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

Beyond the annotations (which only say it is a non-read-only, open-world, non-destructive write), the description discloses the crucial behavioral facts: the first call is free, charging only occurs on the second call with `payment`, and idempotency context matters for retries. It also discloses the non-obvious delivery semantics — PM lands on the wallet bound to this key, not the paying address — and the value constraint that PM is non-redeemable and has no cash value.

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 two-step flow is front-loaded and the value caveats come at the end, which is the right order. It wastes some space repeating "nothing is charged" twice in one sentence and packs several independent facts into a single em-dash chain, but no sentence is expendable.

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?

An output schema exists, so return-shape explanation is rightly omitted, and the description covers the charge model, delivery destination, and redemption limits that an agent needs before spending. The one gap is that `usd` must match a "listed purchase bundle" without saying where those bundles are listed or what the allowed values are, which matters because there are no enums on the parameter.

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%, so all three parameters are already documented in the schema, including the `payment`-omitted first call. The description reinforces the token/quote semantics of `usd` (registry quote, same rate as the card rail) but adds no syntax, format, or bundle-list detail beyond what the schema states. 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?

"Buy PM (Kellian play money) with USDC on Base" states a specific verb, resource, and settlement rail, which no sibling tool covers (the nearest, problee_plan_pm_order, is planning rather than purchase). An agent can distinguish this from balance, plan, and trade tools without opening any 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?

The description gives explicit procedural guidance: call once with no `payment` to receive the x402 requirements, then sign and call again with the base64 payload. That removes the main sequencing ambiguity for a two-phase tool. It stops short of naming alternatives (e.g., problee_plan_pm_order, or the card rail it references) as when-to-use-this-vs-that routing.

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