CAPI Gateway MCP server

CAPI Gateway

Lightweight API Gateway

CAPI is a lightweight API Gateway and load balancer built on Undertow's async proxy architecture. Services register themselves in HashiCorp Consul, and CAPI automatically discovers them, creates proxy handlers, and applies security, throttling, and observability policies — no database required.

Features

• REST, WebSocket, and SSE gateway with fully async proxying (zero threads blocked during backend calls)
• Service discovery via HashiCorp Consul (automatic route creation and removal)
• OAuth2 / OIDC token validation (multi-provider, cookie-based auth supported)
• API Key authentication with per-key throttling (via Consul KV)
• Fine-grained authorization via OPA (Open Policy Agent, async policy evaluation)
• Distributed throttling (Hazelcast, with Kubernetes discovery)
• Load balancing (Round Robin) and Failover
• Distributed tracing (OpenTelemetry / OTLP)
• Prometheus metrics
• OpenAPI spec aggregation from upstream services
• TLS termination and dynamic truststore management (via Consul KV hot-reload)
• CORS management

REST-to-MCP Bridging

CAPI's MCP Gateway turns any existing REST API into an MCP tool — no code changes on your backends.

Register a service in Consul with MCP metadata, and LLM agents (Claude Desktop, Cursor, custom agents) can discover and invoke it immediately. Your REST services don't need to know anything about MCP, JSON-RPC, or tool schemas. CAPI handles the translation:

LLM Agent                          CAPI                          Your REST API
                                                                 (unchanged)
  tools/list  ──────────────►  reads Consul metadata
              ◄──────────────  returns tool catalog

  tools/call  ──────────────►  extracts arguments
              {"name":"..."}   POST /endpoint  ──────────────►  handles request
                               wraps response  ◄──────────────  returns JSON
              ◄──────────────  MCP-formatted result

CAPI also supports real MCP Server backends — services that already speak JSON-RPC 2.0. Register them with mcp-type: server and CAPI discovers their tools automatically via tools/list, then proxies tools/call requests transparently. Both REST and MCP Server backends are aggregated under one unified MCP endpoint.

Unlike dedicated MCP gateways that require all backends to be MCP Servers, CAPI bridges the gap between existing REST infrastructure and LLM-native protocols. See the MCP Gateway docs and the demo for a working example with both backend types.

Quickstart

CAPI requires the CAPICONFIGFILE environment variable pointing to a valid configuration file.

Running from JAR

Shortened. The full README is on GitHub.