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

Goes well beyond the annotations (readOnlyHint=false, openWorldHint=true, idempotentHint=false, destructiveHint=false) by disclosing the delegation model: the link must be shown to the person, the payer chooses quantity and pays in-browser, and the agent never handles payment details. It also discloses asynchronous fulfillment — credits appear only once Stripe confirms.

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 action and resource, then conditions, then exclusions. Every sentence carries distinct information — trigger, handing model, fulfillment timing, alternative, exclusions — with no restatement of the name or title.

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 single-optional-parameter tool with an output schema present, the description covers the decision context, the human-in-the-loop flow, and the confirmation timing. Nothing an agent needs to select and correctly invoke it 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 100% and the single optional 'credits' parameter is already documented, including that Stripe's page lets the payer change the quantity. The description adds only the £10 unit price, which is pricing context rather than parameter meaning, so the 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?

Specific verb and resource: 'Get a Stripe Checkout link to buy Developer credits (£10 each)'. It names the exact trigger condition (402 insufficient_allowance) and explicitly excludes two adjacent cases (Core auto-billing, plan subscription), so an agent can separate it from billing/account tools without opening the 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?

States when to use it (free trial or Developer account runs out, signalled by a 402 with insufficient_allowance) and when not to (not for Core, whose extra credits bill automatically, not for subscribing to a plan). It also names an alternative path — the person adding a card on the billing page for monthly usage billing.

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