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

The description goes well beyond the basic annotations (readOnlyHint=false, destructiveHint=false) by detailing side effects: immediate charging, auto-renewal every 30 days, cancellation behavior via unsubscribe_addon or subscription end, and the grace period for paid cycles. It also discloses prerequisites and permission scoping, providing thorough transparency for a billing-related mutation.

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 efficient, using six concise sentences to cover purpose, billing behavior, renewal, cancellation, prerequisites, RPM specifics, return format, and access control. There is no redundant phrasing, and the most important information is front-loaded in the first sentence, making it easy to parse quickly.

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?

Given the complexity of a billing tool with recurring charges and add-on management, the description is remarkably complete. It covers key aspects: what it does, payment and renewal mechanics, termination conditions, RPM project scoping, expected return fields (compensating for no output schema), and security requirements. It even addresses edge cases like what happens after the parent subscription ends. This level of detail is sufficient for an agent to select and invoke the tool correctly.

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?

The schema already fully documents the single parameter `addonKey` with examples (100% coverage), so the baseline is 3. The description adds meaningful context by explaining that `rpm_*` add-ons are per-project and apply automatically to the current project, which is not captured in the schema. This extra behavioral nuance justifies a 4.

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 clearly states the tool's function: 'Subscribe the project owner to a monthly add-on' with concrete examples. It distinguishes itself from sibling tools like unsubscribe_addon, downgrade_addon, and list_addons by focusing on the activation action for add-ons, and it explicitly mentions the requirement of an active Pro/Scale subscription, setting it apart from broader subscription tools.

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 provides clear context for usage, including prerequisites ('Requires an active Pro/Scale subscription and a saved PM'), permission requirements (owner-only, billing scope), and behavioral notes like per-project RPM application. However, it does not explicitly state when not to use this tool or directly compare to alternatives such as list_addons or downgrade_addon, so it stops short of full alternative guidance.

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