OpenAPI / Swagger diff

Compare two OpenAPI 3.x or Swagger 2.x documents and return a structured diff: added / removed / changed endpoints, with a conservative "is any change breaking?" flag. Send POST /api/openapi-diff with the required fields before and after and pay $0.001 per call over x402 or MPP, or call it free by solving a proof-of-work challenge. It returns a JSON object with added, removed, changed, breaking, breakingCount and 1 more.

Breaking = an endpoint or required-2xx status was removed, a required parameter was added, an optional param became required, a param type changed, or a JSON body field became required. Pure CPU - deterministic, no network, no $ref dereferencing (resolve refs upstream if needed).

Category: Data conversion · Tags: openapi swagger diff breaking-change api

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Parameters

Example request

curl -i -X POST https://agent402.tools/api/openapi-diff \
  -H "Content-Type: application/json" \
  -d '{"before":{"openapi":"3.0.0","paths":{"/users":{"get":{"responses":{"200":{}}}},"/legacy":{"get":{"responses":{"200":{}}}}}},"after":{"openapi":"3.0.0","paths":{"/users":{"get":{"responses":{"200":{}}}},"/admin":{"post":{"responses":{"201":{}}}}}}}'

Without payment this returns HTTP 402 Payment Required with the exact price for openapi-diff; any x402 v2 or MPP client pays it and retries.

Example response

{
  "added": [
    "POST /admin"
  ],
  "removed": [
    "GET /legacy"
  ],
  "changed": [],
  "breaking": true,
  "breakingCount": 1,
  "summary": {
    "added": 1,
    "removed": 1,
    "changed": 0
  }
}

From an MCP client

catalog.call {
  "slug": "openapi-diff",
  "params": {
    "before": {
      "openapi": "3.0.0",
      "paths": {
        "/users": {
          "get": {
            "responses": {
              "200": {}
            }
          }
        },
        "/legacy": {
          "get": {
            "responses": {
              "200": {}
            }
          }
        }
      }
    },
    "after": {
      "openapi": "3.0.0",
      "paths": {
        "/users": {
          "get": {
            "responses": {
              "200": {}
            }
          }
        },
        "/admin": {
          "post": {
            "responses": {
              "201": {}
            }
          }
        }
      }
    }
  }
}

On the hosted connector at https://agent402.tools/mcp, catalog.call runs openapi-diff free (rate-limited, no wallet). Local install: npx -y agent402-mcp.

Errors and behavior

• before and after are required. An input the tool rejects returns an HTTP 4xx whose body carries error, tool, expected, required and example, so the caller can correct it.
• A paid call that ends in any status of 400 or above is not charged over x402, MPP or a prepaid credits key: settlement is cancelled when the tool fails. The exception is a Tempo push credential, a transfer the buyer sent before the call: it settles before the tool runs, so if the tool then fails the payment is recorded as a refund owed to the paying wallet.
• Free tier: no outbound network call leaves the server for this tool, so proof-of-work (16 leading zero bits of sha256) pays for it.
• A GET or HEAD to /api/openapi-diff returns the same 402 quote, so the price can be read without a body.
• An Idempotency-Key header makes a retried paid call replay the first 200 instead of charging again (an answer larger than 1 MB is not replayed).

Paid call (JavaScript agent)

import { wrapFetchWithPayment } from "@x402/fetch";
import { x402Client } from "@x402/core/client";
import { registerExactEvmScheme } from "@x402/evm/exact/client";
import { privateKeyToAccount } from "viem/accounts";

const client = new x402Client();
client.setSpendControls?.(false); // keep your own spending ceiling in code
registerExactEvmScheme(client, { signer: privateKeyToAccount(KEY) });
const payFetch = wrapFetchWithPayment(fetch, client);

const res = await payFetch("https://agent402.tools/api/openapi-diff", {
  method: "POST",
  headers: { "Content-Type": "application/json" },
  body: JSON.stringify({
    "before": {
      "openapi": "3.0.0",
      "paths": {
        "/users": {
          "get": {
            "responses": {
              "200": {}
            }
          }
        },
        "/legacy": {
          "get": {
            "responses": {
              "200": {}
            }
          }
        }
      }
    },
    "after": {
      "openapi": "3.0.0",
      "paths": {
        "/users": {
          "get": {
            "responses": {
              "200": {}
            }
          }
        },
        "/admin": {
          "post": {
            "responses": {
              "201": {}
            }
          }
        }
      }
    }
  }),
});

No wallet? Pay with compute

Fetch a challenge, solve the sha256 puzzle (16 leading zero bits, a fraction of a second of CPU), and resend with the X-Pow-Solution header:

import { createHash } from "node:crypto";
const lz = (b) => { let t = 0; for (const x of b) { if (!x) { t += 8; continue; } t += Math.clz32(x) - 24; break; } return t; };
const c = await (await fetch("https://agent402.tools/api/pow/challenge?slug=openapi-diff")).json();
let n = 0;
while (lz(createHash("sha256").update(c.challenge + ":" + n).digest()) < c.difficulty) n++;
await fetch("https://agent402.tools/api/openapi-diff", { method: "POST", headers: { "X-Pow-Solution": c.token + ":" + n, "Content-Type": "application/json" }, body: JSON.stringify({"before":{"openapi":"3.0.0","paths":{"/users":{"get":{"responses":{"200":{}}}},"/legacy":{"get":{"responses":{"200":{}}}}}},"after":{"openapi":"3.0.0","paths":{"/users":{"get":{"responses":{"200":{}}}},"/admin":{"post":{"responses":{"201":{}}}}}}}) });

Part of these workflows

OpenAPI / Swagger diff is one step in this skill pack, each sold as a single call:

• API contract drift check - The 'did this API contract change in a way that breaks our integration?' workflow. Diff two OpenAPI snapshots structurally, lint the new one for agent-readiness regressions, extract the endpoint surface for inventory comparison, validate a known-good payload against the new contract, and summarize security-relevant changes. Six tools, one go/no-go answer on whether the upstream API broke us.

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