Browser Automation with cmux

Core workflow

Open or target a browser surface, verify navigation with get url, snapshot for fresh element refs, act on refs, wait for the state change, re-snapshot.

cmux --json browser open https://example.com     # returns a surface ref, e.g. surface:7
cmux browser surface:7 get url
cmux browser surface:7 wait --load-state complete --timeout-ms 15000
cmux browser surface:7 snapshot --interactive
cmux browser surface:7 fill e1 "hello"
cmux --json browser surface:7 click e2 --snapshot-after
cmux browser surface:7 snapshot --interactive

If get url is empty or about:blank, navigate first instead of waiting on load state. Re-snapshot after navigation, modal open/close, or major DOM changes; refs go stale.

Surface targeting

browser open targets the workspace of the terminal running the command (CMUX_WORKSPACE_ID), even when another workspace is focused. Override with --workspace / --window:

cmux identify --json
cmux browser open https://example.com --workspace workspace:2 --window window:1 --json

Output defaults to short refs (surface:N, pane:N, workspace:N, window:N); UUIDs are accepted on input, and --id-format uuids|both requests them on output. Keep one surface:N per task.

Wait

cmux browser <surface> wait --selector "#ready" --timeout-ms 10000
cmux browser <surface> wait --text "Success" --timeout-ms 10000
cmux browser <surface> wait --url-contains "/dashboard" --timeout-ms 10000
cmux browser <surface> wait --load-state complete --timeout-ms 15000
cmux browser <surface> wait --function "document.readyState === 'complete'" --timeout-ms 10000

Viewport sizing (WKWebView)

cmux browser <surface> viewport <width> <height> sets an exact logical viewport from 1 to 4096 CSS pixels. The page is aspect-fitted inside its existing pane, so pane layout and focus stay unchanged, and screenshots use the requested logical dimensions. viewport reset returns to native pane sizing.

Close or detach the browser inspector first: its inspector-managed split layout cannot be combined with viewport emulation, and opening or redocking an attached inspector resets emulation to native sizing. Large viewport and page-zoom combinations are bounded; the command returns structured maximum_page_zoom details and leaves the viewport unchanged when the combination exceeds WKWebView render limits.

Limits (WKWebView)

Offline emulation, trace/screencast recording, network route interception/mocking, and low-level raw input injection return not_supported; they depend on Chrome/CDP-only APIs. Use click, fill, press, scroll, wait, snapshot instead.

Troubleshooting js_error

Some complex pages reject the JavaScript behind snapshot --interactive and eval. Recover by checking whether the page actually navigated, then falling back to raw text or HTML:

cmux browser surface:7 get url
cmux browser surface:7 get text body
cmux browser surface:7 get html body

If it still fails, navigate to a simpler intermediate page and retry from there.

Deep-dive references

Ready-to-use templates