Javascript is broken with FastAPI sub-application.

[racinmat]

Describe the bug

When you mount the fastapi app as sub-application, the js in /dev-ui/ stops working.

To Reproduce

1. Make the agent

from pathlib import Path
import uvicorn
from fastapi import FastAPI
from google.adk.cli.fast_api import get_fast_api_app
app = FastAPI()
adk_app = get_fast_api_app(
    agents_dir=str((Path(__file__) / '..').absolute()),
    session_service_uri="sqlite:///:memory:",
    allow_origins=["*"],
    web=True,
)
app.mount("/adk", adk_app)
uvicorn.run(app, host="0.0.0.0", port=8081)

2. Run the code
3. Visit http://localhost:8081/adk/dev-ui/
4. See the error Http failure response for http://localhost:8081/list-apps?relative_path=./: 404 Not Found

Expected behavior

There would be no error, proper subpath would be used.

Screenshots

Desktop (please complete the following information):

• OS: Windows 11
• Python version(python -V): Python 3.12.8
• ADK version(pip show google-adk): 1.7.0

Additional context
Add any other context about the problem here.

[racinmat]

In the end, setting the runtimeConfig could be enough, but I have no idea how to do it from the get_fast_api_app.

[racinmat]

For anyone interested, this is my workaround:

class ConfigInjectingStaticFiles(StaticFiles):
    """
    Custom StaticFiles that injects config.json for dev-ui.
    Fixes https://github.com/google/adk-python/issues/2072
    """
    
    def __init__(self, *, directory, host: str, port: int, **kwargs):
        super().__init__(directory=directory, **kwargs)
        self.host = host
        self.port = port
    
    async def get_response(self, path: str, scope) -> Response:
        # Check if the request is for config.json
        if Path(path).as_posix() == "assets/config/runtime-config.json":
            config = {
                "backendUrl": f"http://{self.host}:{self.port}/adk"
            }
            return JSONResponse(content=config)
        
        # Otherwise, serve static files normally
        return await super().get_response(path, scope)

routes_to_keep = []
static_directory = None

for route in adk_app.routes:
    # Check if this is a Mount route for /dev-ui
    if isinstance(route, Mount) and route.path == '/dev-ui':
        # This is the StaticFiles mount, extract the directory
        if isinstance(route.app, StaticFiles) and hasattr(route.app, 'directory'):
            static_directory = route.app.directory
        continue  # Don't keep this route
    routes_to_keep.append(route)

# Replace routes with filtered list
adk_app.router.routes = routes_to_keep

# Add our custom StaticFiles mount if we found the directory
if static_directory:
    adk_app.mount(
        "/dev-ui/",
        ConfigInjectingStaticFiles(
            directory=static_directory,
            host=<host where it runs>,
            port=<port where it runs>,
            html=True
        ),
        name="custom_static",
    )

[wyf7107]

1. What is you use case? If you want to build and test agents, you could go to the parent folder and run adk web directly. adk web fastapi app has its own internal routing, you would need to customize it to your own needs for your use case.

2. The 404 from list_app is probably due to wrong agent folder, and the loader failed to fetch any agents inside. Could you double check that?

[racinmat]

1. The use-case is: I have FastAPI application where I have my own logic, my own routes, and I want to add the adk under some prefix, e.g. /api/v1/adk, so all adk endpoints would be there, while the non-adk endpoints of my application would have different url prefix.
2. It definitely is not due to agent folder, it really has to with sub-apps and url prefix, I know that because I was able to fix it locally, see the code I posted above or the PR that fixes it: fix: dev ui url works in sub-apps  #2568.