Register a stub transport for ESPHome serial proxies via USB

[puddly]

Proposed change

Integrations that depend on usb but not esphome can load earlier than the ESPHome integration, before the esphome-hass:// URI handler is registered with serialx. This causes them to fail with serialx.common.UnknownUriScheme.

This PR registers a second handler for esphome-hass:// with a lower weight. Integrations that try to connect before ESPHome has loaded will immediately fail with ConfigEntryNotReady. Once ESPHome loads, the real implementation of this serial transport will take over.

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Deprecation (breaking change to happen in the future)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue:
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• Link to developer documentation pull request:
• Link to frontend pull request:

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[home-assistant]

Hey there @bdraco, mind taking a look at this pull request as it has been labeled with an integration (usb) you are listed as a code owner for? Thanks!

Code owner commands

Code owners of usb can trigger bot actions by commenting:

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• @home-assistant unassign usb Removes the current integration label and assignees on the pull request, add the integration domain after the command.
• @home-assistant update-branch Update the pull request branch with the base branch.
• @home-assistant add-label needs-more-information Add a label (needs-more-information, problem in dependency, problem in custom component, problem in config, problem in device, feature-request) to the pull request.
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[home-assistant]

Hey there @jesserockz, @kbx81, @bdraco, mind taking a look at this pull request as it has been labeled with an integration (esphome) you are listed as a code owner for? Thanks!

Code owner commands

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• @home-assistant ready-for-review Remove the draft status from the pull request.
• @home-assistant rename Awesome new title Renames the pull request.
• @home-assistant reopen Reopen the pull request.
• @home-assistant unassign esphome Removes the current integration label and assignees on the pull request, add the integration domain after the command.
• @home-assistant update-branch Update the pull request branch with the base branch.
• @home-assistant add-label needs-more-information Add a label (needs-more-information, problem in dependency, problem in custom component, problem in config, problem in device, feature-request) to the pull request.
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[Copilot]

Pull request overview

This PR ensures esphome-hass:// serialx URLs fail gracefully (with ConfigEntryNotReady) when ESPHome hasn’t loaded yet, by registering a low-priority stub transport from the USB integration.

Changes:

• Add a USB-owned stub esphome-hass:// serialx URI handler that raises ConfigEntryNotReady on open.
• Adjust ESPHome’s register_uri_handler registration to use a distinct unique_scheme so multiple handlers can coexist.
• Add a USB integration test that validates the stub handler is selected and raises the expected exception.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
homeassistant/components/usb/serial_proxy_stub.py	New stub serialx URI handler/transport that raises ConfigEntryNotReady until ESPHome loads.
homeassistant/components/usb/__init__.py	Imports the stub module to register the URI handler during USB integration import.
homeassistant/components/esphome/serial_proxy.py	Updates handler registration to use a different unique_scheme to allow parallel registrations.
tests/components/usb/test_init.py	Adds a test ensuring the stub handler is registered and raises ConfigEntryNotReady when opened.

[bdraco]

we have bluetooth_adapters integration for similar load order problems to make sure components needed for bluetooth load before. Not sure if thats better or worse for this case

[puddly]

It's a bit of a weird situation here because integrations that rely on USB now have an implicit runtime dependency on ESPHome but only if an ESPHome serial proxy is actually used. These integrations typically only have "dependencies": ["usb"]. They'd need to be updated to also have "after_dependencies": ["esphome"], which is unreasonable IMO.

I see a few options:

1. Have some way for integrations to specify forward dependencies (i.e. don't consider usb loaded until esphome also loads), as a way to do this automatically.
2. The slightly messy, double runtime registration dance we do here.
3. Have the USB integration break isolation and register the ESPHome serial proxy itself.

[Copilot]

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

[bdraco]

This works, its not as architecturally pure as I'd like, but the other options aren't particularly better so its pragmatic.