mcp-secure-remote

A stdio ↔ remote bridge for the Model Context Protocol 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

2. How it works

3. Prerequisites

4. Install

5. Generate or obtain client certificates

6. Quick start

7. CLI parameters

8. Environment variables

9. AI agent / IDE integration

   • Claude Desktop

   • Claude Code (CLI)

   • Cursor

   • Windsurf

   • Cline (VS Code)

   • Continue (VS Code / JetBrains)

   • Zed

   • Generic MCP client

10. Testing your setup

11. Security notes

12. Troubleshooting

13. Development

14. License

Related MCP server: Streamable HTTP Bridge

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

# 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:

# 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:

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:

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:

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

mTLS / TLS

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.

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).

{
  "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:

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:

{
  "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).

{
  "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.

{
  "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:

{
  "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).

{
  "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.

{
  "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:

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

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.