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

Although annotations already mark destructiveHint: true, the description adds substantial behavioral detail: the free and immediate effect during trial, prorated charging on the saved payment method for active subscriptions, the confirm requirement, and the one-click Stripe confirmation link. This goes far beyond the generic annotation and gives the agent the necessary caution.

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 compact, front-loads the core action, and packs conditional behavior, confirmation flow, and use cases into about four sentences. It is slightly longer than strictly necessary but every sentence carries operational information, so no filler.

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 mutating billing tool with no output schema, the description covers the essential operational aspects: trial vs. subscription behavior, the confirm gate, plan/interval changes, and typical use cases. It does not explicitly describe the result of the confirm=true call or error conditions (e.g., missing payment method), but it is sufficient for safe invocation.

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 each parameter is already well documented, including confirm's behavior and interval's annual discount. The description restates the confirm workflow in narrative form but does not introduce new parameter semantics beyond the schema, so the 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 action and resource: 'Change the user's plan (Personal, Personal Plus, Team) or billing interval.' It then lists concrete use cases ('Use when a second account of a provider needs Personal Plus...') that distinguish it from sibling tools like account_start_subscription and team_add_seats, even though it does not name them explicitly.

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 explicitly states when to use the tool ('Use when a second account of a provider needs Personal Plus, when the user wants Team for colleagues, or asks to switch monthly/annual') and prescribes a two-step invocation sequence (call without confirm, then with confirm after approval). It lacks explicit when-not-to-use guidance or named alternatives, so it stops 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.