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

Annotations already declare the tool as read-only, idempotent, non-destructive, and closed-world. The description adds useful behavioral context beyond that: it discloses the calculation basis (uses the MDR % the user gives) and states that it does not look up provider rates, which is meaningful for an agent expecting external data retrieval.

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 tightly written sentences with no wasted words. The purpose and outputs are front-loaded, and the rate-lookup caveat follows 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?

Despite having no output schema, the description lists the return values (MDR fee, GST, total cost, effective rate, net settlement), which gives the agent enough to understand the response. The input basis is also clarified, so there are no significant gaps for this 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?

Schema description coverage is 100%, so the schema already documents all six parameters in detail. The description does not add syntax or constraints beyond noting that the MDR percentage is user-provided, so the baseline of 3 is appropriate.

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 (calculates) and resource (MDR charges on card or payment gateway sales), then enumerates the exact outputs: MDR fee, GST on that fee, total cost, effective rate, and net settlement. This is far more specific than a generic calculator and clearly distinguishes the tool from unrelated siblings such as roas_check or break_even.

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 implies usage by naming the domain (card or payment gateway sales) and clarifies that the MDR percentage is user-supplied rather than looked up. However, it gives no explicit when-to-use guidance, no alternatives, and no exclusions beyond the rate lookup caveat.

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