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

Beyond the annotations (readOnlyHint=false, destructiveHint=false), the description discloses key behaviors: it returns a checkoutUrl for human approval, errors on existing subscriptions, and requires billing-scoped tokens. This adds meaningful context about the flow and prerequisites without contradicting annotations.

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 three sentences, front-loaded with the main action. Every sentence adds value: purpose, output, error condition, alternative, and access requirements. No wasted words.

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 simple schema (1 enum param, no output schema), the description is complete. It explains what the tool does, what it returns, when it fails, the alternative, and authorization requirements. Nothing critical 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 coverage is 0% for the single 'plan' parameter. The description compensates by naming the valid values ('Pro or Scale') and mapping them to the subscription tier. It doesn't explicitly state that the parameter is 'plan' or that it is the only parameter, but the schema already provides the enum and required field, so the added semantics are sufficient.

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 uses a specific verb ('Begin') and resource ('Pro or Scale subscription'), clearly stating the tool's function. It distinguishes from sibling tools by explicitly mentioning change_plan as an alternative when the owner is already on a paid plan.

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 explicit when-to-use guidance ('Begin a new Pro or Scale subscription'), when-not-to-use ('Errors if the owner is already on an active paid plan'), and the alternative tool ('use change_plan instead'). It also states prerequisites (Owner-only, requires a token with billing scope), giving clear context.

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