Cmux Browser

Browser Automation with cmux

Use this skill for browser tasks inside cmux webviews.

Core Workflow

1. Open or target a browser surface.
2. Verify navigation with get url before waiting or snapshotting.
3. Snapshot (--interactive) to get fresh element refs.
4. Act with refs (click, fill, type, select, press).
5. Wait for state changes.
6. Re-snapshot after DOM/navigation changes.

cmux --json browser open https://example.com
# use returned surface ref, for example: 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

Surface Targeting

# identify current context
cmux identify --json

# open routed to a specific topology target
cmux browser open https://example.com --workspace workspace:2 --window window:1 --json

Notes:

• CLI output defaults to short refs (surface:N, pane:N, workspace:N, window:N).
• UUIDs are still accepted on input; only request UUID output when needed (--id-format uuids|both).
• Keep using one surface:N per task unless you intentionally switch.

Wait Support

cmux supports wait patterns similar to agent-browser:

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

Common Flows

Form Submit

cmux --json browser open https://example.com/signup
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 "Jane Doe"
cmux browser surface:7 fill e2 "jane@example.com"
cmux --json browser surface:7 click e3 --snapshot-after
cmux browser surface:7 wait --url-contains "/welcome" --timeout-ms 15000
cmux browser surface:7 snapshot --interactive

Clear an Input

cmux browser surface:7 fill e11 "" --snapshot-after --json
cmux browser surface:7 get value e11 --json

Stable Agent Loop (Recommended)

# navigate -> verify -> wait -> snapshot -> action -> snapshot
cmux browser surface:7 get url
cmux browser surface:7 wait --load-state complete --timeout-ms 15000
cmux browser surface:7 snapshot --interactive
cmux --json browser surface:7 click e5 --snapshot-after
cmux browser surface:7 snapshot --interactive

If get url is empty or about:blank, navigate first instead of waiting on load state.

Deep-Dive References

Ready-to-Use Templates

Viewport Sizing (WKWebView)

Use cmux browser <surface> viewport <width> <height> to set an exact logical viewport from 1...4096 CSS pixels. The page is aspect-fitted inside its existing pane, so pane layout and focus stay unchanged; screenshots use the requested logical dimensions. Run cmux browser <surface> viewport reset to follow native pane sizing again. Close or detach the browser inspector first because its inspector-managed split layout cannot be combined with viewport emulation. Large viewport and page-zoom combinations are bounded; the viewport command returns structured maximum_page_zoom details without changing the current viewport when the combination exceeds the WKWebView render limits. Opening or redocking an attached browser inspector resets emulation to native sizing because WebKit owns the attached split geometry.

Limits (WKWebView)

These commands currently return not_supported because they rely on Chrome/CDP-only APIs not exposed by WKWebView:

• offline emulation
• trace/screencast recording
• network route interception/mocking
• low-level raw input injection

Use supported high-level commands (click, fill, press, scroll, wait, snapshot) instead.

Troubleshooting

js_error on snapshot --interactive or eval

Some complex pages can reject or break the JavaScript used for rich snapshots and ad-hoc evaluation.

Recovery steps:

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

• Use get url first so you know whether the page actually navigated.
• Fall back to get text body or get html body when snapshot --interactive or eval returns js_error.
• If the page is still failing, navigate to a simpler intermediate page, then retry the task from there.