MCP Tools

msw-mcp exposes five MCP tools that AI assistants can call to control MSW in the browser.

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msw_add_handlers

Add one or more new request handlers to the browser service worker at runtime.

Parameters:

Parameter	Type	Required	Description
handlers	string[]	Yes	Array of handler code strings
once	boolean	No	If true, each handler only intercepts the first matching request, then deactivates

Available MSW utilities inside handler strings:

• http — http.get, http.post, http.put, http.delete, http.patch, …
• HttpResponse — HttpResponse.json, HttpResponse.text, HttpResponse.error, …
• bypass — bypass the handler and perform the real network request
• passthrough — explicitly pass through to the network
• delay — add a response delay (await delay(500))

Example:

{
  "handlers": [
    "http.get('/api/users', () => HttpResponse.json([{ id: 1, name: 'Alice' }]))",
    "http.post('/api/users', async ({ request }) => { const user = await request.json(); return HttpResponse.json({ ...user, id: Date.now() }, { status: 201 }) })"
  ]
}

One-time handler:

{
  "handlers": [
    "http.get('/api/feature-flag', () => HttpResponse.json({ enabled: true }))"
  ],
  "once": true
}

After the first request, the handler deactivates and subsequent requests pass through.

Handler priority

MSW evaluates handlers in order — the first matching handler wins. msw_add_handlers prepends new handlers in front of existing ones, so they always take priority over base handlers.

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msw_update_handlers

Replace handlers that match specified URL patterns. Atomic operation — matched handlers are removed and new handlers added in one step.

Parameters:

Parameter	Type	Required	Description
patterns	string[]	Yes	URL patterns to match existing handlers
handlers	string[]	Yes	New handler code strings to replace matched handlers
methods	string[]	No	Only update handlers matching these HTTP methods

Update all methods for a pattern:

{
  "patterns": ["/api/users"],
  "handlers": ["http.get('/api/users', () => HttpResponse.json([]))"]
}

Update only GET, leave POST unchanged:

{
  "patterns": ["/api/users"],
  "handlers": ["http.get('/api/users', () => HttpResponse.json([]))"],
  "methods": ["GET"]
}

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msw_remove_handlers

Remove specific handlers by URL patterns and optional HTTP method filter.

Parameters:

Parameter	Type	Required	Description
patterns	string[]	Yes	URL patterns to match handlers for removal
methods	string[]	No	Only remove handlers matching these HTTP methods

{ "patterns": ["/api/users"] }

{ "patterns": ["/api/users"], "methods": ["GET"] }

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msw_reset_handlers

Reset MSW handlers. Without handlers, removes all runtime handlers and restores the initial base handlers. With handlers, replaces the entire set.

Parameters:

Parameter	Type	Required	Description
handlers	string[]	No	New handler code strings to set after reset

{}

{
  "handlers": ["http.get('/api/user', () => HttpResponse.json({ id: 1 }))"]
}

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msw_get_status

Get the current status of the MSW service worker and all active handlers. No parameters.

Response:

{
  "connected": true,
  "workerStatus": "running",
  "activeHandlers": [
    "GET /api/users",
    "POST /api/users",
    "GET /api/products"
  ],
  "handlerCount": 3
}

Field	Type	Description
connected	boolean	Whether a browser tab is connected via WebSocket
workerStatus	string	"running", "stopped", or "unknown"
activeHandlers	string[]	Summary of active handlers (method + URL)
handlerCount	number	Total number of active handlers