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

Annotations already declare readOnlyHint=false, idempotentHint=false and destructiveHint=false, so the safety profile is covered. The description's added value is the unpaid-registration precondition, but it says nothing about invalid/expired codes, whether an existing discount is replaced or stacked (relevant given idempotentHint=false), or whether fees are recalculated.

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?

A single sentence with no wasted words, and the operative verb plus the precondition are front-loaded. Structure is efficient, though the pivotal detail (which business/registration the business_id refers to) is absent rather than merely compressed.

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 mutation tool with no output schema and 0% schema description coverage, the definition covers purpose and one precondition but leaves failure behavior and return expectations unexplained. It is adequate to attempt a call, not complete enough to predict the outcome.

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 0% for both parameters, so the description must carry the burden. It clarifies that the code is user-supplied rather than generated, but never mentions business_id, and its use of "registration" instead of the actual parameter name adds mapping ambiguity rather than removing it.

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?

States a specific verb+resource ("Apply a discount code") and adds the target scope ("a registration that isn't paid yet"), which distinguishes it from remove_discount_code. The only weakness is that it says "registration" while the tool actually keys on a business_id, leaving a small ambiguity about what is being modified.

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?

Gives a real trigger condition ("the user gives you") plus a precondition ("registration that isn't paid yet"), which is enough for an agent to decide when to call it. It does not name alternatives such as remove_discount_code or explain what to do if the registration is already paid, so it stops short of full when/when-not guidance.

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