Kellian

Buy play money with USDC on Base (x402)

kellian_buy_play_money

Buy PM (Kellian play money) with USDC on Base. Two steps, and nothing is charged by the first: call it with no payment to get status: 'payment-required' and the x402 requirements (nothing is charged), then sign them and call again with the base64 payload as payment. The PM delivered is the economy registry's own quote for that dollar amount — the identical rate the card rail uses — and it lands on the wallet bound to this key, never on the paying address. PM has no cash value and cannot be redeemed or withdrawn.

Tool call template

Replace each placeholder with a value matching the input schema. Only required parameters are included; add optional parameters as needed.

{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "arguments": {
      "usd": "<usd>",
      "idempotencyKey": "<idempotencyKey>"
    },
    "name": "kellian_buy_play_money"
  }
}

Input Schema

Name	Required	Description
`usd`	Yes	US dollars to spend. Must equal one of the listed purchase bundles.
`payment`	No	Base64 x402 payment payload — the value an HTTP caller sends as the X-PAYMENT header. Omit it on the first call to receive the requirements to sign.
`idempotencyKey`	Yes	Stable retry key. Reuse only with byte-equivalent tool arguments.

Output Schema

Name	Required	Description	Default
`result`	Yes

Schema Changelog

Changes observed during successful MCP inspections.

AddedNov 5, 2026

TDQS

A4.5/5.0

Behavior5/5

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

Goes well beyond the annotations (readOnlyHint=false, destructiveHint=false, openWorldHint=true): it discloses that the first call is free, where the price comes from (economy registry quote, same as card rail), that delivery goes to the wallet bound to this key rather than the paying address, and that PM is non-redeemable. Those are exactly the facts an agent needs before spending real USDC.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness4/5

Is the description appropriately sized, front-loaded, and free of redundancy?

Four sentences, front-loaded with the core action and the two-step mechanic. The "nothing is charged" caveat is stated twice, which is mild redundancy but arguably deliberate emphasis for a spend operation.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness4/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

For a spend tool with an output schema present, it covers cost, flow, price source, delivery destination, and value limitations. Minor gaps remain: it does not say whether `idempotencyKey` is expected on the first payment-required call or how long requirements stay valid before a re-quote.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters4/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema coverage is 100%, so the baseline is 3; the description adds genuine meaning on top by explaining that `payment` is omitted on the first call and carries a signed base64 payload on the second, and by framing `usd` as a dollar amount whose PM yield is quoted, not fixed. The bundle constraint on `usd` is left to the schema.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

Opens with a specific verb, resource, and settlement rail: "Buy PM (Kellian play money) with USDC on Base." No sibling tool performs a PM purchase, and the two-step x402 flow plus the registry quote make the purpose unambiguous to an agent scanning the tool list.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines4/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

Gives explicit operational sequencing: call with no `payment` to obtain requirements, then sign and call again with the base64 payload. It does not name a sibling or alternative route for acquiring PM (e.g., whether `kellian_plan_pm_order` should be consulted first), so exclusion guidance is absent, but the when/when-not within the call cycle is clear.

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

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