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Remote Server Auth: Architecture

This document describes the internal design of the API key authentication system used when the MCP for Unity server runs in remote-hosted mode. It is intended for contributors and maintainers.

Overview

MCP Client                    MCP Server                      External Auth
(Cursor, etc.)               (Python)                         Service
     |                            |                               |
     |  X-API-Key: abc123        |                               |
     |  POST /mcp (tool call)    |                               |
     |-------------------------->|                               |
     |                           |                               |
     |          UnityInstanceMiddleware.on_call_tool              |
     |                           |                               |
     |                   _resolve_user_id()                      |
     |                           |                               |
     |                           |  POST /validate               |
     |                           |  {"api_key": "abc123"}        |
     |                           |------------------------------>|
     |                           |                               |
     |                           |  {"valid":true,               |
     |                           |   "user_id":"user-42"}        |
     |                           |<------------------------------|
     |                           |                               |
     |                   Cache result (TTL)                      |
     |                           |                               |
     |            ctx.set_state("user_id", "user-42")            |
     |            ctx.set_state("unity_instance", "Proj@hash")   |
     |                           |                               |
     |            PluginHub.send_command_for_instance             |
     |            (user_id scoped session lookup)                 |
     |                           |                               |
     |  Tool result              |                               |
     |<--------------------------|                               |


Unity Plugin                  MCP Server                      External Auth
(C# WebSocket)               (Python)                         Service
     |                            |                               |
     |  WS /hub/plugin            |                               |
     |  X-API-Key: abc123        |                               |
     |-------------------------->|                               |
     |                           |                               |
     |              PluginHub.on_connect                          |
     |                           |  POST /validate               |
     |                           |------------------------------>|
     |                           |  {"valid":true, ...}          |
     |                           |<------------------------------|
     |                           |                               |
     |  accept()                 |                               |
     |  websocket.state.user_id = "user-42"                      |
     |<--------------------------|                               |
     |                           |                               |
     |  {"type":"register", ...} |                               |
     |-------------------------->|                               |
     |                           |                               |
     |          PluginRegistry.register(                          |
     |              ..., user_id="user-42")                      |
     |          _user_hash_to_session[("user-42","hash")] = sid  |
     |                           |                               |
     |  {"type":"registered"}    |                               |
     |<--------------------------|                               |

Components

ApiKeyService

File: Server/src/services/api_key_service.py

Singleton service that validates API keys against an external HTTP endpoint.

  • Singleton access: ApiKeyService.get_instance() / ApiKeyService.is_initialized()
  • Initialization: Constructed in create_mcp_server() when config.http_remote_hosted and config.api_key_validation_url are both set.
  • Validation: async validate(api_key) -> ValidationResult
  • Caching: In-memory dict keyed by raw API key. Entries store (valid, user_id, metadata, expires_at).
  • Retry: 1 retry with 100ms backoff on timeouts and connection errors.
  • Fail-closed: Any unrecoverable error returns ValidationResult(valid=False).

PluginHub (WebSocket Auth Gate)

File: Server/src/transport/plugin_hub.py

The on_connect method validates the API key from the WebSocket handshake headers before accepting the connection.

  • Reads X-API-Key from websocket.headers
  • Validates via ApiKeyService.validate()
  • Stores user_id and api_key_metadata on websocket.state for use during registration
  • Rejects with close codes: 4401 (missing), 4403 (invalid), 1013 (service unavailable)

The _handle_register method reads websocket.state.user_id and passes it to PluginRegistry.register().

The get_sessions(user_id=None) and _resolve_session_id(unity_instance, user_id=None) methods accept an optional user_id to scope session queries in remote-hosted mode.

PluginRegistry (Dual-Index Session Storage)

File: Server/src/transport/plugin_registry.py

In-memory registry of connected Unity plugin sessions. Maintains two parallel index maps:

Index Key Used In
_hash_to_session project_hash -> session_id Local mode
_user_hash_to_session (user_id, project_hash) -> session_id Remote-hosted mode

Both indexes are updated during register() and cleaned up during unregister().

Key methods:

  • register(session_id, project_name, project_hash, unity_version, user_id=None) - Registers a session and updates the appropriate index. If an existing session claims the same key, it is evicted.
  • get_session_id_by_hash(project_hash) - Local-mode lookup.
  • get_session_id_by_hash(project_hash, user_id) - Remote-mode lookup.
  • list_sessions(user_id=None) - Returns sessions filtered by user. Raises ValueError if user_id is None while config.http_remote_hosted is True, preventing accidental cross-user leaks.

UnityInstanceMiddleware

File: Server/src/transport/unity_instance_middleware.py

FastMCP middleware that intercepts all tool and resource calls to inject the active Unity instance and user identity into the request-scoped context.

Entry points:

  • on_call_tool(context, call_next) - Intercepts tool calls.
  • on_read_resource(context, call_next) - Intercepts resource reads.

Both delegate to _inject_unity_instance(context), which:

  1. Calls _resolve_user_id() to extract the user identity from the HTTP request.
  2. If remote-hosted mode is active and no user_id is resolved, raises RuntimeError (surfaces as MCP error).
  3. Sets ctx.set_state("user_id", user_id).
  4. Looks up or auto-selects the active Unity instance.
  5. Sets ctx.set_state("unity_instance", active_instance).

_resolve_user_id_from_request

File: Server/src/transport/unity_transport.py

Extracts the user_id from the current HTTP request's X-API-Key header.

_resolve_user_id_from_request()
  -> if not config.http_remote_hosted: return None
  -> if not ApiKeyService.is_initialized(): return None
  -> get_http_headers() from FastMCP dependencies
  -> extract "x-api-key" header
  -> ApiKeyService.validate(api_key)
  -> return result.user_id if valid, else None

The middleware calls this indirectly through _resolve_user_id(), which adds an early return when not in remote-hosted mode (avoiding the import of FastMCP internals in local mode).

Request Lifecycle

A complete authenticated MCP tool call follows this path:

  1. HTTP request arrives at /mcp with X-API-Key: <key> header.

  2. FastMCP dispatches the MCP tool call through its middleware chain.

  3. UnityInstanceMiddleware.on_call_tool is invoked.

  4. _inject_unity_instance runs:

    • Calls _resolve_user_id(), which calls _resolve_user_id_from_request().
    • The request function imports get_http_headers from FastMCP and reads the x-api-key header.
    • ApiKeyService.validate() checks the cache or calls the external auth endpoint.
    • If valid, user_id is returned. If invalid or missing, None is returned.
    • In remote-hosted mode, None causes a RuntimeError.

  5. user_id stored in context via ctx.set_state("user_id", user_id).

  6. Session key derived by get_session_key(ctx):

    • Priority: client_id (if available) > user:{user_id} > "global".
    • The user:{user_id} fallback ensures session isolation when MCP transports don't provide stable client IDs.

  7. Active Unity instance looked up from _active_by_key dict using the session key. If none is set, _maybe_autoselect_instance is called (but returns None in remote-hosted mode).

  8. Instance injected via ctx.set_state("unity_instance", active_instance).

  9. Tool executes, reading the instance from ctx.get_state("unity_instance").

  10. Command routed through PluginHub.send_command_for_instance(unity_instance, ..., user_id=user_id), which resolves the session using PluginRegistry.get_session_id_by_hash(project_hash, user_id).

WebSocket Auth Flow

When a Unity plugin connects via WebSocket:

Plugin -> WS /hub/plugin (with X-API-Key header)
  |
  v
PluginHub.on_connect()
  |
  +-- config.http_remote_hosted && ApiKeyService.is_initialized()?
  |     |
  |     +-- No  -> accept() (local mode, no auth needed)
  |     |
  |     +-- Yes -> read X-API-Key from headers
  |           |
  |           +-- No key -> close(4401, "API key required")
  |           |
  |           +-- ApiKeyService.validate(key)
  |                 |
  |                 +-- valid=True  -> websocket.state.user_id = user_id
  |                 |                  accept()
  |                 |
  |                 +-- valid=False, "unavailable" in error
  |                 |                -> close(1013, "Try again later")
  |                 |
  |                 +-- valid=False -> close(4403, "Invalid API key")

After acceptance, when the plugin sends a register message, _handle_register reads websocket.state.user_id and passes it to PluginRegistry.register().

Session Registry Design

Local Mode

project_hash  ->  session_id
"abc123"      ->  "uuid-1"
"def456"      ->  "uuid-2"

A single _hash_to_session dict. Any user can see any session. list_sessions(user_id=None) returns all sessions.

Remote-Hosted Mode

(user_id, project_hash)    ->  session_id
("user-A", "abc123")       ->  "uuid-1"
("user-B", "abc123")       ->  "uuid-3"   (same project, different user)
("user-A", "def456")       ->  "uuid-2"

A separate _user_hash_to_session dict with composite keys. Two users working on cloned repos (same project_hash) get independent sessions.

Reconnect Handling

When a Unity editor reconnects (e.g., after domain reload), register() detects the existing mapping for the same key and evicts the old session before inserting the new one. This ensures the latest WebSocket connection always wins.

list_sessions Guard

list_sessions(user_id=None) raises ValueError when config.http_remote_hosted is True. This prevents code paths from accidentally listing all users' sessions. Every call site in remote-hosted mode must pass an explicit user_id.

Caching Strategy

ApiKeyService maintains an in-memory cache:

# api_key -> (valid, user_id, metadata, expires_at)
_cache: dict[str, tuple[bool, str | None, dict | None, float]]

What Gets Cached

Response Cached? Rationale
200 + valid: true Yes Definitive valid result
200 + valid: false Yes Definitive invalid result
401 status Yes Definitive rejection
5xx status No Transient; retry on next request
Timeout No Transient; retry on next request
Connection error No Transient; retry on next request
Unexpected exception No Transient; retry on next request

Non-cacheable results use ValidationResult(cacheable=False).

Cache Lifecycle

  • TTL: Configurable via --api-key-cache-ttl (default: 300 seconds).
  • Expiry: Checked on read. Expired entries are deleted and re-validated.
  • Invalidation: invalidate_cache(api_key) removes a single key. clear_cache() removes all.
  • Concurrency: Protected by asyncio.Lock.

Revocation Latency

A revoked key continues to work for up to cache_ttl seconds. Lower the TTL for faster revocation at the cost of more validation requests.

Fail-Closed Behaviour

The system fails closed at every boundary:

Component Failure Behaviour
ApiKeyService._validate_external Timeout after retries valid=False, cacheable=False
ApiKeyService._validate_external Connection error after retries valid=False, cacheable=False
ApiKeyService._validate_external 5xx status valid=False, cacheable=False
ApiKeyService._validate_external Unexpected exception valid=False, cacheable=False
PluginHub.on_connect Auth service unavailable Close 1013 (retry hint)
UnityInstanceMiddleware._inject_unity_instance No user_id in remote-hosted mode RuntimeError

API keys are never logged in full. Keys longer than 8 characters are redacted to xxxx...yyyy in log messages.

Session Key Derivation

UnityInstanceMiddleware.get_session_key(ctx) determines which dict key to use for storing/retrieving the active Unity instance per session:

1. client_id (string, non-empty)  ->  return client_id
2. ctx.get_state("user_id")       ->  return "user:{user_id}"
3. fallback                       ->  return "global"
  • client_id: Stable per MCP client connection. Preferred when available.
  • user:{user_id}: Used in remote-hosted mode when the MCP transport doesn't provide a stable client ID. Ensures different users don't share instance selections.
  • "global": Local-dev fallback for single-user scenarios. Unreachable in remote-hosted mode because the auth enforcement raises RuntimeError before this point if no user_id is available.

Disabled Features in Remote-Hosted Mode

Feature Local Mode Remote-Hosted Mode Reason
Auto-select sole instance Enabled Disabled Implicit behaviour is dangerous with multiple users
CLI REST routes Enabled Disabled No auth layer on these endpoints
list_sessions(user_id=None) Returns all Raises ValueError Prevents accidental cross-user session leaks

Configuration Flow

CLI args / env vars
       |
       v
main.py: parser.parse_args()
       |
       +-- config.http_remote_hosted = args or env
       +-- config.api_key_validation_url = args or env
       +-- config.api_key_login_url = args or env
       +-- config.api_key_cache_ttl = args or env (float)
       +-- config.api_key_service_token_header = args or env
       +-- config.api_key_service_token = args or env
       |
       +-- Validate: remote-hosted requires validation URL
       |     (exits with code 1 if missing)
       |
       v
create_mcp_server()
       |
       +-- get_unity_instance_middleware()  ->  registers middleware
       |
       +-- if remote-hosted + validation URL:
       |     ApiKeyService(
       |       validation_url, cache_ttl,
       |       service_token_header, service_token
       |     )
       |
       +-- WebSocketRoute("/hub/plugin", PluginHub)
       |
       +-- if not remote-hosted:
             register CLI routes (/api/command, /api/instances, /api/custom-tools)

Key Files

File Role
Server/src/core/config.py ServerConfig dataclass with auth fields
Server/src/main.py CLI argument parsing, startup validation, service initialization
Server/src/services/api_key_service.py API key validation singleton with caching and retry
Server/src/transport/plugin_hub.py WebSocket auth gate, user-scoped session queries
Server/src/transport/plugin_registry.py Dual-index session storage (local + user-scoped)
Server/src/transport/unity_instance_middleware.py Per-request user_id and instance injection
Server/src/transport/unity_transport.py _resolve_user_id_from_request helper