Remote Server API Key Authentication

When running the MCP for Unity server as a shared remote service, API key authentication ensures that only authorized users can access the server and that each user's Unity sessions are isolated from one another.

This guide covers how to configure, deploy, and use the feature.

Prerequisites

External Auth Service

You need an external HTTP endpoint that validates API keys. The server delegates all key validation to this endpoint rather than managing keys itself.

The endpoint must:

• Accept POST requests with a JSON body: {"api_key": "<key>"}
• Return a JSON response indicating validity and the associated user identity
• Be reachable from the MCP server over the network

See Validation Contract for the full request/response specification.

Transport Mode

API key authentication is only available when running with HTTP transport (--transport http). It has no effect in stdio mode.

Server Configuration

CLI Arguments

Environment variables take effect when the corresponding CLI argument is not provided. For boolean flags, set the env var to true, 1, or yes.

Startup Validation

The server validates its configuration at startup:

• If --http-remote-hosted is set but --api-key-validation-url is not provided (and the env var is also unset), the server logs an error and exits with code 1.

Example

python -m src.main \
  --transport http \
  --http-host 0.0.0.0 \
  --http-port 8080 \
  --http-remote-hosted \
  --api-key-validation-url https://auth.example.com/api/validate-key \
  --api-key-login-url https://app.example.com/api-keys \
  --api-key-cache-ttl 120

Or using environment variables:

export UNITY_MCP_TRANSPORT=http
export UNITY_MCP_HTTP_HOST=0.0.0.0
export UNITY_MCP_HTTP_PORT=8080
export UNITY_MCP_HTTP_REMOTE_HOSTED=true
export UNITY_MCP_API_KEY_VALIDATION_URL=https://auth.example.com/api/validate-key
export UNITY_MCP_API_KEY_LOGIN_URL=https://app.example.com/api-keys

python -m src.main

Service Token (Optional)

If your auth service requires the MCP server to authenticate itself (server-to-server auth), configure a service token:

--api-key-service-token-header X-Service-Token \
--api-key-service-token "your-server-secret"

This adds the specified header to every validation request sent to the auth endpoint.

We strongly recommend using this feature because it ensures that the entity requesting validation is the MCP server itself, not an imposter.

Unity Plugin Setup

When connecting to a remote-hosted server, Unity users need to provide their API key:

1. Open the MCP for Unity window in the Unity Editor.
2. Select HTTP Remote as the connection mode.
3. Enter the API key in the API Key field. The key is stored in EditorPrefs (per-machine, not source-controlled).
4. Click Get API Key to open the login URL in a browser if you need a new key. This fetches the URL from the server's /api/auth/login-url endpoint.

The API key is a one-time entry per machine. It persists across Unity sessions until explicitly cleared.

MCP Client Configuration

When an API key is configured, the Unity plugin's MCP client configurators automatically include the X-API-Key header in generated configuration files.

Example generated config for Cursor (~/.cursor/mcp.json):

{
  "mcpServers": {
    "mcp-for-unity": {
      "url": "http://remote-server:8080/mcp",
      "headers": {
        "X-API-Key": "<your-api-key>"
      }
    }
  }
}

Example for Claude Code (CLI):

claude mcp add --transport http mcp-for-unity http://remote-server:8080/mcp \
  --header "X-API-Key: <your-api-key>"

Similar header injection works for VS Code, Windsurf, Cline, and other supported MCP clients.

Behaviour Changes in Remote-Hosted Mode

Enabling --http-remote-hosted changes several server behaviours compared to the default local mode:

Authentication Enforcement

All MCP tool and resource calls require a valid API key. The X-API-Key header must be present on every HTTP request to the /mcp endpoint. If the key is missing or invalid, the middleware raises a RuntimeError that surfaces as an MCP error response.

WebSocket Auth Gate

Unity plugins connecting via WebSocket (/hub/plugin) are validated during the handshake:

Session Isolation

Each user can only see and interact with their own Unity instances. When User A calls set_active_instance or lists instances, they only see Unity editors that connected with User A's API key. User B's sessions are invisible to User A.

Auto-Select Disabled

In local mode, the server automatically selects the sole connected Unity instance. In remote-hosted mode, this auto-selection is disabled. Users must explicitly call set_active_instance with a Name@hash from the mcpforunity://instances resource.

CLI Routes Disabled

The following REST endpoints are disabled in remote-hosted mode to prevent unauthenticated access:

• POST /api/command
• GET /api/instances
• GET /api/custom-tools

Endpoints Always Available

These endpoints remain accessible regardless of auth:

Validation Contract

Request

POST <api-key-validation-url>
Content-Type: application/json

{
  "api_key": "<the-api-key>"
}

If a service token is configured, an additional header is sent:

<service-token-header>: <service-token-value>

Response (Valid Key)

{
  "valid": true,
  "user_id": "user-abc-123",
  "metadata": {}
}

• valid (bool, required): Must be true.
• user_id (string, required): Stable identifier for the user. Used for session isolation.
• metadata (object, optional): Arbitrary metadata stored alongside the validation result.

Response (Invalid Key)

{
  "valid": false,
  "error": "API key expired"
}

• valid (bool, required): Must be false.
• error (string, optional): Human-readable reason.

Response (HTTP 401)

A 401 status code is also treated as an invalid key (no body parsing required).

Timeouts and Retries

• Request timeout: 5 seconds
• Retries: 1 (with 100ms backoff)
• Failure mode: deny by default (treated as invalid on any error)

Transient failures (5xx, timeouts, network errors) are not cached, so subsequent requests will retry the auth service.

Error Reference

Troubleshooting

"API key authentication required" error on every tool call

The server is in remote-hosted mode but no API key is being sent. Ensure the MCP client configuration includes the X-API-Key header, or set it in the Unity plugin's connection settings.

Server exits immediately with code 1

The --http-remote-hosted flag requires --api-key-validation-url. Provide the URL via CLI argument or UNITY_MCP_API_KEY_VALIDATION_URL environment variable.

WebSocket connection closes with 4401

The Unity plugin is not sending an API key. Enter the key in the MCP for Unity window's connection settings.

WebSocket connection closes with 1013

The external auth service is unreachable. Check network connectivity between the MCP server and the validation URL. The Unity plugin can retry the connection.

User cannot see their Unity instance

Session isolation is active. The Unity editor and the MCP client must use API keys that resolve to the same user_id. Verify that the Unity plugin's WebSocket connection and the MCP client's HTTP requests use the same API key.

Stale auth after key rotation

Validated keys are cached for --api-key-cache-ttl seconds (default: 300). After rotating or revoking a key, there is a delay equal to the TTL before the old key stops working. Lower the TTL for faster revocation at the cost of more frequent validation requests.