

It gives clear usage context ('Use for invoices, reports and reconciliation') but does not state when not to use it or explicitly direct the agent to get_latest_rate for current rates. This is clear context without exclusions.

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