Charge per tool callon your MCP server.
Drop a one-line middleware onto any MCP tool registration. Agents pay before the call returns. USDC settled on Base in about two seconds, with a signed receipt every time. The MCP catalog turns into a market.
The Model Context Protocol gave agents a clean way to call tools. What it didn't ship is a way for tool authors to get paid — MCP monetization is that missing payment layer. Most MCP servers are free because there was no payment primitive to wrap a tool registration. Mailgent puts a one-line paywall in front of any MCP tool — same idea as the REST middleware, same project wallet, same audit trail. The agent's first call returns a payment requirement, the agent's wallet signs and retries, your tool runs and returns. The whole exchange happens before the LLM sees the result.
How it works
Four steps. Most fire in under a second.
Install the middleware
Install @mailgent/sdk and import the paywall middleware for MCP. Bind it to your project's API key at server startup. One factory call.
Wrap each paid tool
Add requirePayment({ amount: '0.01' }) to any tool registration. Different tools can have different prices. The wrapper composes with whatever logic you had — it only adds the payment gate in front.
Agent pays before the tool runs
Buyer agent calls the tool with no payment. Mailgent returns the payment requirement. The agent's wallet signs and retries. Mailgent verifies, settles USDC on Base, then your handler runs.
Receipt + log
Your tool returns; the response includes a signed receipt. The activity log records the call with the agent identity, tool name, amount, and chain link. Webhooks fire if the project has one configured.
What you get
Built for developers who already shipped.
One line per tool
Wrap any tool registration with requirePayment(). Different tools can have different prices. Same pattern as the REST middleware — share the same project wallet, scopes, and activity timeline.
Settles before the response
Agent calls the tool, Mailgent verifies the signed payment, settles USDC on Base, then your handler runs. Failed payments never reach your business logic.
Listed in the Mailgent Index
Opt your MCP into the Mailgent Index. Buyer agents query paid tools, price-compare, and start paying immediately. Distribution included with the paywall.
Streamable HTTP + stdio
Works with both transports the MCP spec defines today. No protocol changes — the paywall is purely additive and invisible to MCP clients that already speak the spec.
Signed receipts on every call
Every paid tool call returns an Ed25519-signed receipt with the tool name, arguments hash, amount, and on-chain transaction. Auditors get a chain of evidence; consumers can verify a tool actually ran.
Refund + dispute primitives
If a tool errors, return the standard MCP error and Mailgent automatically refunds the buyer (the settle had not happened yet, so it's instant). Failed calls never charge.
What people build
Real-world mcp server monetization flows shipping today.
Search MCP
Wrap a web-search or vector-search tool. Charge per query. Agents that hammer free-tier endpoints become real revenue.
Extraction MCP
PDF parsing, OCR, structured-data extraction. Per-document pricing matches actual cost — no monthly tier nobody fits.
Inference MCP
Wrap a model inference tool. Charge per token, per generation, or per call. Pass-through model cost plus a margin.
Premium data MCP
Financial quotes, on-chain analytics, weather, satellite imagery. Per-lookup pricing turns expensive upstream feeds into sustainable resale.
Free MCP server vs MCP + Mailgent paywall
What the difference looks like.
| Feature | Free MCP server | MCP + Mailgent paywall |
|---|---|---|
| Revenue model | Donations, ads, or a separate API gate | Per-call USDC, settled before the tool runs |
| Integration | Roll your own API key + invoicing | One line per tool registration |
| Settlement | Per-month invoice, dispute window | Per-call USDC on Base, ~2 second settle, no chargebacks |
| Discovery | Your own catalog and outreach | Listed in the Mailgent Index, ranked by reputation |
| Receipts | Stripe invoice or none | Ed25519 signed per call, with chain link |
FAQ
Things builders ask.
Does MCP monetization require forking the MCP spec?
What happens if an MCP client doesn't support payments?
How do I set pricing for MCP server monetization?
Do failed MCP tool calls charge the buyer?
Does Mailgent host my MCP server?
How do I list my paid MCP server in the Mailgent Index?
Which MCP transports does Mailgent monetization support?
How much does MCP server monetization with Mailgent cost?
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