fix: dev ui url works in sub-apps

contributing/samples/a2a_auth/README.md

@@ -185,7 +185,7 @@ When deploying the remote BigQuery A2A agent to different environments (e.g., Cl
 }
 ```
 
-**Important:** The `url` field in `remote_a2a/bigquery_agent/agent.json` must point to the actual RPC endpoint where your remote BigQuery A2A agent is deployed and accessible.
+**Important:** The `url` field in `remote_a2a/bigquery_agent/agent.json` must point to the actual RPC endpoint where your remote BigQuery A2A agent is deployed and accessible. If the `url` field is an empty string, it will be automatically filled by the base URL provided to `get_fast_api_app`.
 
 ## Troubleshooting
 

contributing/samples/a2a_basic/README.md

@@ -135,7 +135,7 @@ When deploying the remote A2A agent to different environments (e.g., Cloud Run,
 }
 ```
 
-**Important:** The `url` field in `remote_a2a/check_prime_agent/agent.json` must point to the actual RPC endpoint where your remote A2A agent is deployed and accessible.
+**Important:** The `url` field in `remote_a2a/check_prime_agent/agent.json` must point to the actual RPC endpoint where your remote A2A agent is deployed and accessible. If the `url` field is an empty string, it will be automatically filled by the base URL provided to `get_fast_api_app`.
 
 ## Troubleshooting
 

contributing/samples/a2a_human_in_loop/README.md

@@ -144,7 +144,7 @@ When deploying the remote approval A2A agent to different environments (e.g., Cl
 }
 ```
 
-**Important:** The `url` field in `remote_a2a/human_in_loop/agent.json` must point to the actual RPC endpoint where your remote approval A2A agent is deployed and accessible.
+**Important:** The `url` field in `remote_a2a/human_in_loop/agent.json` must point to the actual RPC endpoint where your remote approval A2A agent is deployed and accessible. If the `url` field is an empty string, it will be automatically filled by the base URL provided to `get_fast_api_app`.
 
 ## Troubleshooting
 

src/google/adk/cli/adk_web_server.py

@@ -18,6 +18,7 @@
 from contextlib import asynccontextmanager
 import logging
 import os
+from pathlib import Path
 import time
 import traceback
 import typing
@@ -31,6 +32,7 @@
 from fastapi import HTTPException
 from fastapi import Query
 from fastapi.middleware.cors import CORSMiddleware
+from fastapi.responses import JSONResponse
 from fastapi.responses import RedirectResponse
 from fastapi.responses import StreamingResponse
 from fastapi.staticfiles import StaticFiles
@@ -44,6 +46,8 @@
 from opentelemetry.sdk.trace import TracerProvider
 from pydantic import Field
 from pydantic import ValidationError
+from starlette.responses import Response
+from starlette.staticfiles import PathLike
 from starlette.types import Lifespan
 from typing_extensions import override
 from watchdog.observers import Observer
@@ -197,6 +201,28 @@ class GetEventGraphResult(common.BaseModel):
   dot_src: str
 
 
+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: PathLike | None = None, base_url: Optional[str] = None, **kwargs):
+    super().__init__(directory=directory, **kwargs)
+    if base_url is None:
+      base_url = ""
+    self.base_url = base_url
+
+  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": self.base_url}
+      return JSONResponse(content=config)
+
+    # Otherwise, serve static files normally
+    return await super().get_response(path, scope)
+
+
 class AdkWebServer:
   """Helper class for setting up and running the ADK web server on FastAPI.
 
@@ -288,6 +314,7 @@ def get_fast_api_app(
           [Observer, "AdkWebServer"], None
       ] = lambda o, s: None,
       register_processors: Callable[[TracerProvider], None] = lambda o: None,
+      base_url: Optional[str] = None,
   ):
     """Creates a FastAPI app for the ADK web server.
 
@@ -304,6 +331,8 @@ def get_fast_api_app(
       tear_down_observer: Callback for cleaning up the file system observer.
       register_processors: Callback for additional Span processors to be added
         to the TracerProvider.
+      base_url: The base URL for the web-ui, useful if fastapi app is mounted as
+        a sub-application. If none is provided, the host is used in the frontend.
 
     Returns:
       A FastAPI app instance.
@@ -1024,7 +1053,12 @@ async def redirect_dev_ui_add_slash():
 
       app.mount(
           "/dev-ui/",
-          StaticFiles(directory=web_assets_dir, html=True, follow_symlink=True),
+          ConfigInjectingStaticFiles(
+              directory=web_assets_dir,
+              base_url=base_url,
+              html=True,
+              follow_symlink=True,
+          ),
           name="static",
       )
 

src/google/adk/cli/cli_tools_click.py

@@ -23,6 +23,7 @@
 import os
 import tempfile
 from typing import Optional
+from urllib.parse import urlparse
 
 import click
 from click.core import ParameterSource
@@ -616,18 +617,12 @@ def fast_api_common_options():
 
   def decorator(func):
     @click.option(
-        "--host",
+        "--base_url",
         type=str,
-        help="Optional. The binding host of the server",
-        default="127.0.0.1",
+        help="Optional. The base URL of the server.",
+        default="http://127.0.0.1:8000",
         show_default=True,
     )
-    @click.option(
-        "--port",
-        type=int,
-        help="Optional. The port of the server",
-        default=8000,
-    )
     @click.option(
         "--allow_origins",
         help="Optional. Any additional origins to allow for CORS.",
@@ -719,8 +714,7 @@ def cli_web(
     eval_storage_uri: Optional[str] = None,
     log_level: str = "INFO",
     allow_origins: Optional[list[str]] = None,
-    host: str = "127.0.0.1",
-    port: int = 8000,
+    base_url="http://127.0.0.1:8000",
     trace_to_cloud: bool = False,
     reload: bool = True,
     session_service_uri: Optional[str] = None,
@@ -741,6 +735,9 @@ def cli_web(
     adk web --session_service_uri=[uri] --port=[port] path/to/agents_dir
   """
   logs.setup_adk_logger(getattr(logging, log_level.upper()))
+  parsed_url = urlparse(base_url)
+  host = parsed_url.hostname
+  port = parsed_url.port
 
   @asynccontextmanager
   async def _lifespan(app: FastAPI):
@@ -777,8 +774,7 @@ async def _lifespan(app: FastAPI):
       trace_to_cloud=trace_to_cloud,
       lifespan=_lifespan,
       a2a=a2a,
-      host=host,
-      port=port,
+      base_url=base_url,
       reload_agents=reload_agents,
   )
   config = uvicorn.Config(
@@ -810,8 +806,7 @@ def cli_api_server(
     eval_storage_uri: Optional[str] = None,
     log_level: str = "INFO",
     allow_origins: Optional[list[str]] = None,
-    host: str = "127.0.0.1",
-    port: int = 8000,
+    base_url="http://127.0.0.1:8000",
     trace_to_cloud: bool = False,
     reload: bool = True,
     session_service_uri: Optional[str] = None,
@@ -833,6 +828,9 @@ def cli_api_server(
   """
   logs.setup_adk_logger(getattr(logging, log_level.upper()))
 
+  parsed_url = urlparse(base_url)
+  host = parsed_url.hostname
+  port = parsed_url.port
   session_service_uri = session_service_uri or session_db_url
   artifact_service_uri = artifact_service_uri or artifact_storage_uri
   config = uvicorn.Config(
@@ -846,8 +844,7 @@ def cli_api_server(
           web=False,
           trace_to_cloud=trace_to_cloud,
           a2a=a2a,
-          host=host,
-          port=port,
+          base_url=base_url,
           reload_agents=reload_agents,
       ),
       host=host,

src/google/adk/cli/fast_api.py

@@ -64,8 +64,7 @@ def get_fast_api_app(
     allow_origins: Optional[list[str]] = None,
     web: bool,
     a2a: bool = False,
-    host: str = "127.0.0.1",
-    port: int = 8000,
+    base_url: str = "http://127.0.0.1:8000",
     trace_to_cloud: bool = False,
     reload_agents: bool = False,
     lifespan: Optional[Lifespan[FastAPI]] = None,
@@ -189,7 +188,9 @@ def _parse_agent_engine_resource_name(agent_engine_id_or_resource_name):
   )
 
   # Callbacks & other optional args for when constructing the FastAPI instance
-  extra_fast_api_args = {}
+  extra_fast_api_args = dict(
+      base_url=base_url,
+  )
 
   if trace_to_cloud:
     from opentelemetry.exporter.cloud_trace import CloudTraceSpanExporter
@@ -352,6 +353,8 @@ async def _get_a2a_runner_async() -> Runner:
         logger.info("Setting up A2A agent: %s", app_name)
 
         try:
+          a2a_rpc_path = f"{base_url}/a2a/{app_name}"
+
           agent_executor = A2aAgentExecutor(
               runner=create_a2a_runner_loader(app_name),
           )
@@ -363,6 +366,8 @@ async def _get_a2a_runner_async() -> Runner:
           with (p / "agent.json").open("r", encoding="utf-8") as f:
             data = json.load(f)
             agent_card = AgentCard(**data)
+            if agent_card.url == "":    # empty url is a placeholder to be filled with the provided url
+                agent_card.url = a2a_rpc_path
 
           a2a_app = A2AStarletteApplication(
               agent_card=agent_card,