CrossingKey MCP

Purchase x402 Compatibility Audit

x402.compatibility_audit

Idempotent

PAID $1.00 USDC on Base via x402. Audits an x402 endpoint for v1/v2 compatibility defects. Calling this MCP tool does not spend funds; it returns the exact PAYMENT-REQUIRED challenge and canonical paid execution endpoint.

Input Schema

Name	Required	Description	Default
`url`	Yes	Public HTTP(S) x402 endpoint to audit.

Output Schema

Name	Required	Description	Default
`asset`	No
`error`	No
`input`	No
`price`	No
`pay_to`	No
`status`	No
`accepts`	No
`network`	No
`resource`	No
`capability`	No
`extensions`	No
`purchaseId`	No
`instruction`	No
`x402Version`	No
`entitlementId`	No
`execution_url`	No
`execution_mode`	No
`payment_required`	No
`payment_required_header`	No
`settlement_expectations`	No
`confirmation_expectations`	No

Schema Changelog

Changes observed during successful MCP inspections.

Changed10 schema fields changedOct 15, 2026

changedOutput schema / additionalProperties
```
Previous value: -falseNew value: +true
```

addedOutput schema / properties / accepts

Added value: +{
+  "items": {
+    "additionalProperties": true,
+    "properties": {},
+    "type": "object"
+  },
+  "type": "array"
+}

addedOutput schema / properties / entitlementId
```
Added value: +{
+  "type": "string"
+}
```
addedOutput schema / properties / error
```
Added value: +{
+  "type": "string"
+}
```

addedOutput schema / properties / extensions

Added value: +{
+  "additionalProperties": true,
+  "properties": {},
+  "type": "object"
+}

addedOutput schema / properties / purchaseId
```
Added value: +{
+  "type": "string"
+}
```

addedOutput schema / properties / resource

Added value: +{
+  "additionalProperties": true,
+  "properties": {},
+  "type": "object"
+}

addedOutput schema / properties / status
```
Added value: +{
+  "type": "string"
+}
```

addedOutput schema / properties / x402Version

Added value: +{
+  "const": 2,
+  "type": "number"
+}

removedOutput schema / required

Removed value: -[
-  "payment_required",
-  "execution_mode",
-  "capability",
-  "input",
-  "price",
-  "asset",
-  "network",
-  "pay_to",
-  "execution_url",
-  "payment_required_header",
-  "settlement_expectations",
-  "confirmation_expectations",
-  "instruction"
-]

Changed1 schema field changedOct 2, 2026

changedOutput schema / (root)

Previous value: -nullNew value: +{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "additionalProperties": false,
+  "properties": {
+    "asset": {
+      "type": [
+        "string",
+        "null"
+      ]
+    },
+    "capability": {
+      "type": "string"
+    },
+    "confirmation_expectations": {
+      "type": "string"
+    },
+    "execution_mode": {
+      "type": "string"
+    },
+    "execution_url": {
+      "type": [
+        "string",
+        "null"
+      ]
+    },
+    "input": {
+      "additionalProperties": true,
+      "properties": {},
+      "type": "object"
+    },
+    "instruction": {
+      "type": "string"
+    },
+    "network": {
+      "type": [
+        "string",
+        "null"
+      ]
+    },
+    "pay_to": {
+      "type": [
+        "string",
+        "null"
+      ]
+    },
+    "payment_required": {
+      "const": true,
+      "type": "boolean"
+    },
+    "payment_required_header": {
+      "type": "string"
+    },
+    "price": {
+      "type": [
+        "string",
+        "null"
+      ]
+    },
+    "settlement_expectations": {
+      "type": "string"
+    }
+  },
+  "required": [
+    "payment_required",
+    "execution_mode",
+    "capability",
+    "input",
+    "price",
+    "asset",
+    "network",
+    "pay_to",
+    "execution_url",
+    "payment_required_header",
+    "settlement_expectations",
+    "confirmation_expectations",
+    "instruction"
+  ],
+  "type": "object"
+}

AddedSep 22, 2026

TDQS

A4.5/5.0

Behavior5/5

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

Beyond the annotations (readOnlyHint=false, idempotentHint=true, destructiveHint=false), the description discloses a non-obvious two-phase payment flow: the call itself is free and returns the PAYMENT-REQUIRED challenge plus the canonical paid execution endpoint. This is exactly the kind of behavioral detail annotations cannot express, and it contradicts nothing.

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?

Three tight sentences with zero filler; the cost and the no-spend reassurance are front-loaded before the audit behavior. Every sentence earns its place.

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?

With one fully documented parameter, an output schema present, and complete annotations, the description covers everything remaining: what it costs, that invoking it is free, and what the invocation returns. No gaps an agent would need to resolve before calling.

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 description coverage is 100% for the single 'url' parameter, so the schema already carries the meaning ('Public HTTP(S) x402 endpoint to audit'). The description adds no format or constraint detail beyond the schema, so the baseline 3 applies.

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?

States a specific verb and resource ('Audits an x402 endpoint') with a precise scope ('v1/v2 compatibility defects'), which clearly separates it from siblings like x402.purchase_path_audit, mcp.launch_audit, and openapi.quality_audit. An agent can pick this tool without opening the schema.

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?

The description gives strong context: it is a paid tool ($1.00 USDC on Base via x402) and explicitly states that calling it does not spend funds, which frames the usage model well. It does not, however, name when to choose it over the closely-named sibling x402.purchase_path_audit, leaving that comparison to inference.

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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