Callx402 MCP server README

Payload — Developer infrastructure for x402, agent payments, and programmable revenue. PAYLOAD → VEYLINE (flagship) → CALLX402 (action layer) → REVRULE (separate) → developer products → free utilities. This repo: callx402 by Payload — the universal action layer into Veyline's x402 infrastructure.

Powered by Veyline. When x402 breaks, callx402.

callx402 is the universal action layer into Veyline's x402 infrastructure. Say what you need done in plain language (or hit an HTTP endpoint) and callx402 routes it to the production system that does the real work: diagnose a broken x402 payment, rescue a failed transaction, route an agent job to the cheapest viable path, resolve settlement state from on-chain evidence, or execute under an explicit budget with fail-closed safety rules.

Why it exists: x402 failures are expensive and opaque. A settlement attempt ends in settlement_pending and nobody knows whether the money moved. A 402 response your wallet misreads. A retry that signs a second authorization for the same intent and pays twice. callx402 exists for exactly those moments: diagnose pins the failure to a stage, rescue triages the incident, resolve settles the question from evidence, route finds the cheapest viable path, execute runs under a hard budget.

Try it (two minutes, no setup, no account):

npm install -g callx402
callx402 status
callx402 'complete this job for under $1'

status prints an honest per-subsystem reachability report, no credentials needed. The intent line parses and plans your request, then stops before executing anything: with no live tool endpoint wired it reports "no executable route available", so no money moves. (Use single quotes: in double quotes your shell would eat the $1.)

Free read-only checks against the live Payload rail:

curl https://payload-rail.fly.dev/v1/callx402/actions
curl 'https://payload-rail.fly.dev/v1/callx402/quote?action=diagnose'

The first returns the paid action catalog with prices; the second returns a free price quote for one action. Invoking a rail action is paid per action via checkout (see https://payloadhq.github.io/agents.json); these commands never pay anything.

If it saves you one debugging session, star the repo and read on.

MCP server: connect in 60 seconds

This repo ships a zero-dependency, read-only MCP server (mcp/index.js, node stdlib only, stdio transport) exposing six x402_* diagnostic tools. Nothing here charges, executes, retries, or repays.

git clone https://github.com/Payloadhq/callx402
# no npm install needed for the MCP server

Add this block to your MCP client config (replace the path), then restart the client:

{
  "mcpServers": {
    "callx402": {
      "command": "node",
      "args": ["/absolute/path/to/callx402/mcp/index.js"]
    }
  }
}

Ask the client to list its MCP tools: the six x402_* tools should appear. Per-client notes and the raw stdio test handshake: integrations/mcp-clients/.

Note: all six free MCP tools respond on a clean npm install using conservative local, read-only assessment of supplied evidence. Without an authoritative source they do not verify settlement, access a private ledger, or execute recovery. Existing configured local/remote subsystem integrations remain available.

The relationship

• PAYLOAD = the parent company.
• VEYLINE = production infrastructure for x402 + MCP. The flagship brand.
• CALLX402 BY PAYLOAD = the x402 response/action layer, powered by Veyline. The entry action into Veyline's production infrastructure, not the flagship name. Nothing here renames Veyline.

Discovery path: a developer searching for x402 finds callx402, uses it, and learns Veyline. Payload is not affiliated with the x402 Foundation.

What it does

• Diagnose an x402 or MCP failure
• Rescue a failed transaction (auth-gated)
• Route a paid agent job to the cheapest viable path
• Resolve settlement state from on-chain evidence
• Execute under an explicit budget, with fail-closed safety rules

Behavioral usage language (not trademark claims):

• "Need to diagnose x402? callx402."
• "Need to rescue a transaction? callx402."
• "Need to route a paid agent job? callx402."
• "Need settlement certainty? callx402."
• "Need to execute under a budget? callx402."

Quickstart

1. Install

npm install -g callx402

Or locally in a project: npm install callx402 (the binary is then at node_modules/.bin/callx402).

Fallback, install direct from GitHub:

npm install Payloadhq/callx402

Local development:

git clone https://github.com/Payloadhq/callx402
cd callx402
npm install
npm link        # exposes the `callx402` command

2. First run: what works with zero setup

callx402 status
callx402 'complete this job for under $1'

status is the honest starting point: a per-subsystem reachability report. On a bare npm install it reads 0/15 subsystems reachable, which is expected (see step 3). The intent line plans in plain language and stops before execution, so it is always safe to run; use single quotes so your shell does not eat $1.

Free read-only checks against the live Payload rail (no account, no keys):

curl https://payload-rail.fly.dev/v1/callx402/actions
curl 'https://payload-rail.fly.dev/v1/callx402/quote?action=diagnose'

These return the paid action catalog with prices and a free price quote for one action. Invoking a rail action is paid per action via checkout (machine front door: https://payloadhq.github.io/agents.json); nothing here pays anything.

3. Self-hosted runtime (advanced, optional)

By default every subsystem command routes to the hosted rail — no setup needed. If you prefer to run everything on your own machine, set CALLX402_LOCAL=1 and point CALLX402_V2_ROOT at the v2.0.0 tree (the x402 Paid API Starter Kit, a separate product):

export CALLX402_LOCAL=1
export CALLX402_V2_ROOT=/path/to/x402-paid-api-starter-kit/v2.0.0
callx402 status    # now: 15/15 reachable, 0/15 enabled

Without CALLX402_LOCAL=1, these commands use the hosted rail instead — that is the standard path and needs nothing installed beyond the npm package.

(The default is a x402-paid-api-starter-kit/v2.0.0 directory sitting next to your callx402 checkout.)

Subsystems are disabled by default; status names the flag that enables each one. Set a flag, then run its command:

export PAYLOAD_MCP_DOCTOR=1
callx402 diagnose --target 'https://api.example.com/x402/pay'

More examples (each needs its subsystem flag enabled; run callx402 status to see the flag names):

callx402 rescue --incident inc_123
callx402 route --goal 'fetch 100 product prices for under 0.50 USD'
callx402 resolve --evidence '{"txHash":"0xabc..."}'

Note the single quotes: the goal text contains $-style amounts that a shell would expand inside double quotes.

Alternative to the local tree: run in remote mode against your own callx402 server (server/index.js):

export CALLX402_MODE=remote
export CALLX402_REMOTE_URL=http://127.0.0.1:8787

JavaScript SDK

const { callx402 } = require('callx402');

const result = await callx402({
  intent: 'complete this job for under $1',
  maxBudget: 1.00,
  networks: ['base'],
  assets: ['USDC'],
  approvalThreshold: 5.00,
  idempotencyKey: 'job-42',
});

Python SDK

import callx402

result = callx402.callx402(
    intent="complete this job for under $1",
    max_budget=1.00,
    networks=["base"],
    assets=["USDC"],
    approval_threshold=5.00,
    idempotency_key="job-42",
)

HTTP server

CALLX402_PORT=8787 node server/index.js
curl -X POST http://127.0.0.1:8787/call \
  -H 'Content-Type: application/json' \
  -d '{"intent":"complete this job for under $1","maxBudget":1.00,"dryRun":true}'

Full HTTP surface: server/openapi.yaml (served live at GET /openapi.json).

Commands

Subsystem commands (diagnose, rescue, route, resolve, doctor, monitor, preflight, inspect) need the v2.0.0 tree plus the matching feature flag (see "Full subsystem commands" above); without them they exit 3 and do nothing. status, intent mode, and config work with zero setup.

Execution modes

callx402 works out of the box. There are two modes:

A bare npm install callx402 gives you the hosted mode — no separate tree, no config, no keys. The v2 tree is never required for standard use.

How payment works

Hosted paid actions settle through Payload's signed-redemption flow, not a generic x402 facilitator:

1. Get a free quote for the action: the exact price, before any payment.
2. Call the action and read the x402 v2 402 challenge: USDC on Base, the amount, and the payTo address.
3. Pay the quoted amount to payTo from your own wallet.
4. Redeem the original quote with the payment tx hash and an EIP-191 signed authorization from the paying wallet. A public tx hash alone never authorizes an action.
5. The rail verifies the payment exactly once on-chain and returns the result.

The full redemption contract is documented in docs/signed-redemption.md. The free local tier above never charges and never moves money.

Paid one-off diagnostics (no subscription)

The CLI and MCP tools above are the free local tier: read-only diagnostics that never charge, never execute, and never move money. When a free diagnostic is not enough — you need a paid, metered, auditable invocation with quote-before-payment and exactly-once payment semantics — each action is also available as a paid one-off rail action. No subscription, no account: quote, then pay deliberately.

What paying buys: the rail authorizes the action, validates the payment exactly once on-chain, meters the invocation against your org, records it for audit, enforces governor/quota rules for subscribers — and then executes the read-only diagnostic server-side and returns the result. No local setup, no separate tree, no keys. The rail never fakes execution: every result is produced by the diagnostic modules over the evidence you supply.

The hosted quote-before-payment flow:

# 1. Obtain a free quote. No money moves.
curl "https://payload-rail.fly.dev/v1/callx402/quote?action=resolve&path=x402"

# 2. Run the interactive client. Pay explicitly by card or with USDC.
#    For USDC redemption, the paying wallet must EIP-191-sign the exact
#    authorization message displayed by the CLI.
callx402 resolve --evidence '{"txHash":"0x..."}'

# 3. Agents with an external wallet-produced signature can redeem the
#    ORIGINAL canonical quote with a signed authorization file:
callx402 resolve --evidence '{"txHash":"0x..."}' \
  --tx-hash 0xYOUR_SETTLED_PAYMENT_HASH \
  --payer-auth @signed-auth.json --approve

A public transaction hash alone never authorizes an action. The signed authorization binds the paying wallet, action, transaction hash, quote, recipient, network, and request inputs. Keep the original quote_inputs with the signed authorization; requesting a new quote does not authorize an older payment. Never give Payload a private key, seed phrase, or wallet recovery phrase. EIP-1271 contract-wallet redemption is not currently enabled. Stripe credits use a separate redemption path.

Live fee schedule: GET https://payload-rail.fly.dev/v1/callx402/actions (responds "model":"paid on-demand per action; no subscription required"). There are two prices per action, and they are not interchangeable: the card price is the Stripe schedule fee; the USDC price on the x402 path is exactly that fee divided by 20 (the free quote endpoint is the source of truth for the USDC column). Both, side by side:

Rail evidence and explain read back a prior paid invocation (the request carries that invocation's invocation_id), so they cannot be used cold; the free local forms read your local operation ledger instead.

Not sold on the rail: safe_retry, rescue, monitor, and execute are local-only. They run in the self-hosted runtime (v2.0.0 tree), are absent from the rail catalog above, and the rail refuses them with 422 HOSTED_ACTION_UNAVAILABLE — there is no Stripe or x402 purchase path for them. For the "is this retry safe?" question, the free local recover verdict is the available read-only answer.

Never blind-retry a payment to reach a paid action: get the quote first.

Problem to action map (which paid action answers which failure, with the free CLI/MCP path for each): docs/problem-map.md.

Public RevRule evaluation (0.01 USDC)

The rail also exposes one public RevRule evaluation endpoint. It is a RevRule demo, not a callx402 diagnostic action: POST https://payload-rail.fly.dev/v1/x402/public/evaluate evaluates one economic event against the fixed, transparent public demo graph (platform 5% of gross; creator 70% of the remainder; referrer 25% of the remainder when a referrer is named in the event) for 0.01 USDC on Base.

The full, correct request body is:

{
  "event": {
    "eventId": "my-event-001",
    "type": "SALE_COMPLETED",
    "rail": "stripe",
    "amountMicros": 100000000,
    "currency": "USD",
    "occurredAt": "2026-10-13T00:00:00.000Z"
  },
  "txHash": "0x<your settled 0.01 USDC payment on Base>"
}

• event.eventId (any unique non-empty string) and event.rail are required, even though the 402's advertised bazaar schema omits both. Valid rail values: stripe, ach, x402, manual, stablecoin.
• event.type must be one of: SALE_COMPLETED, API_PAYMENT, LICENSE_PAID, SUBSCRIPTION_PAYMENT, REFERRAL_CONVERTED, ROYALTY_RECEIVED, CAMPAIGN_MILESTONE, MARKETPLACE_TRANSACTION, TOUCH.
• The event is validated before payment: a malformed body gets 400 INVALID_EVENT, not a 402 (verified live: the body above, minus txHash, returns the 402). The endpoint fills in the rest itself — graphId is fixed to the public demo graph, and currency defaults to "USD" and occurredAt to now when omitted.
• Omit txHash to receive the x402 v2 402 challenge; pay 0.01 USDC, then retry the same body with the hash. Each txHash is single-use.

Integrations

Working entry points for agent frameworks, automation, MCP clients, and x402 facilitators — all in integrations/ and tested against the live rail:

• LangChain — 9 tools (integrations/langchain/): diagnose, recover, resolve, evidence, explain, safe-retry, duplicate-payment risk, preflight, plus the free live fee schedule
• CrewAI — the same actions as CrewAI Tools (integrations/crewai/)
• n8n — importable incident-guard workflow (integrations/n8n/): maps an incident to a callx402 action, fetches the free live quote, invokes when credentialed, otherwise emits payment instructions — never auto-pays
• MCP clients — Claude Desktop / Cursor / Windsurf setup for the free read-only MCP server (integrations/mcp-clients/)
• x402 facilitators — settle-guard.js (integrations/facilitator/): resolve settlement state from evidence before re-broadcasting a payment

Problem → action map: docs/problem-map.md. Machine front door for agents: https://payloadhq.github.io/agents.json.

Money-safety rules

• Settlement UNKNOWN is never auto-retried and never repaid.
• Over-budget intents are refused with zero side effects.
• Approval thresholds fail closed.
• Idempotency keys dedupe — repeats return the original, never re-execute.
• Disabled or unreachable subsystems fail closed — success is never faked.
• Non-custodial — callx402 dispatches work; it never holds funds or private keys.

What callx402 is not

• Not the flagship product name. The product is Veyline.
• Not a broker, negotiator, or custodian. It dispatches; it never holds funds.
• The phrases above are usage language, not exclusivity claims.

Links

• Canonical docs: https://payloadhq.github.io/
• Payload org profile: https://github.com/Payloadhq/Payloadhq
• RevRule by Payload (separate product): https://github.com/Payloadhq/revrule-console

Full subsystem reference: docs/README.md. Behavioral language: docs/ACTION_LANGUAGE.md. Design spec: SPEC.md.

License

MIT. See LICENSE.

More from Payload · payloadhq.github.io · all Payload repos

Related: x402-manifest-check · x402-observatory · flow-agentic-demo