Document setting user agent

man/guides/browsers.md

@@ -1,3 +1,24 @@
 # Browsers
 
-...
+## Setting a custom user agent
+
+It's possible to set the user agent to a custom value via `BrowserContext` or `Browser`
+### With Browser
+
+```elixir
+page = Playwright.Browser.new_page(browser, %{"userAgent" => "My Custom Agent"})
+```
+
+### With BrowserContext
+
+```elixir
+context = Browser.new_context(browser, %{"userAgent" => "Special Agent"})
+```
+
+## Custom Agent and Phoenix / Ecto
+
+Setting a custom agent can be particularly useful when running Playwright in tests with the database involved.
+
+Follow https://hexdocs.pm/phoenix_ecto/Phoenix.Ecto.SQL.Sandbox.html and set the `userAgent` to the result of `Phoenix.Ecto.SQL.Sandbox.metadata_for(YourApp.Repo, pid)`
+
+