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

Annotations only convey readOnlyHint=false and destructiveHint=false, so the description carries the full burden of behavioral disclosure. It thoroughly covers the immediate charge, 30-day auto-renewal, cancellation semantics, per-project RPM behavior, and the meaning of chargeStatus. This goes well beyond what the annotations provide.

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 dense but every sentence carries necessary information: billing, renewal, prerequisites, return shape, field semantics, and authorization. It is slightly long but not wasteful, and the core purpose is front-loaded.

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?

With a single parameter and no output schema, the description fully compensates by documenting the returned object, explaining transactionId and chargeStatus edge cases, and specifying access requirements. Nothing needed to call the tool correctly is missing.

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% and the addonKey parameter is already described as a 'Stable key from list_addons' with examples. The description reinforces the source of the key and mentions rpm_* per-project behavior, but does not fundamentally add new parameter-level meaning beyond the schema, so baseline 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 opens with a specific verb and resource: 'Subscribe the project owner to a monthly add-on', followed by concrete examples of add-ons. This clearly identifies the tool's function and distinguishes it from sibling operations like unsubscribe_addon, downgrade_addon, or list_addons.

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 states when the tool is appropriate: requires an active Pro/Scale subscription, a saved payment method, owner-only access, and a billing-scoped token. It also explains billing behavior and warns not to retry when the charge is pending. It does not explicitly reference alternatives, so it falls just short of a 5.

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