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

Annotations already declare readOnlyHint=false and openWorldHint=true, and the description adds the non-obvious behavior: the call itself is free, it is a human-in-the-loop handoff, and credits only arrive after payment so the caller must retry. It does not say what happens on repeated calls (non-idempotent, could double-charge), which is the one meaningful gap for a paid flow.

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?

Three short sentences, front-loaded with the cheapest fact ('Free') and then the value, the human handoff, and the alternative. No filler or repetition of the 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?

With no parameters and no output schema, the description covers everything an agent needs: cost, what is returned (a checkout URL), who must act on it, when credits appear, and which sibling to use for other purchase sizes.

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?

The tool takes zero parameters, so there is nothing for the description to disambiguate; the fixed 500-credit/$5 quantity is stated instead. Baseline 4 applies for a parameterless tool.

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?

States a specific verb+resource ('Returns a checkout URL that buys 500 credits for $5 for this exact key'), including the exact price and scope. It explicitly names the sibling create_checkout as the tool for bigger packs, so the agent can route without opening either 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?

Gives the preconditions ('works on self-serve keys, no sign-in'), the handoff step ('Show the URL to your human'), the post-payment retry instruction, and the exclusion ('For bigger packs or plans use create_checkout after the key is claimed'). When-to-use and when-not-to-use are both explicit.

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