Add envvar overrides for mcp app (#606)

Needed for deployed MCP servers so that the Engine can override port,
host, and transport.

libs/arcade-mcp-server/arcade_mcp_server/mcp_app.py

@@ -6,6 +6,7 @@ Provides a clean, minimal API for building MCP servers with lazy initialization.
 
 from __future__ import annotations
 
+import os
 import sys
 from pathlib import Path
 from typing import Any, Callable, Literal, ParamSpec, TypeVar
@@ -210,6 +211,8 @@ class MCPApp:
 
         logger.info(f"Starting {self.name} v{self.version} with {len(self._catalog)} tools")
 
+        host, port, transport = MCPApp._get_configuration_overrides(host, port, transport)
+
         if transport in ["http", "streamable-http", "streamable"]:
             run_arcade_mcp(
                 catalog=self._catalog,
@@ -232,6 +235,37 @@ class MCPApp:
         else:
             raise ServerError(f"Invalid transport: {transport}")
 
+    @staticmethod
+    def _get_configuration_overrides(
+        host: str, port: int, transport: TransportType
+    ) -> tuple[str, int, TransportType]:
+        """Get configuration overrides from environment variables."""
+        if envvar_transport := os.getenv("ARCADE_SERVER_TRANSPORT"):
+            transport = envvar_transport
+            logger.debug(
+                f"Using '{transport}' as transport from ARCADE_SERVER_TRANSPORT environment variable"
+            )
+
+        # host and port are only relevant for HTTP Streamable transport
+        if transport in ["http", "streamable-http", "streamable"]:
+            if envvar_host := os.getenv("ARCADE_SERVER_HOST"):
+                host = envvar_host
+                logger.debug(f"Using '{host}' as host from ARCADE_SERVER_HOST environment variable")
+
+            if envvar_port := os.getenv("ARCADE_SERVER_PORT"):
+                try:
+                    port = int(envvar_port)
+                except ValueError:
+                    logger.warning(
+                        f"Invalid port: '{envvar_port}' from ARCADE_SERVER_PORT environment variable. Using default port {port}"
+                    )
+                else:
+                    logger.debug(
+                        f"Using '{port}' as port from ARCADE_SERVER_PORT environment variable"
+                    )
+
+        return host, port, transport
+
 
 class _ToolsAPI:
     """Unified tools API for MCPApp (build-time and runtime)."""

libs/tests/arcade_mcp_server/test_mcp_app.py

@@ -179,3 +179,58 @@ class TestMCPApp:
         # Test removing a resource at runtime
         removed_resource = await mcp_app.resources.remove("file:///test.txt")
         assert removed_resource.uri == "file:///test.txt"
+
+    def test_get_configuration_overrides(self, monkeypatch):
+        """Test configuration overrides from environment variables."""
+        # Ensure environment variables are clear at the start
+        monkeypatch.delenv("ARCADE_SERVER_TRANSPORT", raising=False)
+        monkeypatch.delenv("ARCADE_SERVER_HOST", raising=False)
+        monkeypatch.delenv("ARCADE_SERVER_PORT", raising=False)
+
+        # Test default values (no environment variables)
+        host, port, transport = MCPApp._get_configuration_overrides("127.0.0.1", 8000, "http")
+        assert host == "127.0.0.1"
+        assert port == 8000
+        assert transport == "http"
+
+        # Test transport override
+        monkeypatch.setenv("ARCADE_SERVER_TRANSPORT", "stdio")
+        host, port, transport = MCPApp._get_configuration_overrides("127.0.0.1", 8000, "http")
+        assert transport == "stdio"
+        monkeypatch.delenv("ARCADE_SERVER_TRANSPORT")
+
+        # Test host override (only works with HTTP transport)
+        monkeypatch.setenv("ARCADE_SERVER_TRANSPORT", "http")
+        monkeypatch.setenv("ARCADE_SERVER_HOST", "192.168.1.1")
+        host, port, transport = MCPApp._get_configuration_overrides("127.0.0.1", 8000, "http")
+        assert host == "192.168.1.1"
+        assert transport == "http"
+        monkeypatch.delenv("ARCADE_SERVER_HOST")
+        monkeypatch.delenv("ARCADE_SERVER_TRANSPORT")
+
+        # Test port override (only works with HTTP transport)
+        monkeypatch.setenv("ARCADE_SERVER_PORT", "9000")
+        host, port, transport = MCPApp._get_configuration_overrides("127.0.0.1", 8000, "http")
+        assert port == 9000
+        monkeypatch.delenv("ARCADE_SERVER_PORT")
+
+        # Test invalid port value
+        monkeypatch.setenv("ARCADE_SERVER_TRANSPORT", "http")
+        monkeypatch.setenv("ARCADE_SERVER_PORT", "invalid_port")
+        host, port, transport = MCPApp._get_configuration_overrides("127.0.0.1", 8000, "http")
+        assert port == 8000  # Should keep the default value
+        monkeypatch.delenv("ARCADE_SERVER_PORT")
+        monkeypatch.delenv("ARCADE_SERVER_TRANSPORT")
+
+        # Test host/port with stdio transport
+        monkeypatch.setenv("ARCADE_SERVER_TRANSPORT", "stdio")
+        monkeypatch.setenv("ARCADE_SERVER_HOST", "192.168.1.1")
+        monkeypatch.setenv("ARCADE_SERVER_PORT", "9000")
+        host, port, transport = MCPApp._get_configuration_overrides("127.0.0.1", 8000, "http")
+        # For stdio, host and port are still returned but not used by the server
+        assert host == "127.0.0.1"  # Host should remain unchanged for stdio transport
+        assert port == 8000  # Port should remain unchanged for stdio transport
+        assert transport == "stdio"
+        monkeypatch.delenv("ARCADE_SERVER_HOST")
+        monkeypatch.delenv("ARCADE_SERVER_PORT")
+        monkeypatch.delenv("ARCADE_SERVER_TRANSPORT")