DropEngine x402 Agent Services

invoice_preflight

Paid invoice check ($0.065 USDC on Base): UBL XML or complete PDF/image invoices up to 3 pages. Compares expected purchase fields and line items; returns evidence, amount checks and match/discrepancy/needs_review. No payment or bookkeeping action is taken.

Agent entry points: risky external action → use Agent Action Gate when a combined action-specific safety assessment is needed; unclear capability/provider → use free /api/discover; constrained comparison between callable alternatives → use paid route_tool; known single capability → call the direct tool; multi-step objective → read the matching workflow recipe and call only relevant steps. Workflows are stateless recipes, not executors; calls are paid separately and are not bundle charged. Agent Action Gate does not approve payment, sign or broadcast transactions, evaluate seller reputation, replace transaction simulation, replace route_tool, or replace human approval.

Input Schema

TableJSON Schema

Name	Required	Description	Default
`expected`	No
`mime_type`	No
`document_url`	No
`document_xml`	No
`document_base64`	No

Output Schema

TableJSON Schema

Name	Required	Description	Default
`meta`	No
`next`	No
`error`	No
`checks`	Yes
`result`	No
`status`	No
`invoice`	Yes
`payment`	No
`agent_ux`	No
`decision`	Yes
`evidence`	Yes
`warnings`	Yes
`source_sha256`	Yes
`pages_processed`	No

Schema Changelog

Changes observed during successful MCP inspections.

Changed7 schema fields changedOct 8, 2026

addedOutput schema / properties / agent_ux

Added value: +{
+  "additionalProperties": false,
+  "properties": {
+    "error": {
+      "anyOf": [
+        {},
+        {
+          "type": "null"
+        }
+      ]
+    },
+    "meta": {
+      "additionalProperties": {},
+      "propertyNames": {
+        "type": "string"
+      },
+      "type": "object"
+    },
+    "next": {
+      "items": {},
+      "type": "array"
+    },
+    "payment": {
+      "additionalProperties": false,
+      "properties": {
+        "execution_status": {
+          "enum": [
+            "not_started",
+            "succeeded",
+            "failed",
+            "unknown"
+          ],
+          "type": "string"
+        },
+        "status": {
+          "enum": [
+            "not_required",
+            "required",
+            "settled",
+            "failed",
+            "unknown"
+          ],
+          "type": "string"
+        }
+      },
+      "required": [
+        "status",
+        "execution_status"
+      ],
+      "type": "object"
+    },
+    "result": {},
+    "status": {
+      "enum": [
+        "succeeded",
+        "failed",
+        "partial",
+        "outcome_unknown"
+      ],
+      "type": "string"
+    }
+  },
+  "required": [
+    "status",
+    "result",
+    "error",
+    "meta",
+    "payment",
+    "next"
+  ],
+  "type": "object"
+}

addedOutput schema / properties / error

Added value: +{
+  "anyOf": [
+    {},
+    {
+      "type": "null"
+    }
+  ]
+}

addedOutput schema / properties / meta

Added value: +{
+  "additionalProperties": {},
+  "propertyNames": {
+    "type": "string"
+  },
+  "type": "object"
+}

addedOutput schema / properties / next

Added value: +{
+  "items": {},
+  "type": "array"
+}

addedOutput schema / properties / payment

Added value: +{
+  "additionalProperties": false,
+  "properties": {
+    "execution_status": {
+      "enum": [
+        "not_started",
+        "succeeded",
+        "failed",
+        "unknown"
+      ],
+      "type": "string"
+    },
+    "status": {
+      "enum": [
+        "not_required",
+        "required",
+        "settled",
+        "failed",
+        "unknown"
+      ],
+      "type": "string"
+    }
+  },
+  "required": [
+    "status",
+    "execution_status"
+  ],
+  "type": "object"
+}

addedOutput schema / properties / result
```
Added value: +{}
```

addedOutput schema / properties / status

Added value: +{
+  "enum": [
+    "succeeded",
+    "failed",
+    "partial",
+    "outcome_unknown"
+  ],
+  "type": "string"
+}

First observedSep 28, 2026

TDQS

A3.9/5.0

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 it does well: discloses the exact price ($0.065 USDC on Base), input formats and a page cap, the nature of the result, and explicitly states no payment or bookkeeping action is taken. It does not describe how payment is authorized/charged or failure modes.

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

Conciseness3/5

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

The opening paragraph is tight and front-loaded. The second paragraph is largely generic routing boilerplate that recurs across tools and is only partially specific to this tool, diluting the signal without adding invoice-specific value.

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

Completeness3/5

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

An output schema exists, so return values need not be spelled out. However, for a 5-parameter nested tool with zero schema documentation and a payment component, the description leaves the document-input alternatives, requiredness, and payment mechanics underspecified.

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 0% across 5 parameters including nested objects, so the description must compensate. It conveys meaning for the expected-fields object (purchase fields, line items) and the document input formats (XML vs complete PDF/image), but does not clarify that document_url/document_xml/document_base64 are alternative transports or how mime_type relates to them.

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+resource (invoice check) with scope: UBL XML or PDF/image up to 3 pages, comparing expected fields and line items, returning match/discrepancy/needs_review. The 3-page limit and 'paid' qualifier cleanly separate it from the sibling invoice_preflight_10_pages and invoice_preflight_50_pages.

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 'Agent entry points' block explicitly routes: known single capability to the direct tool, unclear capability to /api/discover, constrained comparison to route_tool, combined safety assessment to Agent Action Gate, and multi-step to workflow recipes. It lacks a direct 'use this over the 10/50-page variants when the invoice is <=3 pages' statement, but alternatives and selection conditions are named.

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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