fix: filter out empty string arguments for Playwright MCP server

- Fixes issue #8251 where Playwright MCP server fails to start with empty string arguments
- Filters out both empty strings and arguments ending with = (like --browser=)
- Added comprehensive tests to verify the fix works correctly
- Does not affect other MCP servers, only applies to Playwright

Author: roomote

src/services/marketplace/SimpleInstaller.ts

@@ -226,6 +226,20 @@ export class SimpleInstaller {
 		const filePath = await this.getMcpFilePath(target)
 		const mcpData = JSON.parse(contentToUse)
 
+		// Fix for Playwright MCP server: filter out empty string arguments and arguments with empty values
+		// Issue #8251: Empty string arguments and arguments ending with '=' cause the Playwright MCP server to fail
+		if (item.id === "playwright" && mcpData.args && Array.isArray(mcpData.args)) {
+			// Filter out empty string arguments and arguments that end with '=' (empty value arguments)
+			mcpData.args = mcpData.args.filter((arg: any) => {
+				if (typeof arg !== "string") return true
+				// Remove empty strings
+				if (arg === "") return false
+				// Remove arguments that end with '=' (like --browser=, --headless=, --viewport-size=)
+				if (arg.endsWith("=")) return false
+				return true
+			})
+		}
+
 		// Read existing file or create new structure
 		let existingData: any = { mcpServers: {} }
 		try {

src/services/marketplace/__tests__/SimpleInstaller.playwright.spec.ts

@@ -0,0 +1,172 @@
+// npx vitest services/marketplace/__tests__/SimpleInstaller.playwright.spec.ts
+
+import { describe, it, expect, vi, beforeEach } from "vitest"
+import * as vscode from "vscode"
+import * as fs from "fs/promises"
+import * as path from "path"
+import { SimpleInstaller } from "../SimpleInstaller"
+import type { MarketplaceItem } from "@roo-code/types"
+
+// Mock vscode
+vi.mock("vscode", () => ({
+	workspace: {
+		workspaceFolders: [{ uri: { fsPath: "/test/workspace" } }],
+	},
+	ExtensionContext: vi.fn(),
+}))
+
+// Mock fs/promises
+vi.mock("fs/promises", () => ({
+	readFile: vi.fn(),
+	writeFile: vi.fn(),
+	mkdir: vi.fn(),
+}))
+
+// Mock globalContext
+vi.mock("../../../utils/globalContext", () => ({
+	ensureSettingsDirectoryExists: vi.fn().mockResolvedValue("/test/settings"),
+}))
+
+describe("SimpleInstaller - Playwright MCP Fix", () => {
+	let installer: SimpleInstaller
+	let mockContext: vscode.ExtensionContext
+
+	beforeEach(() => {
+		vi.clearAllMocks()
+		mockContext = {} as vscode.ExtensionContext
+		installer = new SimpleInstaller(mockContext)
+	})
+
+	it("should filter out empty string arguments for Playwright MCP server", async () => {
+		const playwrightItem: MarketplaceItem = {
+			id: "playwright",
+			name: "Playwright MCP Server",
+			type: "mcp",
+			description: "Browser automation MCP server",
+			author: "Playwright Team",
+			url: "https://github.com/playwright/mcp",
+			content: JSON.stringify({
+				command: "npx",
+				args: ["-y", "@playwright/mcp@latest", "--browser=", "--headless=", "--viewport-size="],
+			}),
+		}
+
+		// Mock file doesn't exist (new installation)
+		vi.mocked(fs.readFile).mockRejectedValue({ code: "ENOENT" })
+		vi.mocked(fs.mkdir).mockResolvedValue(undefined)
+		vi.mocked(fs.writeFile).mockResolvedValue(undefined)
+
+		await installer.installItem(playwrightItem, { target: "global" })
+
+		// Check that writeFile was called with filtered args
+		expect(fs.writeFile).toHaveBeenCalledWith(
+			expect.stringContaining("mcp_settings.json"),
+			expect.any(String),
+			"utf-8",
+		)
+
+		// Get the actual content that was written
+		const writeCall = vi.mocked(fs.writeFile).mock.calls[0]
+		const writtenContent = JSON.parse(writeCall[1] as string)
+
+		// Verify that empty string arguments were filtered out
+		expect(writtenContent.mcpServers.playwright.args).toEqual(["-y", "@playwright/mcp@latest"])
+
+		// Verify command is preserved
+		expect(writtenContent.mcpServers.playwright.command).toBe("npx")
+	})
+
+	it("should not modify args for non-Playwright MCP servers", async () => {
+		const otherItem: MarketplaceItem = {
+			id: "other-server",
+			name: "Other MCP Server",
+			type: "mcp",
+			description: "Another MCP server",
+			author: "Other Team",
+			url: "https://example.com/other-server",
+			content: JSON.stringify({
+				command: "node",
+				args: ["server.js", "--option=", ""],
+			}),
+		}
+
+		// Mock file doesn't exist (new installation)
+		vi.mocked(fs.readFile).mockRejectedValue({ code: "ENOENT" })
+		vi.mocked(fs.mkdir).mockResolvedValue(undefined)
+		vi.mocked(fs.writeFile).mockResolvedValue(undefined)
+
+		await installer.installItem(otherItem, { target: "global" })
+
+		// Get the actual content that was written
+		const writeCall = vi.mocked(fs.writeFile).mock.calls[0]
+		const writtenContent = JSON.parse(writeCall[1] as string)
+
+		// Verify that args were NOT modified for non-Playwright servers
+		expect(writtenContent.mcpServers["other-server"].args).toEqual(["server.js", "--option=", ""])
+	})
+
+	it("should handle Playwright MCP server with no args gracefully", async () => {
+		const playwrightItem: MarketplaceItem = {
+			id: "playwright",
+			name: "Playwright MCP Server",
+			type: "mcp",
+			description: "Browser automation MCP server",
+			author: "Playwright Team",
+			url: "https://github.com/playwright/mcp",
+			content: JSON.stringify({
+				command: "npx",
+				// No args property
+			}),
+		}
+
+		// Mock file doesn't exist (new installation)
+		vi.mocked(fs.readFile).mockRejectedValue({ code: "ENOENT" })
+		vi.mocked(fs.mkdir).mockResolvedValue(undefined)
+		vi.mocked(fs.writeFile).mockResolvedValue(undefined)
+
+		await installer.installItem(playwrightItem, { target: "global" })
+
+		// Should not throw an error
+		expect(fs.writeFile).toHaveBeenCalled()
+
+		// Get the actual content that was written
+		const writeCall = vi.mocked(fs.writeFile).mock.calls[0]
+		const writtenContent = JSON.parse(writeCall[1] as string)
+
+		// Verify command is preserved and no args property exists
+		expect(writtenContent.mcpServers.playwright.command).toBe("npx")
+		expect(writtenContent.mcpServers.playwright.args).toBeUndefined()
+	})
+
+	it("should handle Playwright MCP server with mixed empty and non-empty args", async () => {
+		const playwrightItem: MarketplaceItem = {
+			id: "playwright",
+			name: "Playwright MCP Server",
+			type: "mcp",
+			description: "Browser automation MCP server",
+			author: "Playwright Team",
+			url: "https://github.com/playwright/mcp",
+			content: JSON.stringify({
+				command: "npx",
+				args: ["-y", "", "@playwright/mcp@latest", "--browser=", "--headless=false", "", "--viewport-size="],
+			}),
+		}
+
+		// Mock file doesn't exist (new installation)
+		vi.mocked(fs.readFile).mockRejectedValue({ code: "ENOENT" })
+		vi.mocked(fs.mkdir).mockResolvedValue(undefined)
+		vi.mocked(fs.writeFile).mockResolvedValue(undefined)
+
+		await installer.installItem(playwrightItem, { target: "project" })
+
+		// Check that writeFile was called
+		expect(fs.writeFile).toHaveBeenCalledWith(expect.stringContaining("mcp.json"), expect.any(String), "utf-8")
+
+		// Get the actual content that was written
+		const writeCall = vi.mocked(fs.writeFile).mock.calls[0]
+		const writtenContent = JSON.parse(writeCall[1] as string)
+
+		// Verify that only empty string arguments were filtered out
+		expect(writtenContent.mcpServers.playwright.args).toEqual(["-y", "@playwright/mcp@latest", "--headless=false"])
+	})
+})