Blackwall MCP Server

blackwall-mcp

A guardrail for AI agents, as an MCP server. Your agent calls one tool — forecast — before any irreversible action (send email, move money, run SQL, delete data, post content). It gets back a risk score (0–100), a reversibility class, a GO / CAUTION / STOP recommendation, and named red flags in a few seconds (~4-8s).

Works in any MCP host: Claude Desktop, Claude Code, Cursor, Windsurf, and any agent framework with MCP support.

The wall between your agent and disaster. A BLUETIER product.

---

1. Get an API key

Sign up free at https://blackwalltier.com → Dashboard → API keys → Create key. Free tier: ~100 forecasts/month, no card. Your key looks like bw_live_….

2. Add the server to your MCP host

Claude Desktop

Edit claude_desktop_config.json (Settings → Developer → Edit Config):

{
  "mcpServers": {
    "blackwall": {
      "command": "npx",
      "args": ["-y", "blackwall-mcp"],
      "env": { "BLACKWALL_API_KEY": "bw_live_your_key_here" }
    }
  }
}

Restart Claude Desktop. You'll see a forecast tool available.

Cursor

Settings → MCP → Add new global MCP server, then in mcp.json:

{
  "mcpServers": {
    "blackwall": {
      "command": "npx",
      "args": ["-y", "blackwall-mcp"],
      "env": { "BLACKWALL_API_KEY": "bw_live_your_key_here" }
    }
  }
}

Claude Code

claude mcp add blackwall -e BLACKWALL_API_KEY=bw_live_your_key_here -- npx -y blackwall-mcp

Run locally (any host / testing)

BLACKWALL_API_KEY=bw_live_your_key_here npx -y blackwall-mcp

3. Use it

Once added, instruct your agent: "Before any irreversible action, call the forecast tool and stop if it returns STOP." The model will call it automatically when it's about to do something risky.

---

The forecast tool

Returns: recommendation (GO/CAUTION/STOP), risk_score (0–100), reversibility (class + rollback cost), gate (proceed/confirm/human-required), confidence, red_flags[], predicted_result, alternative_actions[].

Example

Agent about to run DELETE FROM users; (no WHERE clause) →

🛑 BLACK_WALL: STOP — risk 99/100
Red flags:
  • [CRITICAL] SQL_NO_WHERE — deletes the entire table, not one row
  • [CRITICAL] INTENT_MISMATCH — intent was "remove a single test row"
  • [CRITICAL] IRREVERSIBLE_NO_BACKUP — no recovery path
Guidance: DO NOT take this action. Surface the red flags to the user.

---

Observe mode — try it with zero risk

Not ready to let a guardrail block your agents? Start in observe mode. It scores and logs every action but never tells the agent to stop — your agents behave exactly as they do today. After a week, review your dashboard and see what it would have caught.

{
  "mcpServers": {
    "blackwall": {
      "command": "npx",
      "args": ["-y", "blackwall-mcp"],
      "env": {
        "BLACKWALL_API_KEY": "bw_live_your_key_here",
        "BLACKWALL_MODE": "observe"
      }
    }
  }
}

Then see "what your agents almost did" in your dashboard. Flip BLACKWALL_MODE to enforce (or just remove it — enforce is the default) when you're ready to actually block.

Two tools

The server exposes two MCP tools:

• forecast — pre-action risk check. Returns GO / CAUTION / STOP, risk score, named red flags, reversibility class, and a verifiable receipt.
• observe — post-action outcome report. Tells BLACK_WALL what actually happened after the action ran (or after the agent obeyed a STOP verdict). Closes the loop so the system can track prediction accuracy over time. FREE — no tokens charged.

Wire your agent to call forecast before any irreversible action, then call observe afterwards with the forecast_id from the original response. observe accepts an outcome_class (matched / over_scope / under_scope / no_op / diverged / aborted) and optional divergence_severity and details. See the forecast example below; the same wiring applies to observe.

Use it in code — the gate() control (any JS/TS agent)

Running an agent in Node (LangChain, a custom loop, ElizaOS, a cron job)? You don't need an MCP host — call BLACK_WALL straight from the library, and let gate() make the check impossible to skip. One wrap forecasts the action, enforces the verdict (fails closed on STOP / unknown / unreachable), runs your side effect only when allowed, and reports the real outcome with observe automatically.

npm i blackwall-mcp

import { gate, BlackWallBlocked } from 'blackwall-mcp/lib/gate';

// Wrap ANY risky action in a few lines. BLACKWALL_API_KEY lives in the env.
try {
  const { result } = await gate(
    { action: 'run_sql', inputs: { statement: sql }, context: { user_intent } },
    () => db.query(sql),                        // your real side effect — only runs if allowed
    { onCaution: (v) => confirmWithHuman(v) },  // CAUTION needs a yes; default = block
  );
  // ...use result
} catch (e) {
  if (e instanceof BlackWallBlocked) {
    // STOP, unconfirmed CAUTION, or forecast unavailable → the action NEVER ran
    console.error('Blocked:', e.reason, e.verdict?.red_flags);
  } else throw e; // a real error thrown by your action
}

Fails closed by design. If no verdict can be obtained (network / auth / timeout), the action does not run unless you explicitly pass failOpen: true. A risk gate that fails open is not a risk gate. The loop closes itself — gate() calls observe with the actual outcome (matched / diverged / aborted), so your forecasts sharpen over time.

Prefer the lower-level pieces? They're exported too:

import { forecast, observe } from 'blackwall-mcp/lib';

const v = await forecast({ action: 'make_payment', inputs: { amount_usd: 50000 } });
if (v.recommendation === 'STOP') throw new Error('halt');
// ... take the action ...
await observe(v.id, { outcome_class: 'matched' });

Runnable demo: examples/gate-quickstart.mjs.

Decision receipts (cryptographic, verifiable offline)

Every forecast response now includes a receipt field — an Ed25519 signature over canonical SHA-256 hashes of the request + response. Anyone with the published public key can verify offline that BLACK_WALL signed off on a specific (request, response) pair, without trusting our servers.

• Published keys: https://blackwalltier.com/.well-known/blackwall-signing-keys.json (stable, cacheable)
• Stateless verify endpoint: POST https://blackwalltier.com/api/v1/receipts/verify with { envelope, request_body, response_body }
• Hashes only — BLACK_WALL never stores the raw request/response bodies, so receipts give cryptographic audit without payload exposure
• Free-tier retention: 90 days. Paid: indefinite.

The MCP server surfaces the receipt id in its tool output so your agent can log it for later replay / audit.

Config reference

Links

• Site & docs: https://blackwalltier.com
• Get a key: https://blackwalltier.com/dashboard/keys

MIT licensed.