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

Annotations only flag destructiveHint=true and openWorldHint=true; the description goes far beyond by disclosing that a card is entered via an external Stripe Checkout url, that the first payment is deferred by a 7-day trial, that upgrades charge a prorated difference immediately, and that downgrades defer to period end. That is exactly the financial consequence an agent needs before invoking a destructive billing tool.

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?

Front-loaded with the primary action, then branch behaviors, then pricing context; no filler sentences. The final sentence on features and overage is slightly tangential to the call mechanics but still helps the agent explain the outcome to a user.

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 three-parameter destructive mutation with no output schema, the description covers return values (Checkout url vs. billing summary), charge timing, trial, and proration. It omits permission/ownership requirements and what happens on failure, which are the remaining gaps for an open-world billing tool.

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% with enums on all three parameters, so the schema already documents interval, planSlug, and returnTo including omission semantics. The description adds business meaning to the plan choice (every plan has every feature; a paid plan adds monthly AI replies with overage) but adds nothing about parameter formats beyond the schema, so baseline 3 applies.

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 names a specific verb and resource ('Start a paid plan, or switch the plan or billing interval') and immediately distinguishes the two modes (first-time checkout vs. plan change). An agent can tell this apart from update_workspace or set_inbox_ai without opening any 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?

It clearly states the two conditions that select different behavior: no paid plan yet yields a Checkout url, while an existing plan applies the change immediately or at period end. It does not name sibling alternatives or exclusions, but the when-to-use context is unambiguous.

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