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

Adds value beyond the annotations by disclosing the return payload (checkout_url, url_type) and the non-obvious branching where a domain_transfer product returns url_type "domain-transfer" plus a transfer-flow link because an authorization code is needed. It is consistent with readOnlyHint=true and destructiveHint=false, but the trailing "Through Royally… runs straight away" sentence is tooling noise rather than behavioral disclosure, and auth/account requirements are unstated.

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 numbered list of three modes is well front-loaded and scannable, and the transfer caveat is placed before the mode enumeration where it matters. A few clauses duplicate the schema ("up to 10 items", quantity wording) and the final "Royally" sentence adds confusion rather than information.

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?

There is no output schema, and the description compensates by naming the returned fields and the alternate return shape for transfers. Combined with full mode routing and the slug convention, an agent has everything needed to pick a mode and build a correct call.

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 coverage is 100%, so the baseline is 3, and the description earns an increment by adding the plan-slug convention not present in the schema: Premium's slug is "value_bundle" with an underscore while all other plan slugs use hyphens. It also restates the mode exclusivity and the max-10 cart size, which are already in the schema, so it stops short of a 5.

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 and resource ("Generates a WordPress.com checkout link") and enumerates the exact domains it covers: plan purchases, domain registrations, subscription renewals. It also distinguishes the domain_transfer special case and points to the wpcom/plans-list sibling, so an agent can separate it from wpcom-domain-purchase without opening a schema.

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?

Explicitly says "provide exactly one" of three modes and describes the condition selecting each: products for a new cart, subscription_id when the ID is known, renewal_product + wpcom_site when only product and site are known. It also names an alternative source of truth ("prefer those returned by wpcom/plans-list"), which is exactly the when-to-use guidance an agent needs.

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