zFinia Intelligence

x402 Metadata Drift

x402_metadata_drift

Did this paid endpoint change its terms since you last looked? Give it a previous challenge as the baseline and it returns every difference: price changes with old and new amounts, offers added or removed, a protocol version change, and a payee_changed flag when a new recipient address appears. Never pays the endpoint. Pay 6000 atomic USDC on Base via exact x402. MCP does not pay; it provides a challenge handoff.

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": {
      "url": "<url>",
      "baseline": "<baseline>"
    },
    "name": "x402_metadata_drift"
  }
}

Input Schema

Name	Required	Description
`url`	Yes	HTTPS URL of an x402 endpoint on a reviewed origin. Credentials, ports and fragments are refused.
`body`	No	Optional JSON body to send with a POST probe.
`method`	No	Method to probe with. Defaults to GET.
`baseline`	Yes	A challenge object from an earlier preflight or decode, used as the comparison point.

Schema Changelog

Changes observed during successful MCP inspections.

AddedNov 2, 2026

TDQS

A4.1/5.0

Behavior4/5

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

With no annotations, the description carries the full burden and does substantial work: it discloses that the tool never pays the endpoint, that the endpoint is only probed, and it enumerates the returned diff fields. The payment sentence ('Pay 6000 atomic USDC on Base via exact x402... MCP does not pay; it provides a challenge handoff') is ambiguous about whether it describes the tool's own price or the endpoint's, which is the one gap.

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?

The question-then-answer structure front-loads the purpose well and the difference enumeration is dense and useful. The payment/handoff sentence sits awkwardly at the end and reads ambiguously, costing a little clarity in an otherwise tight description.

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?

There is no output schema, so the description wisely enumerates what the diff returns (old/new amounts, offers added/removed, version change, payee_changed), which is what an agent most needs to interpret results. Nested baseline object and the probe-vs-pay distinction are covered; error or refusal behavior for unreviewed origins is only implied by the schema.

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

Parameters3/5

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

Schema coverage is 100%, so url, body, method and baseline are already fully documented structurally. The description reinforces the role of `baseline` as a prior challenge snapshot, but adds no syntax, format or edge-case detail (e.g. what happens if baseline is malformed or comes from a different endpoint) beyond 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 concrete question that names the resource (a paid endpoint's terms) and the operation (compare against a previous challenge), and the enumeration of returned differences (price, offers, version, payee_changed) pins down exactly what 'drift' means. An agent can tell this apart from x402_endpoint_preflight/health, which inspect endpoints rather than diff them against a baseline.

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?

'Give it a previous challenge as the baseline' plus the schema wording 'from an earlier preflight or decode' establishes the precondition and effectively routes the agent through x402_endpoint_preflight first. It stops short of an explicit when-not or a named sibling alternative, so it is clear context rather than full routing guidance.

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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