Playwright MCP in Claude Code: 'Browser is already in use' error

Question

When using the Playwright MCP server (microsoft/playwright-mcp) with Claude Code 1.0.43 inside a Debian 12 devcontainer with X11, every browser navigation fails with: 'Error: Browser is already in use for /home/node/.cache/ms-playwright/mcp-chromium-profile, use --isolated to run multiple instances of the same browser.' Running npx playwright open --browser chromium directly works, so the MCP server is the only affected path. How can I fix this?

Originally asked on GitHub: https://github.com/microsoft/playwright-mcp/issues/636

AgentArmorHQ team · Accepted Answer

This is explained behavior, not a bug: playwright-mcp uses a persistent Chromium profile (normally at /home/node/.cache/ms-playwright/mcp-chromium-profile) by default, so a second instance cannot start while another browser process holds that profile. A maintainer confirmed this in the thread and said to use --isolated for non-persistent profiles.

Add --isolated to the server args. If you use a project-level .mcp.json, the config suggested in the thread is:

{
  "mcpServers": {
    "playwright": {
      "type": "stdio",
      "command": "npx",
      "args": ["@playwright/mcp@latest", "--isolated"],
      "env": {}
    }
  }
}

If you registered the server with the CLI as in the issue report, add "--isolated" to the args array of the same claude mcp add-json command.

Until the flag is in place, the workarounds that worked for people in the thread are: rebuild the container, or run docker compose up --force-recreate if you use docker-compose. Closing the browser window and deleting the cache directory did not clear the lock, and neither did uninstalling the browser or the plugin.

General advice: in a devcontainer the profile lives inside the container, so a leftover browser process from an earlier MCP session is the usual cause. The thread does not contain a report of someone confirming that --isolated resolved the error, only the maintainer's recommendation to use it.

AgentArmorHQ team · Answer

**What a GitHub user tried on GitHub** (Didn't work)

Registered the MCP server in Claude Code with `claude mcp add-json` using args ['@playwright/mcp@latest', '--browser', 'chromium'] in a Debian devcontainer. Standalone `npx playwright open --browser chromium` worked, but MCP navigation failed with 'Browser is already in use'.

Source: https://github.com/microsoft/playwright-mcp/issues/636

AgentArmorHQ team · Answer

**What @Jorisslagter tried on GitHub** (Outcome unclear)

Suggested configuring the server in a project-level .mcp.json with args ['@playwright/mcp@latest', '--isolated'], based on the docs. A maintainer later confirmed --isolated is the right mode for non-persistent profiles, but nobody in the thread reported back whether it fixed the error.

Source: https://github.com/microsoft/playwright-mcp/issues/636#issuecomment-3065265553

AgentArmorHQ team · Answer

**What @SEI-John tried on GitHub** (Worked)

Rebuilt the container to clear the stale profile lock; that was the only fix that worked. Closing the local browser window and deleting the cache directory did not help.

Source: https://github.com/microsoft/playwright-mcp/issues/636#issuecomment-3057790177

AgentArmorHQ team · Answer

**What @ElanHasson tried on GitHub** (Worked)

Ran `docker compose up --force-recreate` to force-recreate the container, which cleared the error.

Source: https://github.com/microsoft/playwright-mcp/issues/636#issuecomment-3124112826

AgentArmorHQ team · Answer

**What @GdmnKomo tried on GitHub** (Didn't work)

Uninstalled the browser, deleted the registry, removed the Playwright plugin, and reinstalled everything; the error persisted.

Source: https://github.com/microsoft/playwright-mcp/issues/636#issuecomment-3061344736

Playwright MCP in Claude Code: 'Browser is already in use' error

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Playwright MCP in Claude Code: 'Browser is already in use' error

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