Policy enforcement for custom MCP integrations. MCPRegistry defines which servers and tools may be reached; WardenGateway checks each call for allowlist, path, OAuth, rate-limit, lateral-movement, and policy violations before your MCP client executes it.

Install

npm install @stlw/warden-mcp-gateway @stlw/warden

Quick start

Create a registry, construct the gateway with a ledger and context manager, then wrap an allowed server:

import { ContextManager, MemoryLedgerStore, TrustLevel } from "@stlw/warden";
import { MCPRegistry, WardenGateway } from "@stlw/warden-mcp-gateway";

const registry = new MCPRegistry([
  {
    name: "filesystem",
    type: "local",
    transport: "stdio",
    allowedTools: ["read_file"],
    authRequired: false,
  },
]);

const gateway = new WardenGateway({
  config,
  ledger: new MemoryLedgerStore(),
  contextManager: new ContextManager(),
  registry,
});

const filesystem = gateway.wrapMCP("filesystem", {
  serverName: "filesystem",
  allowedTools: ["read_file"],
  trustLevel: TrustLevel.TOOL,
  maxCallsPerMinute: 120,
});

const decision = await filesystem.onToolCall(
  "read_file",
  { path: "./README.md" },
  "session-1",
  "task-1",
);
if (decision.action !== "ALLOW") throw new Error(decision.reason);

config is the PolicyConfig loaded from your Warden configuration. Keep the registry restrictive: an unlisted server or tool is denied before policy evaluation. Use allowedPaths for filesystem boundaries and authRequired: true for servers that need OAuth credentials.

Discovery and approvals

Use gateway.listTools(serverName, upstreamTools) (or the wrapped server's listTools) when proxying MCP tools/list. It retains only tools allowed by the registry whose policy result is ALLOW. A client must still pass normal call-time authorization; discovery is not an authorization grant.

Gateway policy evaluation, rate limits, and ledger entries use the canonical action ID mcp.<server>.<tool> (for example, mcp.filesystem.read_file). Existing <server>__<tool> policy rules remain supported during migration.

For a multi-step approval UX, call requestApprovalGrant from a trusted gateway-side approval adapter after the configured approval channel is available. The returned grant can be supplied as the final onToolCall argument. It is valid for 60 seconds and is single-use, bound to the session, task, canonical action, and exact tool input. Never expose the approval API to an unauthenticated MCP client.

Prefer the CLI when possible

If you do not need a custom MCP client, @stlw/warden-cli provides the ready-to-run warden proxy stdio server. See the public manual for client configuration and the CLI package.