Error: No browsers were started. Browser launcher configuration

Answer

Karma does not auto‑install missing browser binaries and provides no flag to skip a single unavailable launcher; the run stops as soon as any entry in the browsers array cannot be matched to an installed launcher plugin.

Likely explanation

The error appears when Karma resolves each name in browsers against the launcher plugins it has loaded (e.g., karma-chrome-launcher, karma-firefox-launcher). If a name has no matching plugin—or the plugin exists but the underlying browser binary cannot be found—Karma treats the whole configuration as invalid and exits.

Confirmed facts

• Karma has no built‑in auto‑download or auto‑install of Chrome, Firefox, etc.; installing launchers and the browser executables is left to the user or CI environment.
• There is no per‑browser “skip if missing” option; a missing launcher aborts the entire test run.
• The only way to continue with the available browsers is to compute the browsers array dynamically, filtering out names whose launchers or binaries are not present.

Steps to resolve this case

1. Verify that the launcher plugin for each browser you intend to use is installed: npm ls karma-<browser>-launcher (or check package.json).
2. If you have an explicit plugins array in karma.conf.js, make sure the missing launcher is listed there; otherwise Karma will not auto‑load it.
3. Confirm the browser binary is reachable (e.g., which google-chrome or the CHROME_BIN environment variable).
4. If you want the run to proceed when a particular browser is absent, replace the static browsers list with a dynamic one, for example:

const fs = require('fs');
function hasLauncher(name) {
  try {
    return require.resolve(`karma-${name}-launcher`);
  } catch (_) {
    return false;
  }
}
function binaryExists(path) {
  try {
    return fs.existsSync(path) && fs.statSync(path).isFile();
  } catch (_) {
    return false;
  }
}
const desired = ['Chrome', 'Firefox'];
const browsers = desired.filter(name => {
  const launcher = hasLauncher(name.toLowerCase());
  const bin = process.env[`${name.toUpperCase()}_BIN`] || (
    name.toLowerCase() === 'chrome' ? '/usr/bin/google-chrome' :
    name.toLowerCase() === 'firefox' ? '/usr/bin/firefox' : null
  );
  return launcher && (bin ? binaryExists(bin) : true);
});
module.exports = function(config) {
  config.set({
    // … other config …
    browsers,
    // …
  });
};

Missing diagnostic detail

Are you explicitly defining the plugins array in your karma.conf.js? Knowing this determines whether you need to add the missing launcher to that array or simply install the plugin via npm.