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Build: DDEV Add-on for Local MCP Server Testing with Playwright

· 2 min read
Victor Jimenez
Victor Jimenez
Software Engineer & AI Agent Builder

I built a DDEV add-on that runs Playwright checks against local MCP endpoints using internal DDEV DNS names like web. This solves flaky localhost-based testing and gives one command for repeatable MCP smoke tests.

The Problem​

Teams testing MCP servers locally usually mix host networking, ad-hoc scripts, and one-off commands. That breaks quickly when ports change or when tests run from a different container context.

Pain pointTypical failure
Host-only URLs (localhost)Fails inside containers where service DNS is required
Manual Playwright setup per projectInconsistent versions and missing browser deps
No MCP-specific smoke commandHealth-check regressions are discovered too late

Related context: Build: MCP Web Setup, Build: Drupal MCP Toolkit, Drupal Context MCP Integration.

The Solution​

The add-on installs:

  • A dedicated mcp-playwright service via Docker Compose.
  • DDEV commands: ddev mcp-pw-smoke and ddev mcp-pw-test.
  • A Playwright smoke test for GET /health with configurable MCP_BASE_URL.

Code from the project:

# docker-compose.mcp-playwright.yaml
services:
  mcp-playwright:
    image: mcr.microsoft.com/playwright:v1.56.1-noble
    working_dir: /var/www/html/.ddev/mcp-playwright
# commands/host/mcp-pw-smoke
ddev exec -s mcp-playwright "cd /var/www/html/.ddev/mcp-playwright && npm ci --silent && MCP_BASE_URL=\${MCP_BASE_URL:-http://web:3100} npx playwright test tests/mcp-health.spec.js --reporter=line"
// mcp-playwright/tests/mcp-health.spec.js
const response = await request.get(`${baseUrl}/health`);
expect(response.ok()).toBeTruthy();

What I Learned​

  • web DNS names inside DDEV are safer than localhost for cross-container test reliability.
  • A tiny helper (resolveMcpBaseUrl) plus tests prevents subtle URL-format bugs.
  • Dedicated smoke commands are worth adding early when MCP endpoints are a release gate.

References​