Shinjuku Shielded MCP

The private x402 facilitator, inside your agent.

Shinjuku Shielded settles x402 payments from a shielded USDC balance on Solana: on chain, a shielded payment does not show who paid. This MCP server is how your agent uses it. It runs on your machine, under caps that only you set.

• Self-custody. The server runs on your machine. Your keys never leave it. We never run it, and we never hold your money.
• Shielded payments. Your agent pays from a shielded balance. Add --tor and no server sees your IP.
• One command to set up. npx -y shinjuku-shielded mcp offers the setup on a machine with no wallet. It makes the wallet, a private passphrase file, and the checked proof tools.
• Caps that only you set. Spending limits are optional; by default there is none (only the balance bounds a payment). Setup records the ones you set. A tool call can only lower them, never raise them.
• Find sellers. x402_discover searches public x402 listings (Coinbase x402 Bazaar, PayAI, Dexter, and others) on your machine.
• Pay any Solana exact seller. A seller on Coinbase CDP, PayAI, or any other facilitator is paid through Shinjuku: your wallet pays us privately, and we pay the seller from our own float. Nothing to fill or set aside.
• Look before you pay. x402_preview shows the price and whether your caps allow it. It pays nothing.
• No SOL. wallet_shield needs only USDC. Our facilitator’s relayer pays the network fee.
• Never twice. A retry with the same request_id resumes the same payment, deposit, or unshield. It never starts a second one.
• No RPC, no account, no API key. Reads go through our relay by default. Your own RPC always wins if you give one.

Every Shinjuku Shielded service joins this MCP as it ships.

The server is the mcp command of shinjuku-wallet, one file. This repository holds the setup guide and examples. The wallet file, its SHA-256, and its release history are in ShinjukuStaition/shinjuku-shielded.

Talk to it in plain words

Ask your agent the way you would ask a person:

Each answer tells the agent the exact next call. The wallet reads the chain and writes an encrypted backup by itself, so you never run a command for upkeep.

Tools

SETUP

On a machine with no wallet, the server starts in setup mode.

In setup mode, every money tool answers wallet_not_set_up and does nothing.

SHIELD

PAY

UNSHIELD

ACCOUNT

Also: wallet_cancel closes one unfinished payment, so its reserved balance comes back.

wallet_shield and wallet_unshield always appear in the tool list. When one is off, it answers "error": "mcp_tool_disabled" and tells the agent what to ask you.

What it pays:

• shielded-exact, from your shielded balance, whenever the seller offers it.
• Standard Solana exact, through Shinjuku (float pay). For a seller on any facilitator (Coinbase CDP, PayAI, or any other), x402_pay pays us privately from your shielded balance: the seller’s price plus a flat fee (feeAtomic, 0.001 USDC on 2026-10-11). We pay the seller at once from our own shared float. The chain shows Shinjuku paying the seller, never your wallet. max_payment_atomic must cover price plus fee. The largest price that one request can pay is our largest float pocket: largestAtomic in GET https://pay.shinjukustaition.com/api/x402/float/config (3 USDC on 2026-10-11).
• A retry with the same request_id never pays twice.

Hidden amounts (scheme confidential, the V0b vault) are retired: the V0b program was closed on chain on 2026-10-11.

Value that an older version put in pockets still counts in the one balance. When the server has --proof-tools, --exit-proof-tools, and --profile, it returns that value to the shielded balance by itself while it is idle. shinjuku-wallet pocket reshield does the same by hand.

A balance split over several accounts: one payment spends one account plus at most 3 notes. When no single account covers a price, x402_pay says so and names shinjuku-wallet merge for you to run. After it, and the next pool anchor (usually within a minute), the agent pays again.

Privacy modes

Setup

You need Node.js 22 or later, on Linux, or on Windows with WSL. macOS is not supported: the provers are Linux programs. The current wallet release is 08a4b56f (npm [email protected]). It keeps the pool of 0.3.5 and uses proof tools 47678912.

Update from 0.3.7 or earlier. On 2026-10-11 the pool program was upgraded (faster pool anchors), and 0.3.7 and earlier refuse payments. Run npx -y shinjuku-shielded@latest setup once: it pins the new program and gets the new proof tools. Your wallet, its address, and its balance stay the same.

Update from a version before 0.3.5. On 2026-10-10 the pool program was upgraded and the shielded pool moved to 45mhosfA2MuygZWeqYRVcEZX2Q3gA8KR8FwP6AZkK1zR. The old pool AAG16mNWTtC1sBeu2tTVTwWjqWXCefLTCByVPj5X3kV8 is retired, and earlier versions refuse payments. Run npx -y shinjuku-shielded@latest setup once: it pins the new program and sets up a wallet for the new pool. That wallet has a new address: ask your agent for it before you send USDC.

1. Add the server to your agent

Claude Code:

claude mcp add --scope user shinjuku-shielded -- npx -y shinjuku-shielded mcp

Claude Desktop (claude_desktop_config.json), Cursor (~/.cursor/mcp.json), and other JSON-config hosts:

{
  "mcpServers": {
    "shinjuku-shielded": {
      "command": "npx",
      "args": ["-y", "shinjuku-shielded", "mcp"]
    }
  }
}

2. Start it and accept the setup

The first time, the server has no wallet, so it starts in setup mode. Ask your agent “what is my balance”. Your MCP client asks you to confirm: create a Shinjuku Shielded wallet on this machine. Accept, and the setup runs (about 25 s in our test, with the proof-tool download of about 150 MB). Then the same server switches to the wallet tools. You do not restart it.

In a terminal, npx -y shinjuku-shielded mcp asks “Set up Shinjuku Shielded now? (y/n)”. Or run the setup by itself:

npx -y shinjuku-shielded setup

It asks for the caps (Enter keeps the default) and Tor. --yes asks nothing. --add-to claude-code (or claude-desktop, cursor) also adds the server to that host’s config.

Setup writes to ~/.carbon-shielded-wallet (or SHIELDED_WALLET_HOME):

• The wallet, and a recovery file with the seed and the key. Copy the recovery file to two offline places, then delete it from this machine.
• passphrase.txt: a new random passphrase, readable by you alone. It is never printed, and no tool takes or returns it.
• The proof tools. Setup checks every file against the set that this wallet release pins.
• config.json: the caps, the paths, and Tor. With it, mcp needs no flags. A flag on the command line still wins.

Spending limits are optional and off by default: with none, only the balance bounds a payment. To set them, give --max-payment, --max-session, or --max-total (in atomic USDC: 50000 = 0.05 USDC). A setup rerun lifts the old default caps (0.05 / 1 / 20 USDC) of earlier versions. --prefer-tor records Tor (see Privacy modes).

You can run setup again at any time. It never changes an existing wallet, and it checks every proof-tool file again.

3. Fund it

Ask your agent “what is my wallet address?”. Send USDC (Solana SPL) to that address. You do not need SOL. Then say “shield 1 USDC”. The steps are in First 10 minutes.

Advanced: download the file and check it

Download the release file and check its SHA-256 before you run it. The current file and hash are also in the Releases table and in section 7b of https://shinjukustaition.com/skill.md.

curl -fsSLO https://shinjukustaition.com/wallet/08a4b56f/shinjuku-wallet.mjs
echo "08a4b56f6452268ffdd513e67326de8782b53dcec16ba88adaddf16793581206  shinjuku-wallet.mjs" | sha256sum -c -
node shinjuku-wallet.mjs setup

With the file, replace npx -y shinjuku-shielded with node /abs/path/shinjuku-wallet.mjs. To pin the npm version, write [email protected].

Advanced: flags instead of config.json

mcp reads config.json only when you give no --pool and no --wallet. To set every flag yourself (for example, for a wallet that you made with init before 0.2.0):

claude mcp add --scope user shinjuku-shielded -- npx -y shinjuku-shielded mcp \
  --max-payment 50000 --max-session 200000 --tor \
  --pool 45mhosfA2MuygZWeqYRVcEZX2Q3gA8KR8FwP6AZkK1zR \
  --program 8PYPw3FSFTMbvSneXdcoH6jNoN4VD23nHwPY2A2riUy1 \
  --memo-service https://pay.shinjukustaition.com \
  --proof-tools /abs/path/shinjuku-proof-tools/proof-tools.json \
  --exit-proof-tools /abs/path/shinjuku-proof-tools/exit-proof-tools.json \
  --profile /abs/path/shinjuku-proof-tools/public-profile.json \
  --passphrase-file /abs/path/passphrase.txt

Use absolute paths: a host starts the server from its own folder. On Windows, write C:/Users/you/.... --proof-tools turns wallet_shield on. --exit-proof-tools and --profile turn wallet_unshield on.

For an older wallet, you can also run setup once with SHIELDED_WALLET_HOME set to your wallet folder and --passphrase-file set to your passphrase file. It keeps your wallet and writes config.json beside it.

A send to a new address needs a host that supports MCP elicitation (it shows you the confirmation). With a host that does not, name each address with --unshield-to.

Examples

examples/ has complete configs and a walkthrough:

• Claude Code: the claude mcp add command, the first start, Tor, each flag explained, and how to check it works.
• Claude Desktop and Cursor: complete JSON configs.
• Hermes Agent: the config.yaml entry.
• Vercel AI SDK: the server’s tools in generateText through @ai-sdk/mcp, with no other package.
• Solana Agent Kit: npm solana-agent-kit-plugin-shinjuku, the wallet as Solana Agent Kit v2 actions.
• First 10 minutes: add the server, set it up, fund it, shield, pay, send, and check the balance.
• Plain requests: 11 requests, the tool each one calls, and the shape of the answer.

Caps and flags

After setup, config.json holds the caps and the paths. You can add any of these flags to the mcp command. A flag wins over config.json.

The passphrase comes from --passphrase-file or SHIELDED_WALLET_PASSPHRASE at start, never from a tool call. A refusal is a normal answer ({"ok": false, "error", "detail", "next"}); the agent does what next says. A cap refusal tells the agent to ask you: only you set the caps.

An unshield is your own money, so it does not count against the payment budget of the wallet file (init --max-payment, --max-cumulative). Payments to sellers still count. To show, change, or remove those wallet-file caps, run shinjuku-wallet limits (none removes a cap). A refusal by one of them tells the agent to ask you for this.

Upkeep is automatic. After a deposit lands, and before a payment or unshield when the local balance is stale, the server reads the chain itself. By default it reads the pool history through our facilitator’s https history proxy: a new wallet is ready in about 60 s. After each shield and unshield, the wallet writes an encrypted backup to /auto-backups//. It keeps the newest 5 complete, verified backups. They are on the same disk as the wallet: copy one to another place.

What others can see

• The seller sees your request, the price, the time, and your IP unless you use --tor.
• Your agent host and its model provider see every URL, body, price, paid answer, amount, and unshield address the agent handles. A local model removes that observer.
• Your MCP client app (Claude Code, Claude Desktop, Cursor, …) answers the confirmation of a send, not the model. So a prompt-injected agent cannot approve a send. But the wallet trusts the client app to show the dialog to you: a malicious or modified client app could answer “accept” by itself. For a strict setup, name your addresses with --unshield-to and use a client without elicitation. Then any other address is refused.
• Our facilitator processes your payment. It keeps no access logs.
• On chain, a shielded-exact payment does not show who paid. A float pay payment is a pool payment to us (the payer hidden), then a normal USDC transfer from a Shinjuku float pocket to the seller. Many buyers pay from the same float pockets. The transfer lands seconds after the pool payment, so time can pair the two.
• For a float payment, our server sees which private payment paid which seller, the price, the URL, and the time, until the payment is final or refunded. It does not see your wallet.
• wallet_shield is a public step: the chain shows USDC leave the wallet’s own key, the amount, and the time.
• wallet_unshield is a public step: the chain shows the amount, the address that receives it, and the time.
• The UNSHIELDED line of wallet_balance reads the wallet’s own key. The RPC (our relay by default) sees which key it reads.
• Privacy needs a crowd. With few users, timing can still link payments.

Status

Listed in the official MCP Registry as io.github.ShinjukuStaition/shinjuku-mcp. npm: shinjuku-shielded, published from this repository’s workflow with npm provenance (from 0.1.1).

Live on Solana mainnet. The current wallet release is 08a4b56f (npm 0.3.8). Float pay paid two stock sellers on the PayAI facilitator on production on 2026-10-11, 0.10 USDC and 1.50 USDC, and a stock seller on our own facilitator, 0.01 USDC twice: the transactions are in proof.md.

These transactions were made with real money through this MCP server, against our production facilitator, on the earlier pool AAG16…3kV8 (retired 2026-10-10). The first two came from a new wallet with the default setup and no SOL at any time (wallet release 94a357bc). The others used wallet release 1a336885 (2026-10-08).

No unshield transaction contains the wallet that shielded; our facilitator paid every network fee of each unshield.

Full guide: https://shinjukustaition.com/skill.md · Onion: http://2kfhlfuyuwvhmibjrpxqsg4nrbcxasjgjq7kmnfzgfwzptkhhznz3dad.onion/skill.md · X: @Shin_StAItion