Observability

Playwright Backend Mocks can show you the HTTP and WebSocket traffic your Node app sends while tests run — which test owned it, and what that test did (fulfill, continue, abort, or passthrough).

Observability is read-only and in-memory. History lives in the proxy process and clears when the proxy stops. It never changes routing.

Pieces

Piece	Package	Role
Proxy + REST	@playwright-backend-mocks/proxy	Stores history; exposes the REST API
Dashboard	@playwright-backend-mocks/dashboard	Optional Vue UI pointed at the proxy

There is no MCP server. Local coding agents can use the same REST API (see below).

Quick start

# terminal 1 — proxy (prints connect + REST URLs on startup)
playwright-backend-mocks-proxy --host 127.0.0.1 --port 4310

# terminal 2 — dashboard (optional)
playwright-backend-mocks-dashboard --host 127.0.0.1 --port 4311 \
  --proxy-url http://127.0.0.1:4310

Open http://127.0.0.1:4311/. The dashboard polls the proxy every 2 seconds when auto-refresh is on (default).

Playwright webServer

Running both as Playwright web servers is a good default:

webServer: [
  {
    command: "playwright-backend-mocks-proxy --host 127.0.0.1 --port 4310",
    url: "http://127.0.0.1:4310/health",
    reuseExistingServer: !process.env.CI,
  },
  {
    command:
      "playwright-backend-mocks-dashboard --host 127.0.0.1 --port 4311 --proxy-url http://127.0.0.1:4310",
    url: "http://127.0.0.1:4311/health",
    reuseExistingServer: !process.env.CI,
  },
  // …your app under test
];

What you can see

• HTTP — timeline of requests/responses; action; owning test title and path; request/response bodies; continue overrides; short per-request timeline
• WebSockets — each connection, outcome (matched / passthrough / error), and a bidirectional event timeline (live only — no WS file download)
• ambiguous_route — history records the competing tests; the dashboard shows a clear callout and links to troubleshooting
• Connections — connected Node agents and Playwright workers

HAR download (single request → routeFromHAR)

Export one HTTP request as a HAR file, then replay it with routeFromHAR:

curl -OJ "http://127.0.0.1:4310/api/history/<requestId>/har"

In the dashboard, select a request and use the download control in the detail panel. HAR export is HTTP only (one entry per file). There is no WebSocket HAR download.

Capture modes

Control how much the proxy stores:

playwright-backend-mocks-proxy --history-capture all      # default
playwright-backend-mocks-proxy --history-capture handled  # only test-acted traffic
playwright-backend-mocks-proxy --history-capture none

Mode	Keeps
all	Every coordinated HTTP request and WebSocket connection
handled	Only traffic a test claimed/acted on (fulfill / continue / abort, matched sockets, errors). Passthrough omitted
none	Nothing in history (health/connections still work)

Related flags: --history-limit (HTTP ring size), --ws-history-limit (WebSocket connection ring size). See Proxy.

Using this with coding agents

There is no MCP package. If a local agent is writing or running tests against this proxy, give it:

1. The REST API documentation (this site’s REST page), and
2. The proxy base URL (default http://127.0.0.1:4310).

The agent can then call GET /api/history, GET /api/ws, and GET /api/history/:id/har to inspect what tests intercepted — the same data the dashboard shows. The dashboard also has small copy controls for URLs and full history JSON. Dropping the REST page into the agent’s context is usually enough.

Related

• Dashboard
• REST API
• Proxy
• Spying and waiting