# mcp-secure-remote

A stdio ↔ remote bridge for the [Model Context Protocol](https://modelcontextprotocol.io)
with first-class **mTLS (mutual TLS) client-certificate authentication**.

Works with any MCP-capable AI agent or IDE — Claude Desktop, Claude Code,
Cursor, Windsurf, Cline, Continue, Zed, VS Code MCP extensions, and any
custom client that speaks the MCP stdio transport.

---

## Contents

1. [What it does](#what-it-does)
2. [How it works](#how-it-works)
3. [Prerequisites](#prerequisites)
4. [Install](#install)
5. [Generate or obtain client certificates](#generate-or-obtain-client-certificates)
6. [Quick start](#quick-start)
7. [CLI parameters](#cli-parameters)
8. [Environment variables](#environment-variables)
9. [AI agent / IDE integration](#ai-agent--ide-integration)
   - [Claude Desktop](#claude-desktop)
   - [Claude Code (CLI)](#claude-code-cli)
   - [Cursor](#cursor)
   - [Windsurf](#windsurf)
   - [Cline (VS Code)](#cline-vs-code)
   - [Continue (VS Code / JetBrains)](#continue-vs-code--jetbrains)
   - [Zed](#zed)
   - [Generic MCP client](#generic-mcp-client)
10. [Testing your setup](#testing-your-setup)
11. [Security notes](#security-notes)
12. [Troubleshooting](#troubleshooting)
13. [Development](#development)
14. [License](#license)

---

## What it does

`mcp-secure-remote` spawns as a local stdio MCP server and forwards every
JSON-RPC message to a remote MCP server over HTTPS. Every outbound request
carries a client certificate you supply, so the remote server sees a
cryptographically authenticated connection — no OAuth dance, no bearer
tokens on the wire, no shared API keys.

```
┌──────────────┐   stdio    ┌────────────────────┐   HTTPS + mTLS   ┌───────────────┐
│ MCP client   │───────────▶│ mcp-secure-remote  │─────────────────▶│ Remote MCP    │
│ (Claude,     │            │ (this proxy)       │                  │ server        │
│  Cursor, …)  │◀───────────│                    │◀─────────────────│               │
└──────────────┘            └────────────────────┘                  └───────────────┘
```

## How it works

1. AI agent launches `mcp-secure-remote` as a local subprocess and talks to
   it over stdio (the transport every MCP client already supports).
2. Proxy builds an undici HTTPS dispatcher seeded with your client cert,
   private key, and trusted CA bundle.
3. Proxy opens either a Streamable HTTP or SSE transport to the remote
   server (configurable). TLS handshake presents the client cert; server
   validates it before forwarding the MCP session.
4. JSON-RPC frames flow bidirectionally. All proxy logging goes to stderr
   so the stdio channel stays clean.

## Prerequisites

- Node.js **≥ 18** (for built-in `fetch`, `undici`, and native TLS features).
- A client certificate + private key issued by a CA the remote MCP server
  trusts (or a PKCS#12 bundle containing both).
- The CA bundle used by the remote server, if it is not in your OS trust
  store (private/corporate CAs almost always need this).
- The remote MCP server URL (typically `https://host/mcp` or
  `https://host/sse`).

## Install

```bash
# global
npm install -g mcp-secure-remote

# or ephemeral (recommended for agent configs)
npx mcp-secure-remote <server-url> [options]
```

## Generate or obtain client certificates

If your team already issues client certs, skip this section. For local
testing, generate a throw-away CA + client cert pair with OpenSSL:

```bash
# CA
openssl req -x509 -newkey rsa:4096 -sha256 -days 365 -nodes \
  -keyout ca.key -out ca.crt -subj "/CN=dev-ca"

# client key + CSR
openssl req -newkey rsa:4096 -nodes \
  -keyout client.key -out client.csr -subj "/CN=dev-client"

# sign client cert with CA
openssl x509 -req -in client.csr -CA ca.crt -CAkey ca.key -CAcreateserial \
  -out client.crt -days 365 -sha256
```

Configure the remote MCP server to require client certs signed by `ca.crt`.
Point the proxy at `client.crt` + `client.key` + the server's CA bundle.

## Quick start

Cert + key pair:

```bash
npx mcp-secure-remote https://mcp.example.com/mcp \
  --tls-cert ./certs/client.crt \
  --tls-key  ./certs/client.key \
  --tls-ca   ./certs/ca-bundle.pem
```

PKCS#12 bundle:

```bash
npx mcp-secure-remote https://mcp.example.com/mcp \
  --tls-pfx       ./certs/client.p12 \
  --tls-passphrase "$P12_PASSPHRASE" \
  --tls-ca        ./certs/ca-bundle.pem
```

Force SSE transport + pin minimum TLS:

```bash
npx mcp-secure-remote https://mcp.example.com/sse \
  --transport sse-only \
  --tls-min-version TLSv1.3 \
  --tls-cert ./certs/client.crt \
  --tls-key  ./certs/client.key \
  --tls-ca   ./certs/ca-bundle.pem
```

## CLI parameters

Usage: `mcp-secure-remote <server-url> [options]`

`<server-url>` is a positional argument (required). Everything else is a
named flag.

### General

| Flag | Type | Default | Description |
| --- | --- | --- | --- |
| `<server-url>` | string (URL) | — | **Required.** Remote MCP endpoint. Must be `https://…` unless `--allow-http` is set. |
| `--header "Name: value"` | string (repeatable) | — | Extra HTTP header on every outbound request. Repeat the flag for multiple headers. |
| `--transport <strategy>` | enum | `http-first` | Transport negotiation. One of `http-first`, `sse-first`, `http-only`, `sse-only`. `-first` variants try the preferred transport then fall back; `-only` variants never fall back. |
| `--allow-http` | boolean | `false` | Permit plain `http://` URLs. Off by default; mTLS is meaningless over HTTP. |
| `--debug` | boolean | `false` | Verbose logging to stderr (parsed args, per-message trace, transport selection). |
| `-h`, `--help` | boolean | — | Print usage and exit. |

### mTLS / TLS

| Flag | Type | Default | Description |
| --- | --- | --- | --- |
| `--tls-cert <path>` | path | — | PEM client certificate (leaf, optionally followed by chain intermediates). |
| `--tls-key <path>` | path | — | PEM private key matching `--tls-cert`. Must be supplied together with `--tls-cert`. |
| `--tls-ca <path>` | path | — | PEM CA bundle used to verify the remote server. Required for private CAs not in the OS trust store. |
| `--tls-pfx <path>` | path | — | PKCS#12 (`.pfx` / `.p12`) bundle. Mutually exclusive with `--tls-cert`/`--tls-key`. |
| `--tls-passphrase <value>` | string | — | Passphrase protecting the private key or PFX bundle. Prefer the env var to keep secrets off the command line. |
| `--tls-servername <name>` | string | URL hostname | SNI override. Use when the server cert's SAN differs from the URL host (e.g. IP literal, internal DNS). |
| `--tls-min-version <ver>` | enum | Node default | Minimum TLS version: `TLSv1.2` or `TLSv1.3`. |
| `--tls-insecure-skip-verify`, `--tls-no-verify` | boolean | `false` | Disable server certificate validation. **Dev only.** Proxy prints a warning when enabled. |

### Parameter rules

- `--tls-cert` and `--tls-key` must appear together.
- `--tls-pfx` cannot combine with `--tls-cert`/`--tls-key`.
- `--allow-http` is required for any `http://` URL. Supplying mTLS flags
  with `http://` triggers a warning (cert is not sent over plain HTTP).
- Unknown `--flags` cause parse failure with exit code 2.
- Argument errors exit with code 2; runtime errors exit with code 1.

## Environment variables

Every TLS flag has an env-var fallback so secrets can stay out of shell
history and MCP client configs.

| Variable | Equivalent flag | Values |
| --- | --- | --- |
| `MCP_REMOTE_TLS_CERT` | `--tls-cert` | path |
| `MCP_REMOTE_TLS_KEY` | `--tls-key` | path |
| `MCP_REMOTE_TLS_CA` | `--tls-ca` | path |
| `MCP_REMOTE_TLS_PFX` | `--tls-pfx` | path |
| `MCP_REMOTE_TLS_PASSPHRASE` | `--tls-passphrase` | string |
| `MCP_REMOTE_TLS_SERVERNAME` | `--tls-servername` | string |
| `MCP_REMOTE_TLS_MIN_VERSION` | `--tls-min-version` | `TLSv1.2` \| `TLSv1.3` |
| `MCP_REMOTE_TLS_INSECURE` | `--tls-insecure-skip-verify` | `1` / `true` / `yes` to disable verify |

Precedence: explicit CLI flag overrides env var.

## AI agent / IDE integration

Every agent below launches the proxy as a local stdio MCP server. Pattern
is identical — only the config file format differs. **Use absolute paths**;
agents do not inherit your shell's working directory.

### Claude Desktop

File: `~/Library/Application Support/Claude/claude_desktop_config.json`
(macOS) or `%APPDATA%\Claude\claude_desktop_config.json` (Windows).

```json
{
  "mcpServers": {
    "example": {
      "command": "npx",
      "args": [
        "mcp-secure-remote",
        "https://mcp.example.com/mcp",
        "--tls-cert", "/absolute/path/client.crt",
        "--tls-key",  "/absolute/path/client.key",
        "--tls-ca",   "/absolute/path/ca-bundle.pem"
      ]
    }
  }
}
```

Restart Claude Desktop after editing.

### Claude Code (CLI)

Add a server via the `claude mcp add` command or edit
`~/.claude.json` / project `.mcp.json`:

```bash
claude mcp add example npx -- mcp-secure-remote \
  https://mcp.example.com/mcp \
  --tls-cert /absolute/path/client.crt \
  --tls-key  /absolute/path/client.key \
  --tls-ca   /absolute/path/ca-bundle.pem
```

Or in `.mcp.json`:

```json
{
  "mcpServers": {
    "example": {
      "command": "npx",
      "args": [
        "mcp-secure-remote",
        "https://mcp.example.com/mcp",
        "--tls-cert", "/absolute/path/client.crt",
        "--tls-key",  "/absolute/path/client.key",
        "--tls-ca",   "/absolute/path/ca-bundle.pem"
      ]
    }
  }
}
```

### Cursor

File: `~/.cursor/mcp.json` (global) or `.cursor/mcp.json` (per project).

```json
{
  "mcpServers": {
    "example": {
      "command": "npx",
      "args": [
        "mcp-secure-remote",
        "https://mcp.example.com/mcp",
        "--tls-cert", "/absolute/path/client.crt",
        "--tls-key",  "/absolute/path/client.key",
        "--tls-ca",   "/absolute/path/ca-bundle.pem"
      ],
      "env": {
        "MCP_REMOTE_TLS_PASSPHRASE": "…optional…"
      }
    }
  }
}
```

### Windsurf

File: `~/.codeium/windsurf/mcp_config.json`.

```json
{
  "mcpServers": {
    "example": {
      "command": "npx",
      "args": [
        "mcp-secure-remote",
        "https://mcp.example.com/mcp",
        "--tls-cert", "/absolute/path/client.crt",
        "--tls-key",  "/absolute/path/client.key",
        "--tls-ca",   "/absolute/path/ca-bundle.pem"
      ]
    }
  }
}
```

### Cline (VS Code)

Cline reads `cline_mcp_settings.json` from its extension storage. Open
the Cline MCP panel → "Configure MCP Servers" or edit the file directly:

```json
{
  "mcpServers": {
    "example": {
      "command": "npx",
      "args": [
        "mcp-secure-remote",
        "https://mcp.example.com/mcp",
        "--tls-cert", "/absolute/path/client.crt",
        "--tls-key",  "/absolute/path/client.key",
        "--tls-ca",   "/absolute/path/ca-bundle.pem"
      ],
      "disabled": false,
      "autoApprove": []
    }
  }
}
```

### Continue (VS Code / JetBrains)

File: `~/.continue/config.json` (or `config.yaml`).

```json
{
  "experimental": {
    "modelContextProtocolServers": [
      {
        "transport": {
          "type": "stdio",
          "command": "npx",
          "args": [
            "mcp-secure-remote",
            "https://mcp.example.com/mcp",
            "--tls-cert", "/absolute/path/client.crt",
            "--tls-key",  "/absolute/path/client.key",
            "--tls-ca",   "/absolute/path/ca-bundle.pem"
          ]
        }
      }
    ]
  }
}
```

### Zed

File: `~/.config/zed/settings.json`.

```json
{
  "context_servers": {
    "example": {
      "command": {
        "path": "npx",
        "args": [
          "mcp-secure-remote",
          "https://mcp.example.com/mcp",
          "--tls-cert", "/absolute/path/client.crt",
          "--tls-key",  "/absolute/path/client.key",
          "--tls-ca",   "/absolute/path/ca-bundle.pem"
        ]
      }
    }
  }
}
```

### Generic MCP client

Any client that spawns stdio MCP servers works. Required pieces:

- `command`: `npx` (or absolute path to `node` + `dist/proxy.js`).
- `args`: `["mcp-secure-remote", "<server-url>", …flags]`.
- Optional `env` block for `MCP_REMOTE_TLS_*` variables to keep secrets
  out of the args array.

## Testing your setup

Bundled `mcp-secure-remote-client` verifies the TLS handshake and enumerates the
server's capabilities — no real agent needed:

```bash
npx mcp-secure-remote-client https://mcp.example.com/mcp \
  --tls-cert ./certs/client.crt \
  --tls-key  ./certs/client.key \
  --tls-ca   ./certs/ca-bundle.pem
```

Output: negotiated capabilities + lists of tools, resources, prompts.

Add `--debug` for per-message tracing.

## Security notes

- **HTTPS only by default.** `http://` URLs are refused unless
  `--allow-http` is explicitly set. Proxy additionally warns when mTLS
  flags are combined with `http://` because the client cert will not be
  sent.
- **Skip-verify prints a warning.** `--tls-insecure-skip-verify` disables
  server certificate validation; intended for local dev loops only.
- **Prefer env vars for passphrases.** Anything on the CLI may leak into
  process listings, shell history, or agent logs.
- **Proxy logs to stderr.** stdout is reserved for the MCP JSON-RPC stream.
- **No credential persistence.** Proxy does not write certs, keys, or
  tokens to disk.
- **Pin TLS 1.3** (`--tls-min-version TLSv1.3`) when the server supports
  it, to avoid downgrade-prone 1.2 cipher suites.

## Troubleshooting

**`self signed certificate in certificate chain` / `unable to verify the first certificate`**
Point `--tls-ca` at the PEM bundle that signed the remote server's cert.
OS trust store alone is not enough for private CAs.

**`Hostname/IP does not match certificate's altnames`**
Set `--tls-servername` to the SAN the server cert presents.

**`error:0909006C:PEM routines:get_name:no start line`**
Private key file malformed or encrypted. If encrypted, supply
`--tls-passphrase` (or `MCP_REMOTE_TLS_PASSPHRASE`).

**`ERR_SSL_SSLV3_ALERT_HANDSHAKE_FAILURE` / `alert bad certificate`**
Server rejected your client cert. Check:
- Cert signed by a CA the server trusts.
- Key matches cert:
  `openssl x509 -noout -modulus -in client.crt | openssl md5`
  vs. `openssl rsa -noout -modulus -in client.key | openssl md5`.
- Intermediate chain present in `--tls-cert`.

**Agent shows "failed to start server" with no detail.**
Run the exact same command in a terminal to see stderr. Agents hide
subprocess stderr by default.

**Remote transport hangs.**
Try `--transport sse-only` or `--transport http-only` to isolate which
transport the server actually implements. Add `--debug`.

**`already started` error in `mcp-secure-remote-client`.**
Upgrade — prior versions double-started the transport. Fixed in current
release.

## Development

```bash
npm install
npm run typecheck
npm run build
```

Build artifacts land in `dist/`. `dist/proxy.js` and `dist/client.js` are
the two bin entrypoints.

## License

MIT — see [LICENSE](./LICENSE).