CrossingKey MCP

Purchase MCP Schema Audit

mcp.schema_audit

Idempotent

PAID $1.00 USDC on Base via x402. Scores supplied MCP tool schemas for discoverability, descriptions, and bounded input contracts. 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
`tools`	Yes	MCP tool definitions 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

A3.7/5.0

Behavior4/5

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

Annotations mark this as non-read-only but idempotent; the description resolves the apparent tension by clarifying that calling it does NOT spend funds and that it returns the PAYMENT-REQUIRED challenge plus the canonical paid execution endpoint. That is genuine behavioral context beyond the annotations (price, chain, asset, and two-phase payment flow), though error/rate-limit behavior is unstated.

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?

Three tight sentences, front-loaded with the cost so the agent sees the payment implication first, followed by capability and the crucial 'no funds spent on call' clarification. No filler, though the payment sentence and the capability sentence could be compressed slightly.

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?

With an output schema present, return values need no explanation, and the description supplies cost, payment semantics, and input intent adequately for a one-parameter tool. The remaining gap is sibling disambiguation among the several audit tools, which matters given the crowded namespace.

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% and there is a single required 'tools' array, so the schema already carries the parameter burden. The description's 'supplied MCP tool schemas' merely restates the parameter meaning without adding shape, size limits, or format guidance beyond what the schema provides.

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

Purpose4/5

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

The description states a specific verb and resource: it 'scores supplied MCP tool schemas for discoverability, descriptions, and bounded input contracts', and even enumerates the scoring dimensions. It does not, however, differentiate itself from the close sibling 'mcp.launch_audit' or from the other audit tools (openapi.quality_audit, prompt.audit), which an agent would need to disambiguate.

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

Usage Guidelines3/5

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

Usage is implied by the input ('supplied MCP tool schemas') and the payment-model framing reveals the calling pattern, but there is no explicit when-to-use/when-not statement and no routing to the sibling audits. The reader must infer selection criteria from the tool name.

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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