fix: gradio_api path in mount_gradio_app

[shandowc]

Description

When mounting a Gradio app to a FastAPI application at a subpath using gr.mount_gradio_app(), custom component assets (CSS and JS files) return 404 errors because the frontend requests don't include the mount path prefix.

Closes: #12802

AI Disclosure

We encourage the use of AI tooling in creating PRs, but the any non-trivial use of AI needs be disclosed. E.g. if you used Claude to write a first draft, you should mention that. Trivial tab-completion doesn't need to be disclosed. You should self-review all PRs, especially if they were generated with AI.

• I used AI to write a first draft
• I did not use AI

🎯 PRs Should Target Issues

Before your create a PR, please check to see if there is an existing issue for this change. If not, please create an issue before you create this PR, unless the fix is very small.

Not adhering to this guideline will result in the PR being closed.

Testing and Formatting Your Code

1. PRs will only be merged if tests pass on CI. We recommend at least running the backend tests locally, please set up your Gradio environment locally and run the backed tests: bash scripts/run_backend_tests.sh

2. Please run these bash scripts to automatically format your code: bash scripts/format_backend.sh, and (if you made any changes to non-Python files) bash scripts/format_frontend.sh

[Copilot]

Pull request overview

This pull request fixes a bug where custom component assets (CSS and JS files) return 404 errors when a Gradio app is mounted at a subpath using gr.mount_gradio_app(). The issue occurred because the frontend was incorrectly constructing the API URL using JavaScript's URL constructor, which doesn't properly preserve the mount path.

Changes:

• Introduced a new get_api_url() helper function that correctly concatenates the root path and API prefix
• Replaced direct new URL(config.api_prefix, config.root) calls with the new helper function in two locations within the AppTree class

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[shandowc]

@freddyaboulton could you plz review this pr?

[Florian-BACHO]

Hi,

Any update on this PR?
We would like to migrate to Gradio 6 but this issue is blocking us.

[freddyaboulton]

Will take a look! Sorry for the delay. We were waiting on #12879