Fixes #4739: Add support for VS Code magic variables in MCP configuration

- Enhanced injectVariables function to resolve VS Code magic variables like , , etc.
- Added comprehensive test coverage for variable injection functionality
- Fixed issue where VS Code magic variables in MCP server configurations would cause 'Invalid MCP settings JSON format' errors
- Variables now properly resolve to actual paths instead of remaining as literal strings

Resolves: #4739

Author: roomote

src/services/mcp/__tests__/variable-injection.spec.ts

@@ -0,0 +1,160 @@
+import { describe, it, expect, beforeEach, vi } from "vitest"
+import * as vscode from "vscode"
+import { injectVariables } from "../../../utils/config"
+
+// Mock vscode module
+vi.mock("vscode", () => ({
+	workspace: {
+		workspaceFolders: [
+			{
+				uri: {
+					fsPath: "/test/workspace",
+				},
+			},
+		],
+		getWorkspaceFolder: vi.fn(),
+	},
+	window: {
+		activeTextEditor: {
+			document: {
+				uri: {
+					fsPath: "/test/workspace/src/test.ts",
+				},
+			},
+		},
+	},
+}))
+
+describe("MCP Variable Injection", () => {
+	beforeEach(() => {
+		vi.clearAllMocks()
+
+		// Setup workspace folder mock
+		const mockWorkspaceFolder = {
+			uri: { fsPath: "/test/workspace" },
+		}
+		;(vscode.workspace.getWorkspaceFolder as any).mockReturnValue(mockWorkspaceFolder)
+	})
+
+	it("should resolve workspaceFolder in MCP server configuration", async () => {
+		const mcpConfig = {
+			type: "stdio",
+			command: "node",
+			args: ["${workspaceFolder}/mcp-server/index.js"],
+			env: {
+				CONFIG_PATH: "${workspaceFolder}/.config",
+			},
+		}
+
+		const result = await injectVariables(mcpConfig, {
+			env: process.env,
+			workspaceFolder: vscode.workspace.workspaceFolders?.[0]?.uri.fsPath ?? "",
+		})
+
+		expect(result.args[0]).toBe("/test/workspace/mcp-server/index.js")
+		expect(result.env.CONFIG_PATH).toBe("/test/workspace/.config")
+	})
+
+	it("should resolve fileWorkspaceFolder in MCP server configuration", async () => {
+		const mcpConfig = {
+			type: "stdio",
+			command: "node",
+			args: ["${fileWorkspaceFolder}/server.js"],
+		}
+
+		const result = await injectVariables(mcpConfig, {
+			env: process.env,
+			workspaceFolder: vscode.workspace.workspaceFolders?.[0]?.uri.fsPath ?? "",
+		})
+
+		expect(result.args[0]).toBe("/test/workspace/server.js")
+	})
+
+	it("should handle complex MCP configuration with multiple variables", async () => {
+		const mcpConfig = {
+			type: "stdio",
+			command: "node",
+			args: [
+				"${workspaceFolder}/node_modules/.bin/mcp-server-git",
+				"--repository",
+				"${workspaceFolder}",
+				"--config",
+				"${userHome}/.gitconfig",
+			],
+			env: {
+				GIT_DIR: "${workspaceFolder}/.git",
+				HOME: "${userHome}",
+			},
+		}
+
+		const result = await injectVariables(mcpConfig, {
+			env: process.env,
+			workspaceFolder: vscode.workspace.workspaceFolders?.[0]?.uri.fsPath ?? "",
+		})
+
+		expect(result.args[0]).toBe("/test/workspace/node_modules/.bin/mcp-server-git")
+		expect(result.args[2]).toBe("/test/workspace")
+		expect(result.args[4]).toMatch(/\/.*\/.gitconfig$/) // Should contain userHome path
+		expect(result.env.GIT_DIR).toBe("/test/workspace/.git")
+		expect(result.env.HOME).toMatch(/\/.*/) // Should contain userHome path
+	})
+
+	it("should handle environment variables with env: prefix", async () => {
+		const mcpConfig = {
+			type: "stdio",
+			command: "node",
+			args: ["${workspaceFolder}/server.js"],
+			env: {
+				PATH: "${env:PATH}",
+				NODE_ENV: "${env:NODE_ENV}",
+			},
+		}
+
+		const result = await injectVariables(mcpConfig, {
+			env: {
+				PATH: "/usr/bin:/bin",
+				NODE_ENV: "development",
+			},
+			workspaceFolder: vscode.workspace.workspaceFolders?.[0]?.uri.fsPath ?? "",
+		})
+
+		expect(result.args[0]).toBe("/test/workspace/server.js")
+		expect(result.env.PATH).toBe("/usr/bin:/bin")
+		expect(result.env.NODE_ENV).toBe("development")
+	})
+
+	it("should leave unresolved variables unchanged", async () => {
+		const mcpConfig = {
+			type: "stdio",
+			command: "node",
+			args: ["${workspaceFolder}/server.js", "${unknownVariable}"],
+		}
+
+		const result = await injectVariables(mcpConfig, {
+			env: process.env,
+			workspaceFolder: vscode.workspace.workspaceFolders?.[0]?.uri.fsPath ?? "",
+		})
+
+		expect(result.args[0]).toBe("/test/workspace/server.js")
+		expect(result.args[1]).toBe("${unknownVariable}")
+	})
+
+	it("should handle the exact scenario from the GitHub issue", async () => {
+		// This is the exact configuration that was failing in the issue
+		const mcpConfig = {
+			type: "stdio",
+			command: "node",
+			args: ["${workspaceFolder}/mcp-server-git/index.js"],
+		}
+
+		const result = await injectVariables(mcpConfig, {
+			env: process.env,
+			workspaceFolder: vscode.workspace.workspaceFolders?.[0]?.uri.fsPath ?? "",
+		})
+
+		// This should now work instead of causing "Invalid MCP settings JSON format"
+		expect(result.args[0]).toBe("/test/workspace/mcp-server-git/index.js")
+		expect(result.type).toBe("stdio")
+		expect(result.command).toBe("node")
+	})
+})