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

With no annotations at all, the description carries the full burden and does a lot: no account or human required, bitcoin-only, unique-amount invoice, exact-amount payment requirement, and an asynchronous poll-until-paid state machine with a claim URL. Gaps remain around invoice expiry, what happens on under/overpayment, and failure/refund handling.

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 purpose and the invoice/poll workflow are front-loaded in two dense sentences with no filler. The trailing 'Launch pricing is live: 50% off every plan' is promotional but at least budget-relevant; everything else earns its place.

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 payment-initiating mutation with no annotations and no output schema, the description does well on the external flow but leaves the call itself underspecified: neither input parameter is explained and it never states what the tool actually returns (an invoice token or URL to poll). An agent could start the call but not interpret the response without guessing.

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 0% and both parameters are enums with no documented meaning. The description never mentions 'plan' or 'cycle', does not explain builder vs scale or month vs year, and does not state that 'cycle' is optional or what its default is — so an agent gets no help beyond the bare enum values.

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 concrete verb and resource ('Buy a FactStable API key') and scopes the payment method ('with bitcoin'). It is clearly distinguishable from the fact-lookup siblings, though the related 'pricing' sibling is never referenced to draw the boundary.

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 spells out the post-purchase workflow (invoice issued, pay exact amount, poll the given URL until state=paid, collect key at claim URL), which is useful operational guidance. However it never says when an agent should call this versus checking 'pricing' first, nor any precondition such as confirming the user actually wants to spend money.

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