esm-capability-check-failed: ESM capability check could not complete

Cause

The ESM capability check could not access or initialize its same-origin iframe, an enforced CSP blocked a probe script, or the check did not complete within five seconds. This indicates a restriction on the check, not proof that the browser lacks dynamic multiple-import-map support.

The check cleans up its iframe, event handlers, and temporary blob URLs. Its result is reused within the host document. If the application's configured loadTimeout expires earlier, the host may receive load-timeout first.

Troubleshooting

1. Inspect the console for CSP, Trusted Types, and iframe access errors.
2. Allow creation and access to a same-origin about:blank iframe. It inherits the host origin and CSP and only runs feature detection, not the micro-app.
3. Ensure script policies permit the probe's blob: modules and inline import maps. Check more specific directives such as script-src-elem as well. See Content Security Policy (CSP) for the full requirements.

Resolution

Adjust script and embedding policies to fit your deployment requirements. Until the check passes, show a loading error or select a Classic build entry. This check does not verify micro-app resource CORS, network availability, or every CSP requirement.

Related

• Browser support
• Error codes and solutions
• esm-capability-check-failed：ESM 能力检测未完成