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

The description goes meaningfully beyond the annotations by disclosing a two-phase flow: the call returns a preview plus a confirmation link, and only the merchant on that page can finalize it. That is exactly the kind of side-effect context an agent needs and the annotations cannot convey. There is mild tension with readOnlyHint=false (the text says 'changes nothing'), but the described preview/confirmation mechanism is what reconciles it, not an outright contradiction.

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?

Short and front-loaded: the purpose is the first clause, and the confirmation-flow caveat follows immediately. One sentence ('Money is integer minor units (cents)') has no bearing on a category-scoping tool and is stray boilerplate, which slightly weakens the otherwise tight structure.

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 3-parameter tool with full schema coverage and an output-schema-free surface, the description covers the essentials: what it scopes, that it previews rather than commits, who confirms. It omits what happens on invalid category names or when onlyCategories and exceptCategories are combined, but the schema description partially covers the latter, so the gap is minor rather than disqualifying.

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%, including the important rule that leaving both category lists out or empty resets the code to apply everywhere, so the schema already carries parameter meaning. The description adds nothing about the parameters themselves, and its only supplemental note ('Money is integer minor units') applies to no parameter on this tool, so it is baseline rather than value-add.

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 opens with a concrete verb and resource: 'Choose which categories a coupon code applies to.' That is specific enough that an agent can distinguish it from sibling tools like discounts_coupon (the coupon itself) or discounts_catalog_sale. It stops short of explicitly naming a sibling for comparison, so it lands at 4 rather than 5.

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?

Usage is implied rather than stated: the agent learns this concerns scoping an existing code to categories, and that the change requires merchant confirmation. There is no explicit 'use this when...' or 'use X instead when...' guidance relative to the many sibling discount tools (discounts_coupon, discounts_catalog_sale, promotions_edit).

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