Fix: HTTP/Stdio transport routing and middleware session persistence (#422)

* Fix: HTTP/Stdio transport routing and middleware session persistence

- Ensure session persistence for stdio transport by using stable client_id/global fallback.

- Fix Nagle algorithm latency issues by setting TCP_NODELAY.

- Cleanup duplicate logging and imports.

- Resolve C# NullReferenceException in EditorWindow health check.

* Fix: thread-safe middleware singleton and re-enable repo stats schedule

MCPForUnity/Editor/Windows/MCPForUnityEditorWindow.cs

@@ -214,11 +214,21 @@ namespace MCPForUnity.Editor.Windows
         {
             EditorApplication.delayCall += async () =>
             {
+                // Ensure window and components are still valid before execution
                 if (this == null || connectionSection == null)
                 {
                     return;
                 }
+
+                try
+                {
                     await connectionSection.VerifyBridgeConnectionAsync();
+                }
+                catch (Exception ex)
+                {
+                    // Log but don't crash if verification fails during cleanup
+                    McpLog.Warn($"Health check verification failed: {ex.Message}");
+                }
             };
         }
     }

Server/src/main.py

@@ -22,7 +22,10 @@ from services.resources import register_all_resources
 from core.telemetry import record_milestone, record_telemetry, MilestoneType, RecordType, get_package_version
 from services.tools import register_all_tools
 from transport.legacy.unity_connection import get_unity_connection_pool, UnityConnectionPool
-from transport.unity_instance_middleware import UnityInstanceMiddleware, set_unity_instance_middleware
+from transport.unity_instance_middleware import (
+    UnityInstanceMiddleware,
+    get_unity_instance_middleware
+)
 
 # Configure logging using settings from config
 logging.basicConfig(
@@ -44,22 +47,25 @@ try:
     _fh.setFormatter(logging.Formatter(config.log_format))
     _fh.setLevel(getattr(logging, config.log_level))
     logger.addHandler(_fh)
+    logger.propagate = False  # Prevent double logging to root logger
     # Also route telemetry logger to the same rotating file and normal level
     try:
         tlog = logging.getLogger("unity-mcp-telemetry")
         tlog.setLevel(getattr(logging, config.log_level))
         tlog.addHandler(_fh)
-    except Exception:
+        tlog.propagate = False  # Prevent double logging for telemetry too
+    except Exception as exc:
         # Never let logging setup break startup
-        pass
-except Exception:
+        logger.debug("Failed to configure telemetry logger", exc_info=exc)
+except Exception as exc:
     # Never let logging setup break startup
-    pass
+    logger.debug("Failed to configure main logger file handler", exc_info=exc)
 # Quieten noisy third-party loggers to avoid clutter during stdio handshake
-for noisy in ("httpx", "urllib3"):
+for noisy in ("httpx", "urllib3", "mcp.server.lowlevel.server"):
     try:
         logging.getLogger(noisy).setLevel(
             max(logging.WARNING, getattr(logging, config.log_level)))
+        logging.getLogger(noisy).propagate = False
     except Exception:
         pass
 
@@ -258,8 +264,8 @@ async def plugin_sessions_route(_: Request) -> JSONResponse:
 
 
 # Initialize and register middleware for session-based Unity instance routing
-unity_middleware = UnityInstanceMiddleware()
-set_unity_instance_middleware(unity_middleware)
+# Using the singleton getter ensures we use the same instance everywhere
+unity_middleware = get_unity_instance_middleware()
 mcp.add_middleware(unity_middleware)
 logger.info("Registered Unity instance middleware for session-based routing")
 

Server/src/services/resources/unity_instances.py

@@ -7,7 +7,7 @@ from fastmcp import Context
 from services.registry import mcp_for_unity_resource
 from transport.legacy.unity_connection import get_unity_connection_pool
 from transport.plugin_hub import PluginHub
-from transport.unity_transport import _is_http_transport
+from transport.unity_transport import _current_transport
 
 
 @mcp_for_unity_resource(
@@ -36,7 +36,8 @@ async def unity_instances(ctx: Context) -> dict[str, Any]:
     await ctx.info("Listing Unity instances")
 
     try:
-        if _is_http_transport():
+        transport = _current_transport()
+        if transport == "http":
             # HTTP/WebSocket transport: query PluginHub
             sessions_data = await PluginHub.get_sessions()
             sessions = sessions_data.sessions
@@ -71,7 +72,7 @@ async def unity_instances(ctx: Context) -> dict[str, Any]:
 
             result = {
                 "success": True,
-                "transport": "http",
+                "transport": transport,
                 "instance_count": len(instances),
                 "instances": instances,
             }
@@ -98,7 +99,7 @@ async def unity_instances(ctx: Context) -> dict[str, Any]:
 
             result = {
                 "success": True,
-                "transport": "stdio",
+                "transport": transport,
                 "instance_count": len(instances),
                 "instances": [inst.to_dict() for inst in instances],
             }

Server/src/services/tools/debug_request_context.py

@@ -3,6 +3,7 @@ from typing import Any
 from fastmcp import Context
 from services.registry import mcp_for_unity_tool
 from transport.unity_instance_middleware import get_unity_instance_middleware
+from transport.plugin_hub import PluginHub
 
 
 @mcp_for_unity_tool(
@@ -35,9 +36,16 @@ def debug_request_context(ctx: Context) -> dict[str, Any]:
 
     # Get session state info via middleware
     middleware = get_unity_instance_middleware()
-    derived_key = middleware._get_session_key(ctx)
+    derived_key = middleware.get_session_key(ctx)
     active_instance = middleware.get_active_instance(ctx)
     
+    # Debugging middleware internals
+    with middleware._lock:
+        all_keys = list(middleware._active_by_key.keys())
+
+    # Debugging PluginHub state
+    plugin_hub_configured = PluginHub.is_configured()
+
     return {
         "success": True,
         "data": {
@@ -53,6 +61,9 @@ def debug_request_context(ctx: Context) -> dict[str, Any]:
             "session_state": {
                 "derived_key": derived_key,
                 "active_instance": active_instance,
+                "all_keys_in_store": all_keys,
+                "plugin_hub_configured": plugin_hub_configured,
+                "middleware_id": id(middleware),
             },
             "available_attributes": ctx_attrs,
         },

Server/src/services/tools/set_active_instance.py

@@ -6,7 +6,7 @@ from services.registry import mcp_for_unity_tool
 from transport.legacy.unity_connection import get_unity_connection_pool
 from transport.unity_instance_middleware import get_unity_instance_middleware
 from transport.plugin_hub import PluginHub
-from transport.unity_transport import _is_http_transport
+from transport.unity_transport import _current_transport
 
 
 @mcp_for_unity_tool(
@@ -16,8 +16,10 @@ async def set_active_instance(
         ctx: Context,
         instance: Annotated[str, "Target instance (Name@hash or hash prefix)"]
 ) -> dict[str, Any]:
+    transport = _current_transport()
+
     # Discover running instances based on transport
-    if _is_http_transport():
+    if transport == "http":
         sessions_data = await PluginHub.get_sessions()
         sessions = sessions_data.sessions
         instances = []
@@ -42,26 +44,69 @@ async def set_active_instance(
             "success": False,
             "error": "No Unity instances are currently connected. Start Unity and press 'Start Session'."
         }
-    ids = {inst.id: inst for inst in instances}
-    hashes = {}
-    for inst in instances:
-        # exact hash and prefix map; last write wins but we'll detect ambiguity
-        hashes.setdefault(inst.hash, inst)
+    ids = {inst.id: inst for inst in instances if getattr(inst, "id", None)}
 
-    # Disallow anything except the explicit Name@hash id to ensure determinism
     value = (instance or "").strip()
-    if not value or "@" not in value:
+    if not value:
         return {
             "success": False,
-            "error": "Instance identifier must be Name@hash. "
-                     "Use unity://instances to copy the exact id (e.g., MyProject@abcd1234)."
+            "error": "Instance identifier is required. "
+                     "Use unity://instances to copy a Name@hash or provide a hash prefix."
         }
     resolved = None
+    if "@" in value:
         resolved = ids.get(value)
         if resolved is None:
-        return {"success": False, "error": f"Instance '{value}' not found. Use unity://instances to choose a valid Name@hash."}
+            return {
+                "success": False,
+                "error": f"Instance '{value}' not found. "
+                         "Use unity://instances to copy an exact Name@hash."
+            }
+    else:
+        lookup = value.lower()
+        matches = []
+        for inst in instances:
+            if not getattr(inst, "id", None):
+                continue
+            inst_hash = getattr(inst, "hash", "")
+            if inst_hash and inst_hash.lower().startswith(lookup):
+                matches.append(inst)
+        if not matches:
+            return {
+                "success": False,
+                "error": f"Instance hash '{value}' does not match any running Unity editors. "
+                         "Use unity://instances to confirm the available hashes."
+            }
+        if len(matches) > 1:
+            matching_ids = ", ".join(
+                inst.id for inst in matches if getattr(inst, "id", None)
+            ) or "multiple instances"
+            return {
+                "success": False,
+                "error": f"Instance hash '{value}' is ambiguous ({matching_ids}). "
+                         "Provide the full Name@hash from unity://instances."
+            }
+        resolved = matches[0]
+    
+    if resolved is None:
+        # Should be unreachable due to logic above, but satisfies static analysis
+        return {
+            "success": False,
+            "error": "Internal error: Instance resolution failed."
+        }
 
     # Store selection in middleware (session-scoped)
     middleware = get_unity_instance_middleware()
+    # We use middleware.set_active_instance to persist the selection.
+    # The session key is an internal detail but useful for debugging response.
     middleware.set_active_instance(ctx, resolved.id)
-    return {"success": True, "message": f"Active instance set to {resolved.id}", "data": {"instance": resolved.id}}
+    session_key = middleware.get_session_key(ctx)
+    
+    return {
+        "success": True,
+        "message": f"Active instance set to {resolved.id}",
+        "data": {
+            "instance": resolved.id,
+            "session_key": session_key,
+        },
+    }

Server/src/transport/legacy/unity_connection.py

@@ -60,6 +60,8 @@ class UnityConnection:
                 # Bounded connect to avoid indefinite blocking
                 connect_timeout = float(
                     getattr(config, "connection_timeout", 1.0))
+                # We trust config.unity_host (default 127.0.0.1) but future improvements
+                # could dynamically prefer 'localhost' depending on OS resolver behavior.
                 self.sock = socket.create_connection(
                     (self.host, self.port), connect_timeout)
                 self._prepare_socket(self.sock)

Server/src/transport/unity_instance_middleware.py

@@ -5,21 +5,29 @@ This middleware intercepts all tool calls and injects the active Unity instance
 into the request-scoped state, allowing tools to access it via ctx.get_state("unity_instance").
 """
 from threading import RLock
+import logging
 
 from fastmcp.server.middleware import Middleware, MiddlewareContext
 
 from transport.plugin_hub import PluginHub
 
+logger = logging.getLogger("mcp-for-unity-server")
+
 # Store a global reference to the middleware instance so tools can interact
 # with it to set or clear the active unity instance.
 _unity_instance_middleware = None
+_middleware_lock = RLock()
 
 
 def get_unity_instance_middleware() -> 'UnityInstanceMiddleware':
     """Get the global Unity instance middleware."""
+    global _unity_instance_middleware
     if _unity_instance_middleware is None:
-        raise RuntimeError(
-            "UnityInstanceMiddleware not initialized. Call set_unity_instance_middleware first.")
+        with _middleware_lock:
+            if _unity_instance_middleware is None:
+                # Auto-initialize if not set (lazy singleton) to handle import order or test cases
+                _unity_instance_middleware = UnityInstanceMiddleware()
+        
     return _unity_instance_middleware
 
 
@@ -42,37 +50,36 @@ class UnityInstanceMiddleware(Middleware):
         self._active_by_key: dict[str, str] = {}
         self._lock = RLock()
 
-    def _get_session_key(self, ctx) -> str:
+    def get_session_key(self, ctx) -> str:
         """
         Derive a stable key for the calling session.
 
-        Uses ctx.session_id if available, falls back to 'global'.
+        Prioritizes client_id for stability.
+        If client_id is missing, falls back to 'global' (assuming single-user local mode),
+        ignoring session_id which can be unstable in some transports/clients.
         """
-        session_id = getattr(ctx, "session_id", None)
-        if isinstance(session_id, str) and session_id:
-            return session_id
-
         client_id = getattr(ctx, "client_id", None)
         if isinstance(client_id, str) and client_id:
             return client_id
 
+        # Fallback to global for local dev stability
         return "global"
 
     def set_active_instance(self, ctx, instance_id: str) -> None:
         """Store the active instance for this session."""
-        key = self._get_session_key(ctx)
+        key = self.get_session_key(ctx)
         with self._lock:
             self._active_by_key[key] = instance_id
 
     def get_active_instance(self, ctx) -> str | None:
         """Retrieve the active instance for this session."""
-        key = self._get_session_key(ctx)
+        key = self.get_session_key(ctx)
         with self._lock:
             return self._active_by_key.get(key)
 
     def clear_active_instance(self, ctx) -> None:
         """Clear the stored instance for this session."""
-        key = self._get_session_key(ctx)
+        key = self.get_session_key(ctx)
         with self._lock:
             self._active_by_key.pop(key, None)
 
@@ -82,13 +89,32 @@ class UnityInstanceMiddleware(Middleware):
 
         active_instance = self.get_active_instance(ctx)
         if active_instance:
+            # If using HTTP transport (PluginHub configured), validate session
+            # But for stdio transport (no PluginHub needed or maybe partially configured),
+            # we should be careful not to clear instance just because PluginHub can't resolve it.
+            # The 'active_instance' (Name@hash) might be valid for stdio even if PluginHub fails.
+            
             session_id: str | None = None
+            # Only validate via PluginHub if we are actually using HTTP transport
+            # OR if we want to support hybrid mode. For now, let's be permissive.
             if PluginHub.is_configured():
                 try:
+                    # resolving session_id might fail if the plugin disconnected
+                    # We only need session_id for HTTP transport routing.
+                    # For stdio, we just need the instance ID.
                     session_id = await PluginHub._resolve_session_id(active_instance)
-                except Exception:
-                    self.clear_active_instance(ctx)
-                    return await call_next(context)
+                except Exception as exc:
+                    # If resolution fails, it means the Unity instance is not reachable via HTTP/WS.
+                    # If we are in stdio mode, this might still be fine if the user is just setting state?
+                    # But usually if PluginHub is configured, we expect it to work.
+                    # Let's LOG the error but NOT clear the instance immediately to avoid flickering,
+                    # or at least debug why it's failing.
+                    logger.debug(
+                        "PluginHub session resolution failed for %s: %s; leaving active_instance unchanged",
+                        active_instance,
+                        exc,
+                        exc_info=True,
+                    )
 
             ctx.set_state("unity_instance", active_instance)
             if session_id is not None:

Server/src/transport/unity_transport.py

@@ -19,6 +19,11 @@ def _is_http_transport() -> bool:
     return os.environ.get("UNITY_MCP_TRANSPORT", "stdio").lower() == "http"
 
 
+def _current_transport() -> str:
+    """Expose the active transport mode as a simple string identifier."""
+    return "http" if _is_http_transport() else "stdio"
+
+
 def with_unity_instance(
     log: str | Callable[[Context, tuple, dict, str | None], str] | None = None,
     *,

Server/uv.lock

@@ -1,5 +1,5 @@
 version = 1
-revision = 2
+revision = 3
 requires-python = ">=3.10"
 
 [[package]]
@@ -694,7 +694,7 @@ wheels = [
 
 [[package]]
 name = "mcpforunityserver"
-version = "7.0.0"
+version = "8.1.4"
 source = { editable = "." }
 dependencies = [
     { name = "fastapi" },
@@ -715,7 +715,7 @@ dev = [
 [package.metadata]
 requires-dist = [
     { name = "fastapi", specifier = ">=0.104.0" },
-    { name = "fastmcp", specifier = ">=2.13.0" },
+    { name = "fastmcp", specifier = ">=2.13.0,<2.13.2" },
     { name = "httpx", specifier = ">=0.27.2" },
     { name = "mcp", specifier = ">=1.16.0" },
     { name = "pydantic", specifier = ">=2.12.0" },