Give an RSelenium Browser Its Proxy When Opening the Session

Use extraCapabilities for the browser route, and distinguish session-protocol rejection from a browser that cannot reach the proxy.

Supply the proxy as a browser session capability before driver$open(), in an RSelenium deployment whose WebDriver server accepts its session protocol:

browser_proxy <- list(
  proxyType = 'manual',
  httpProxy = 'proxy.example:8080',
  sslProxy = 'proxy.example:8080'
)

driver <- RSelenium::remoteDriver(
  remoteServerAddr = 'webdriver.example',
  port = 4444L,
  browserName = 'chrome',
  extraCapabilities = list(proxy = browser_proxy)
)
driver$open()

remoteServerAddr identifies the WebDriver service. The capability describes where the browser goes next. An R HTTP proxy setting alone does not configure that browser route.

Use host:port in httpProxy and sslProxy. The latter selects HTTPS destinations; it does not imply a TLS-protected proxy listener. A public HTTP candidate from the HTTP proxy list supplies a host and port for these fields, provided the browser host can reach it.

Which failure happened first?

If opening the session is rejected, check protocol compatibility before changing the proxy endpoint. RSelenium's remoteDriver source submits legacy desiredCapabilities; an arbitrary current WebDriver server may reject that format before a browser exists to test the route.

If the session opens but navigation fails, check reachability from the actual browser host. For a container or Grid node, localhost means that node, not the R workstation.

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