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

Annotations already declare readOnlyHint=true and idempotentHint=true, but the description adds valuable behavioral context by stating that the rate is supplied by the caller and that no rate lookup is performed. This goes beyond the annotations to clarify the tool's pure calculation nature.

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?

The description is two concise sentences with no redundant wording. It is front-loaded with the action ('Convert an amount') and efficiently communicates the key scope constraint ('No rate is ever looked up').

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?

The tool is simple with only two parameters, both fully documented in the schema, and an output schema is present. The description, combined with annotations and schema, provides complete context for correct usage. No additional details like rounding or edge cases are necessary for this straightforward calculation 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 schema already provides 100% parameter description coverage, so the baseline is 3. The description restates the rate unit meaning ('units of target currency per unit of source') which is already in the schema's rate description, adding no new semantic information beyond what the schema provides.

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 clearly states the tool converts an amount using a caller-supplied rate, which is a specific verb+resource. It distinguishes itself from siblings by emphasizing that no rate lookup occurs, which is unique among the financial calculation tools.

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 clearly implies when to use this tool: when you have a rate and want to convert. The phrase 'No rate is ever looked up' provides an exclusionary guideline, signaling that this tool is not for rate retrieval. However, it does not explicitly name alternatives or provide explicit when-not-to-use conditions.

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