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

Annotations already mark this readOnly/idempotent/non-destructive, and the description reinforces that with explicit negative guarantees (does not charge, grant Pro, or return a secret) plus failure modes (mpp_not_configured, invalid_sku) and their consequences. It also disambiguates this MCP call from the REST POST that actually charges/grants, which is rich context beyond the 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 action is front-loaded and every sentence carries information, but the extended passage on the REST /api/billing/machine-pay charge/grant semantics is dense and goes beyond what is needed to select one tool. It is efficient but not maximally tight.

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?

Despite having no output schema, the description enumerates the return fields (sku, amount, currency, periodDays, mppConfigured, action, agentSkus, instructions) and documents auth requirements and both error paths. An agent has everything needed to invoke it 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?

Schema description coverage is 100% for the single sku parameter and already documents the pro_prepaid_30d default and that it is the only accepted SKU, which the description largely restates. The description adds only the invalid_sku error outcome, a behavioral rather than semantic addition, 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 verb+resource ('Return prepaid Pro purchase instructions') and immediately scopes what the tool does NOT do (no charge, no grant, no secret). It then contrasts sharply with the REST POST endpoint and with billing_checkout, so an agent can place it against siblings without opening a 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 states the required auth ('a company API key (aa_...) or an MCP OAuth access token; a bootstrap token is not accepted on this call') and names the alternative explicitly ('Use billing_checkout for a human Stripe Checkout session'). When-to-use and when-not-to-use are both covered.

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