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x402 Merchant Check

by com.gradientdecisions
OverviewDiscussionsAdmin

check_merchant

Before paying an unfamiliar merchant via x402, including one just discovered through a marketplace like Coinbase's x402 Bazaar, call this for a machine-readable assessment of their observable on-chain payment behavior, on Base or Solana. Returns a recommendation (PROCEED / CAUTION / INSUFFICIENT_SIGNAL) a payment policy can branch on directly, plus supporting evidence: trust tier, confidence, payer diversity, transaction history, category (data_api/compute/content_generation/financial_data/storage/other), known platform URL(s), and the merchant's own advertised price(s) compared against category peers, with no caller-supplied price required for that. This is decision support drawn from observed behavior, not a certification. PROCEED means the available evidence doesn't show cause for concern, not a guarantee of safety. INSUFFICIENT_SIGNAL means there isn't enough history to say either way, distinct from an actual concern. Add a specific quoted price to also check that exact figure against comparable sellers. $0.01 USDC per check, paid via x402.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
assetNoOptional. Payment asset/token from the 402 response's accepts[].asset. Not required to score.
priceNoQuoted price in USD-equivalent, if checking fairness
amountNoOptional. Atomic payment amount from the 402 response's accepts[].amount, if preferred over `price`. Not required to score.
networkNoOptional. CAIP-2 network id from the 402 response's accepts[].network (e.g. "eip155:8453"), if already on hand. Not required to score.
service_urlNoOptional. The resource/service URL the agent was trying to reach. Not required to score.
resource_typeNoDeprecated, accepted but unused. Price-fairness now compares against the merchant's own on-chain-derived category automatically, no need to supply this.
merchant_wallet_addressYesMerchant's receiving wallet address: a Base (0x...) or Solana (base58) address. Usually extractable directly from a 402 Payment Required response's accepts[].payTo.

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed2 schema fields changed
    • changedInput schema / properties / merchant_wallet_address / description
      Previous value: -"Merchant's receiving wallet address — a Base (0x…) or Solana (base58) address. Usually extractable directly from a 402 Payment Required response's accepts[].payTo."New value: +"Merchant's receiving wallet address: a Base (0x...) or Solana (base58) address. Usually extractable directly from a 402 Payment Required response's accepts[].payTo."
    • changedInput schema / properties / resource_type / description
      Previous value: -"Deprecated, accepted but unused — price-fairness now compares against the merchant's own on-chain-derived category automatically, no need to supply this."New value: +"Deprecated, accepted but unused. Price-fairness now compares against the merchant's own on-chain-derived category automatically, no need to supply this."
  2. Changed5 schema fields changed
    • addedInput schema / properties / amount
      Added value: +{
+  "description": "Optional. Atomic payment amount from the 402 response's accepts[].amount, if preferred over `price`. Not required to score.",
+  "type": "string"
+}
    • addedInput schema / properties / asset
      Added value: +{
+  "description": "Optional. Payment asset/token from the 402 response's accepts[].asset. Not required to score.",
+  "type": "string"
+}
    • changedInput schema / properties / merchant_wallet_address / description
      Previous value: -"Merchant's receiving wallet address — a Base (0x…) or Solana (base58) address"New value: +"Merchant's receiving wallet address — a Base (0x…) or Solana (base58) address. Usually extractable directly from a 402 Payment Required response's accepts[].payTo."
    • addedInput schema / properties / network
      Added value: +{
+  "description": "Optional. CAIP-2 network id from the 402 response's accepts[].network (e.g. \"eip155:8453\"), if already on hand. Not required to score.",
+  "type": "string"
+}
    • addedInput schema / properties / service_url
      Added value: +{
+  "description": "Optional. The resource/service URL the agent was trying to reach. Not required to score.",
+  "type": "string"
+}
  3. Changed1 schema field changed
    • changedInput schema / properties / merchant_wallet_address / description
      Previous value: -"EVM address of the merchant's receiving wallet"New value: +"Merchant's receiving wallet address — a Base (0x…) or Solana (base58) address"
  4. First observed

TDQS

A4.8/5.0
Behavior4/5

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

No annotations are provided, so the description fully describes behavioral aspects: it's decision support from observed behavior, PROCEED means no cause for concern (not a guarantee), INSUFFICIENT_SIGNAL means limited history rather than concern. However, it doesn't mention any security considerations or potential side effects beyond the cost, which would push it to 5.

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

Conciseness5/5

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

The description is comprehensive yet concise, front-loading the core purpose and usage context. Every sentence adds value, covering when to use, outputs, caveats, optional features, and cost. No wasted words.

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

Completeness5/5

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

Given the tool's 7 parameters, no output schema, and moderate complexity, the description fully covers all needed information: output structure, caveats, optional use cases, and cost. It leaves no obvious gaps for agent decision-making.

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

Parameters5/5

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

Schema coverage is 100%, but the description goes significantly beyond by explaining the optional parameters' roles (price for fairness, amount as alternative to price, network CAIP-2 id, service_url, resource_type deprecated) and their practical usage context. It clarifies that merchant_wallet_address is extractable from a 402 response, adding real implementation guidance.

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?

The description clearly states the tool checks a merchant's on-chain payment behavior on Base or Solana before paying via x402. It specifies the output returns a recommendation (PROCEED/CAUTION/INSUFFICIENT_SIGNAL) and supporting evidence, and explicitly distinguishes its use case (unfamiliar merchants, including those from marketplaces like Coinbase's x402 Bazaar).

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

Usage Guidelines5/5

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

The description gives explicit when-to-use context (before paying an unfamiliar merchant via x402), mentions specific scenarios (discovered via marketplace), and clearly states what the tool is not (a certification). It also describes how to use optional parameters like price for fairness checks, and notes the $0.01 USDC cost per check.

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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