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_….

Related MCP server: MCP Permission Guard

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.