Which path should my agent use?

Chat apps (Cursor, Claude, Codex, ChatGPT) should use MCP: the tool returns a Checkout link, or sends on your subscription page pool. Autonomous scripts that can mint a Stripe Shared Payment Token should call the REST machine endpoint with MPP. Backend jobs should use an API key and prepaid credits.

Can Cursor pay with Stripe MPP by itself?

No. Cursor has no Stripe wallet. Use MCP + Checkout, or install Stripe Link CLI (npx skills add stripe/link-cli) and run link-cli mpp pay against the REST machine URL. Approve each spend in the Link app.

Do I need an account?

MCP uses OAuth, so yes — sign in (or create an account) when the client opens the consent screen. MPP machine sends do not need an API key. REST credits need a dashboard API key.

No. Do not send PHI over MCP, MPP, or the public API. Covered Entities should use HIPAA Secure Mode on the website with an electronic BAA.

MCP · REST · Stripe MPP

Fax from your agent.

Connect Cursor, Claude, Codex, ChatGPT, or VS Code to SingleFax over MCP. The assistant uploads the PDF, quotes the send, and either opens Checkout or uses your page pool.

Add to Cursor Add to VS Code

Cursor and VS Code install in one click. Claude, Codex, and ChatGPT copy the snippet and open the right settings page. Cursor deeplink if the web install page does not open the app.

https://singlefax.com/mcp

Chat apps

Cursor, Claude, Codex, ChatGPT. MCP + OAuth. Human confirms payment unless you have leftover subscription pages.

Scripts & backends

API key + prepaid credits. No checkout redirect. Best for your own workers.

Autonomous pay

Stripe MPP (HTTP 402 + Shared Payment Token). Cursor cannot do this alone — add Link CLI.

Install by client

Streamable HTTP at https://singlefax.com/mcp. After install, the client runs OAuth — sign in with your SingleFax account.

Click Add to Cursor above, or save this in ~/.cursor/mcp.json.
Approve the SingleFax OAuth consent screen.
Ask the agent to send a fax. It will call create_upload then create_fax_draft.

~/.cursor/mcp.json

{
  "mcpServers": {
    "singlefax": {
      "url": "https://singlefax.com/mcp"
    }
  }
}

Cursor does not mint Stripe Shared Payment Tokens. Prefer the Checkout link from create_fax_draft. To pay MPP from a Cursor terminal, see MPP below.

Claude Code

Run the command, then complete OAuth with /mcp in the session.

terminal

claude mcp add --transport http singlefax https://singlefax.com/mcp

Claude.ai / Desktop

Open Customize → Connectors.
Add a custom connector named singlefax with URL https://singlefax.com/mcp.
Connect, then enable the connector in the chat.

Free Claude.ai accounts are limited to one custom connector. Claude has no install deeplink — the button copies the CLI command and opens Connectors.

Paste the commands in a terminal (CLI, IDE extension, and ChatGPT desktop share ~/.codex/config.toml).
codex mcp login singlefax starts OAuth if the add step did not.
In a Codex session, run /mcp to confirm tools.

terminal

codex mcp add singlefax --url https://singlefax.com/mcp
codex mcp login singlefax

~/.codex/config.toml

[mcp_servers.singlefax]
url = "https://singlefax.com/mcp"

Turn on Developer mode (Settings → Security and login).
Open Connectors / Plugins and create a custom connector.
Name: SingleFax. MCP URL: https://singlefax.com/mcp. Authentication: OAuth.
In a chat, enable Developer mode and the SingleFax connector before asking to send a fax.

MCP server URL

https://singlefax.com/mcp

Custom MCP in ChatGPT is for Developer mode, not Instant Checkout. The agent still returns a SingleFax Checkout URL for you to pay.

Click Add to VS Code, or Command Palette → MCP: Add Server.
Or save this in .vscode/mcp.json (workspace) or user MCP config.

.vscode/mcp.json

{
  "servers": {
    "singlefax": {
      "type": "http",
      "url": "https://singlefax.com/mcp"
    }
  }
}

What to say

After the server is connected, a prompt like this is enough. The agent should upload, quote, and give you a payment link (or send if your subscription still has pages).

example prompt

Send this PDF as a fax to +1-415-555-2671 using SingleFax.

create_upload— Issue a short-lived upload token, then PUT the PDF

create_fax_draft— Quote and draft a send — Checkout URL, or send on your page pool

get_fax_status— Poll delivery status for an order you own

get_credits— Read prepaid credit balance

create_machine_fax— MPP challenge JSON — only if the client can mint an SPT

MPP — autonomous payment

Machine Payment Protocol is a buyer protocol: the server returns HTTP 402, the agent attaches a Stripe Shared Payment Token, and retries. That is POST https://singlefax.com/api/v1/machine/faxes, not the chat MCP tools.

Cursor, Claude Code, and Codex can call that URL from a terminal, but they cannot mint an SPT unless you give them a wallet. Stripe’s path is Link CLI (US Link accounts). Approve each spend in the Link app.

Install the skill: npx skills add stripe/link-cli — or add MCP npx @stripe/link-cli --mcp.
Log in, then pay the machine endpoint (valid E.164, real PDF base64 — not a +1555 placeholder).
Do not point mpp pay at /mcp. Link CLI needs a URL that returns HTTP 402 + WWW-Authenticate.

link-cli mpp pay

npx @stripe/link-cli auth login

npx @stripe/link-cli mpp pay https://singlefax.com/api/v1/machine/faxes \
  -X POST \
  -d '{"to":"+14155552671","base64":"<pdf-bytes>","request_id":"mpp-1"}' \
  --context "Send a fax via SingleFax after the user asked this agent to pay for the send."

Full challenge shapes, validate/status/refunds: mpp.md. Fiat SPT needs our Stripe profile (NUXT_STRIPE_PROFILE_ID); without it the endpoint returns 503. Minimum SPT charge is $0.50; SingleFax retail floor is $0.99.

REST API + credits

For your own backend, create a scoped key and prepay credits — no Checkout redirect.

Create an API key OpenAPI 3 spec Agent Auth device approval

FAQ

Agent-readable docs

Plain markdown for crawlers and coding agents. Same facts as this page.

llms.txt agents.md mpp.md openapi.json