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

With readOnlyHint=false and openWorldHint=true already declared, the description adds genuinely new context: no charge is made, no artefact is produced internally, a human completes the payment on the card rail, and downstream work only fires once Stripe records the session as paid. This resolves the apparent tension between 'opens a session' and 'charges nothing'. It stops short of stating that each call creates a new non-idempotent session.

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?

Four short sentences, purpose front-loaded, no repetition. The phrasing 'the thing that taps is a human' is rhetorical but does encode the meaningful fact that no automated charge occurs, so it earns its place, if narrowly.

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-parameter tool with no output schema, the description covers what is returned (payment link, session id), what is not done (no charge, no artefact), and the external dependency (Stripe's paid record gates the redeem path). Only the non-idempotency of repeated calls is left to annotations.

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?

Only one parameter, and schema coverage is 100%, with the sku description already pointing to list_card_packages as the source and giving an example. The tool description adds nothing about sku format or lookup, so the schema carries the load and 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 first sentence gives a specific verb and resource: 'Opens a Stripe Checkout Session for one card package and returns its payment link and its session id.' An agent immediately knows this is a payment-link generator, not a package executor. It does not name a sibling tool for contrast, which keeps it out of 5 territory.

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?

'This tool charges nothing and produces no artefact' plus 'The package's work runs only at its redeem path' implicitly tells the agent when not to expect results here, but no alternative tool (e.g. a redeem/verify sibling) is named and no precondition such as 'call list_card_packages first' appears in the description. Usage is inferable but not explicit.

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