OpenAPI mock response generator

Synthesize a JSON response body for one operation + status code in an OpenAPI 3.x or Swagger 2.x spec. Send POST /api/openapi-mock-response with the required field spec 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 status, contentType, mock and source.

Locate the operation by operationId or method+path. Pick the response by explicit `status`, else the first 2xx, else the first documented response. Generation precedence: operation-level `example`/`examples` > schema `example` > recursive type-inferred walk (objects walk properties, arrays return [mock(items)], enums return the first value, $ref is surfaced literally). Returns a `source` tag so callers know the mock's fidelity. Pure CPU - deterministic, no network, no $ref dereferencing.

Category: Data conversion · Tags: openapi swagger mock response api agent-readiness

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Parameters

Example request

curl -i -X POST https://agent402.tools/api/openapi-mock-response \
  -H "Content-Type: application/json" \
  -d '{"spec":{"openapi":"3.0.0","paths":{"/users/{id}":{"get":{"operationId":"getUser","responses":{"200":{"description":"ok","content":{"application/json":{"schema":{"type":"object","properties":{"id":{"type":"string","example":"u_42"},"name":{"type":"string","example":"Alice"},"active":{"type":"boolean","example":true},"roles":{"type":"array","items":{"type":"string","example":"admin"}}}}}}}}}}}},"operationId":"getUser"}'

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

Example response

{
  "status": "200",
  "contentType": "application/json",
  "mock": {
    "id": "u_42",
    "name": "Alice",
    "active": true,
    "roles": [
      "admin"
    ]
  },
  "source": "generated"
}

From an MCP client

catalog.call {
  "slug": "openapi-mock-response",
  "params": {
    "spec": {
      "openapi": "3.0.0",
      "paths": {
        "/users/{id}": {
          "get": {
            "operationId": "getUser",
            "responses": {
              "200": {
                "description": "ok",
                "content": {
                  "application/json": {
                    "schema": {
                      "type": "object",
                      "properties": {
                        "id": {
                          "type": "string",
                          "example": "u_42"
                        },
                        "name": {
                          "type": "string",
                          "example": "Alice"
                        },
                        "active": {
                          "type": "boolean",
                          "example": true
                        },
                        "roles": {
                          "type": "array",
                          "items": {
                            "type": "string",
                            "example": "admin"
                          }
                        }
                      }
                    }
                  }
                }
              }
            }
          }
        }
      }
    },
    "operationId": "getUser"
  }
}

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

Errors and behavior

• spec is 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-mock-response 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-mock-response", {
  method: "POST",
  headers: { "Content-Type": "application/json" },
  body: JSON.stringify({
    "spec": {
      "openapi": "3.0.0",
      "paths": {
        "/users/{id}": {
          "get": {
            "operationId": "getUser",
            "responses": {
              "200": {
                "description": "ok",
                "content": {
                  "application/json": {
                    "schema": {
                      "type": "object",
                      "properties": {
                        "id": {
                          "type": "string",
                          "example": "u_42"
                        },
                        "name": {
                          "type": "string",
                          "example": "Alice"
                        },
                        "active": {
                          "type": "boolean",
                          "example": true
                        },
                        "roles": {
                          "type": "array",
                          "items": {
                            "type": "string",
                            "example": "admin"
                          }
                        }
                      }
                    }
                  }
                }
              }
            }
          }
        }
      }
    },
    "operationId": "getUser"
  }),
});

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-mock-response")).json();
let n = 0;
while (lz(createHash("sha256").update(c.challenge + ":" + n).digest()) < c.difficulty) n++;
await fetch("https://agent402.tools/api/openapi-mock-response", { method: "POST", headers: { "X-Pow-Solution": c.token + ":" + n, "Content-Type": "application/json" }, body: JSON.stringify({"spec":{"openapi":"3.0.0","paths":{"/users/{id}":{"get":{"operationId":"getUser","responses":{"200":{"description":"ok","content":{"application/json":{"schema":{"type":"object","properties":{"id":{"type":"string","example":"u_42"},"name":{"type":"string","example":"Alice"},"active":{"type":"boolean","example":true},"roles":{"type":"array","items":{"type":"string","example":"admin"}}}}}}}}}}}},"operationId":"getUser"}) });

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