About blackwall-x402-guardrail
blackwall-x402-guardrail is a hosted remote MCP server that provides pre-signature risk verdicts for x402 payments. It returns GO, HOLD, or STOP with signals and a third-party-verifiable signed receipt.
Key features
- 01Return GO, HOLD, or STOP verdicts
- 02Check counterparty settlement reputation
- 03Detect price deviations from the payee's median
- 04Check OFAC sanctions signals
- 05Create Ed25519-signed receipts
Use cases
- 01Assess risk before signing an x402 payment
- 02Review the reasons and signals behind a verdict
- 03Let a third party verify a signed receipt
MCP support
The remote MCP server exposes the forecast_payment tool over Streamable HTTP and forwards requests to an x402 oracle. If the oracle fails or provides no valid verdict, the tool returns an error rather than GO.
Configuration
Remote servers need no local install. Pick your client, then copy the config or install in one click.
https://mcp.blackwalltier.comclaude mcp add --transport http com-blackwalltier-blackwall-x402-guardrail https://mcp.blackwalltier.com{"mcpServers":{"com-blackwalltier-blackwall-x402-guardrail":{"type":"http","url":"https://mcp.blackwalltier.com"}}}Open in Cursor{"servers":{"com-blackwalltier-blackwall-x402-guardrail":{"type":"http","url":"https://mcp.blackwalltier.com"}}}Open in VS Code{"mcpServers":{"com-blackwalltier-blackwall-x402-guardrail":{"type":"http","url":"https://mcp.blackwalltier.com"}}}{"mcpServers":{"com-blackwalltier-blackwall-x402-guardrail":{"serverUrl":"https://mcp.blackwalltier.com"}}}Run commands in a terminal; paste JSON into the client’s MCP config and restart. Servers that need sign-in will prompt on first connect.
Tools
1| Parameter | Type | Required | Description |
|---|---|---|---|
| asset | string | Yes | e.g. USDC |
| chain | string | Yes | e.g. base |
| payer | string | No | agent's EVM wallet (optional; binds settlement) |
| amount | string | Yes | amount as a decimal string, e.g. "0.09" |
| context | object | No | {quoted_price_history, expected_recipient} |
| agent_id | string | No | caller DID/identity |
| resource | string | No | what's being paid for |
| counterparty | string | Yes | recipient wallet from the 402 |
Details
From the project READMEBlack_Wall remote MCP
A hosted remote MCP (Model Context Protocol over HTTP) that gives any MCP-capable agent a pre-signature payment-risk verdict — with no install and no signup. Add one URL and your agent can ask, before it signs an x402 payment, "should I pay this?" and get GO / HOLD / STOP — from counterparty settlement reputation, price-anomaly (quoted vs the payee's own median), and OFAC sanctions — plus a third-party-verifiable Ed25519 signed receipt.
Add it
Point your MCP client at:
https://mcp.blackwalltier.com
Transport: Streamable HTTP. No API key. Verdicts for amounts under $10 at
risk are free; at or above that, the tool returns an x402 payment challenge
your agent settles from its own wallet (pass the settled payment back as an
X-PAYMENT header, then retry).
The tool: forecast_payment
| field | ||
|---|---|---|
counterparty | string, required | recipient wallet from the 402 |
amount | string, required | decimal string, e.g. "0.09" |
asset | string, required | e.g. USDC |
chain | string, required | e.g. base |
payer / resource / agent_id / context | optional | binds settlement / context |
Returns a verdict (GO/HOLD/STOP) + reasons + signals, and a
signed_receipt anyone can verify against the published key at
https://agent-egress-proxy.onrender.com/.well-known/blackwall-receipt-key.json
(any RFC 8032 Ed25519 library) — Black_Wall never has to be in the loop.
How it works
This Worker is a thin transport + proxy: it speaks minimal MCP JSON-RPC
(initialize / tools/list / tools/call / ping) and forwards
forecast_payment calls to the Black_Wall x402 oracle. All decision logic and
the receipt come from the oracle unchanged.
Fail-closed by design: if the oracle errors, is unreachable, or returns a
non-verdict, the tool returns an error — never a phantom GO. A payment
guardrail that can't reach its backend must not imply "safe to pay."
Registry
Published to the official MCP Registry as
com.blackwalltier/blackwall-x402-guardrail.
Develop
node test.mjs # unit tests (mocked oracle)
npx wrangler dev # run locally
npx wrangler deploy # deploy (Cloudflare Workers)
By BlueTier. Verdict only — never takes custody, never in the settlement path. No evidence is not trust.
Comments
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