x402-json-repair-mcp

A small, autonomous, pay-per-call MCP server that AI agents discover and pay for per call, with revenue landing directly in a USDC wallet on Base via the x402 payment protocol.

• One pure-logic tool today: structured_json_repair — deterministic JSON repair + JSON Schema validation/coercion. No model or paid API on the hot path, so each call costs ~nothing to serve (~100% margin).

• Tool-agnostic core: the MCP + payment + deploy skeleton is separate from tool logic. Adding tool #2 is a one-file change.

• Cheap, serverless, near-zero maintenance: runs on Cloudflare Workers (Node fallback included). Stateless. Holds no private keys — only a public payout address.

What it does

structured_json_repair takes a messy/invalid JSON-ish string (the kind LLMs and tools frequently emit) plus an optional JSON Schema, and returns clean, valid, schema-conformant JSON — with structured diagnostics when it can't fully fix it.

Fixes: trailing commas, single-quoted strings, unquoted keys, Python literals (None/True/False), NaN/Infinity, Markdown ``` code-fence wrappers, and truncated/garbled tails. With a schema, it validates (JSON Schema draft 2020-12) and optionally coerces primitives ("36" → 36, "true" → true).

Input (tools/call arguments):

Output (structuredContent):

{
  "ok": true,                 // valid JSON (and schema-valid when a schema was given)
  "data": { "...": "..." },   // the repaired/validated value; null if unfixable
  "changed": true,            // true if any repair or coercion modified the input
  "errors": [],               // actionable messages when ok is false
  "repairs": ["Removed trailing comma(s) before a closing } or ]."]
}

Annotations: readOnlyHint: true, destructiveHint: false, idempotentHint: true, openWorldHint: false.

Related MCP server: Pay MCP Server

Pricing

Discovery is free: initialize, tools/list, and ping are never charged — only a tools/call to a paid tool is.

How it works

agent (x402-enabled MCP client)
        │  POST /mcp  (JSON-RPC: tools/call structured_json_repair)
        ▼
┌─────────────────────────── Hono app (Workers / Node) ───────────────────────────┐
│  PaymentGate.evaluate()                                                          │
│   • initialize / tools/list / ping / free tools ─────────────► run MCP, return   │
│   • paid tools/call:                                                             │
│        no/invalid payment ─► HTTP 402 + x402 "payment-required" envelope         │
│        valid payment      ─► run MCP tool ─► settle ─► result + receipt header   │
└──────────────────────────────────────────────────────────────────────────────────┘

Because every MCP call hits a single /mcp endpoint, the gate inspects the JSON-RPC method/tool name and maps each paid tool to a synthetic x402 route (POST /x402/<tool>). The official x402 engine (@x402/core + @x402/evm) then handles price→atomic conversion, USDC asset resolution, the 402 envelope, facilitator verification, and on-chain settlement. Payment lives entirely at the HTTP layer and is transparent to JSON-RPC.

A real unpaid call returns (verified against the public testnet facilitator):

HTTP/1.1 402 Payment Required
payment-required: <base64 x402 v2 envelope>

// decoded envelope
{ "x402Version": 2,
  "resource": { "url": ".../x402/structured_json_repair", ... },
  "accepts": [{ "scheme": "exact", "network": "eip155:84532",
                "amount": "10000", "asset": "0x036CbD…CF7e" /* USDC on Base Sepolia */,
                "payTo": "0x…", "maxTimeoutSeconds": 300,
                "extra": { "name": "USDC", "version": "2" } }] }

Quick start (local sandbox / testnet)

npm install
cp .dev.vars.example .dev.vars      # then set PAYOUT_WALLET_ADDRESS (a PUBLIC 0x address)
npm run build
PAYOUT_WALLET_ADDRESS=0xYourPublicAddress npm start   # Node server on :8787
# or: npm run dev        (Cloudflare Workers dev server via wrangler)

Exercise it (no payment needed for discovery; the paid call returns 402):

# tools/list — free
curl -s -X POST http://localhost:8787/mcp \
  -H 'content-type: application/json' -H 'accept: application/json, text/event-stream' \
  -d '{"jsonrpc":"2.0","id":1,"method":"tools/list"}'

# paid tool, unpaid — returns HTTP 402 with the x402 envelope in the `payment-required` header
curl -i -s -X POST http://localhost:8787/mcp \
  -H 'content-type: application/json' -H 'accept: application/json, text/event-stream' \
  -d '{"jsonrpc":"2.0","id":2,"method":"tools/call",
       "params":{"name":"structured_json_repair","arguments":{"input":"{a:1,}"}}}'

Inspect interactively with the MCP Inspector:

npm run inspector      # then connect to Streamable HTTP at http://localhost:8787/mcp

Sample agent call

An x402-enabled MCP client pays automatically: it calls the tool, receives the 402, signs a USDC payment from its own wallet, and retries with the payment header — all transparently. With the x402 SDK (@x402/* + a wallet), wrap the HTTP transport so 402s are auto-paid, then use the standard MCP client against https://<host>/mcp. See the x402 client docs: https://docs.x402.org.

Project layout

src/
  index.ts            # Hono app: routes, x402 gate wiring, Workers entry (default export)
  node.ts             # Node/VPS entry (npm start / bin)
  config.ts           # env loading + validation (address-only; refuses private keys)
  mcp/
    server.ts         # MCP server factory; registers tools (tool-agnostic core)
    transport.ts      # stateless Streamable HTTP via @hono/mcp
  tools/
    structuredJsonRepair.ts   # pure tool logic + Zod input/output schemas + registration
    index.ts          # tool registry — add new tools here
    types.ts          # ToolModule contract
  payments/
    x402.ts           # paywall: per-tool routes, facilitator, verify/settle, JSON-RPC gating
test/
  structuredJsonRepair.test.ts
  paywall.test.ts
server.json           # Official MCP Registry metadata
wrangler.toml         # Cloudflare Workers config (sandbox + production envs)
.dev.vars.example     # example env (placeholders only — no secrets)

Adding a tool (tool-agnostic core)

1. Create src/tools/myTool.ts exporting a ToolModule (name, title, description, price, register). Set price: null for a free tool or "$0.05" for a paid one.

2. Append it to the array in src/tools/index.ts. That's the only change — the MCP, payment, and deploy layers pick it up automatically (the gate prices every tool whose price !== null).

Configuration

All config comes from environment variables (Workers [vars] / wrangler secret, or the process env on Node).

Deploy

See OPERATOR_CHECKLIST.md for the exact human-only steps (wallet, accounts, secrets, deploy, publish). In short:

# Sandbox (Base Sepolia, public facilitator)
wrangler secret put PAYOUT_WALLET_ADDRESS
wrangler deploy                       # → https://x402-json-repair-mcp.<subdomain>.workers.dev/mcp

# Production (Base mainnet, Coinbase CDP facilitator)
wrangler secret put PAYOUT_WALLET_ADDRESS --env production
wrangler secret put CDP_API_KEY_ID --env production
wrangler secret put CDP_API_KEY_SECRET --env production
wrangler deploy --env production

Node / VPS fallback: npm run build && PAYOUT_WALLET_ADDRESS=0x… npm start (serves /mcp on :8787; put it behind TLS).

Listing & discovery

• Official MCP Registry: edit server.json (set your OWNER namespace and the deployed URL), then publish with the registry mcp-publisher CLI. The registry feeds Smithery, PulseMCP, etc.

• x402 Bazaar / Agentic.market: the server serves machine-readable discovery at GET /.well-known/x402 (capability + pricing per tool). Submit per current x402 docs.

• npm: npm publish (the package is runnable via npx x402-json-repair-mcp for self-hosting).

Operator-run commands are in OPERATOR_CHECKLIST.md.

Security

• Address only — never a private key. The server reads PAYOUT_WALLET_ADDRESS to tell payers where to send USDC; it never signs or moves funds. config.ts actively refuses anything shaped like a private key (64 hex) or seed phrase.

• No secrets in source. Use .dev.vars locally (gitignored) and wrangler secret in production. Only .dev.vars.example (placeholders) is committed.

• Stateless. Request payloads aren't persisted beyond serving the response.

Engineering notes

• x402 v2. Uses the maintained @x402/core / @x402/evm packages (the v1 x402/x402-hono packages are deprecated). The 402 envelope is delivered in the payment-required response header (v2), which the agent's x402 client reads and pays.

• Validation uses @cfworker/json-schema, not Ajv. Ajv compiles validators at runtime via the Function constructor, which Cloudflare Workers forbid (no eval). Since the schema is a runtime input, it can't be precompiled — so we use the eval-free, Workers-native @cfworker/json-schema (already an MCP SDK peer) plus a small schema-driven coercion pass.

Roadmap

• Phase 2 — vertical document parser (planned, not built): a tool for one document type that general parsers handle poorly (e.g. a specific statement/invoice/receipt format), priced toward the work-done end ($0.05–$0.10/call). It would slot in as another ToolModule with no changes to the MCP/payment/deploy core.

License

MIT