MCP Gateway

The Nexus MCP Gateway acts as a deterministic firewall between your probabilistic AI agents and your mission-critical infrastructure. It performs deep packet inspection on every JSON-RPC tool call before it reaches your backend.

Zero-LLM latency

Unlike prompt-based guardrails that rely on a secondary LLM to evaluate safety, the Nexus Gateway evaluates execution payloads against compiled Network Policy Language (NPL) rules entirely in-memory.

Metric	Value
p50 latency	~160 ms
p99 latency	< 201 ms
Token cost	0

Policies compile from the open npl-spec repository. View open-source spec →

Inspection pipeline

flowchart TD
  A[JSON-RPC request] --> B{Valid RPC 2.0?}
  B -->|no| X[BLOCK: MCP_MALFORMED_RPC]
  B -->|yes| C{Server whitelisted?}
  C -->|no| X2[BLOCK: MCP_SERVER_NOT_WHITELISTED]
  C -->|yes| D{Method blocked?}
  D -->|yes| X3[BLOCK: MCP_METHOD_BLOCKED]
  D -->|no| E{Destructive op?}
  E -->|yes| X4[BLOCK: MCP_DESTRUCTIVE_OP_BLOCKED]
  E -->|no| F{NPL policy}
  F -->|deny| X5[BLOCK + audit]
  F -->|require_human| H[Approval queue]
  F -->|allow| G[Forward to backend]

Decision types

Decision	Behavior
`allow`	Forward to backend (or mock in simulation mode)
`deny`	Block with stable `reasonCode`, audit before agent sees error
`require_human_approval`	Queue for operator review, block until resolved

Inspect an MCP call

curl -X POST "https://api.hardalion.com/api/v1/mcp/gateway/inspect" \
  -H "Authorization: Bearer $API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "agent_id": "agt_prod_f8a92b",
    "execution_id": "exec_01hxyz",
    "server_id": "enterprise-db",
    "tool_name": "query_ledger",
    "execution_mode": "production",
    "request": {
      "jsonrpc": "2.0",
      "id": 1,
      "method": "tools/call",
      "params": {
        "name": "query_ledger",
        "arguments": { "account_id": "ACC-4421" }
      }
    }
  }'

Scope required: agents:execute.

Connecting a LangChain agent

Wrap your tools with the Nexus SDK and provide your AgentIAM passport:

import { NexusGateway } from '@hardalion/sdk'
import { myEnterpriseTools } from './tools'

const securedTools = NexusGateway.wrap(
  { invoke: myEnterpriseTools.invoke.bind(myEnterpriseTools) },
  {
    tenantId: 'ten_acme',
    agentId: 'agt_prod_f8a92b',
    apiKey: process.env.HARDALION_API_KEY,
    baseUrl: 'https://api.hardalion.com',
    policy: 'hardalion://nato-prou-strict-financial@1.0.0',
  },
)

// Pass securedTools to your LangChain or CrewAI instance.

Execution modes

Mode	Behavior
`production`	Inspect and forward allowed calls to real backends
`simulation`	Inspect and return mock responses (no backend contact)
`shadow`	Evaluate policies, log counterfactual blocks, do not enforce (see Shadow mode)

Policy pinning

Pin immutable policy versions with registry URIs:

hardalion://nato-prou-strict-financial@1.0.0

View bundled policies: NPL reference policies.

Gateway policies

Per-tenant MCP gateway policies configure:

Trusted server whitelist
Blocked methods and tools
Destructive operation denial
Data exfiltration heuristics

Configure via the Nexus console or tenant API.