If you're using Google's ADK Web UI for agent development, you've probably run into this: every time you create a new session, you have to manually set up the state your agent needs. Here's a clean way to automate that with a preset file and ASGI middleware.
The Problem
ADK's Web UI is great for testing agents, but injecting initial state into a session is a bit of a hassle.
The current options are:
Add state via the 3-dot menu in the UI → You have to manually enter values every time you create a session
Usebefore_agent_callbackwith a first-run check → Extra code to write and maintain
Instead, I built a setup where you define initial state in a preset.yaml file, and a custom ASGI middleware automatically merges it into the session when you click "New Session" in the Web UI.
How It Works
When you click "New Session" in the Web UI, it internally calls POST /apps/{app_name}/users/{user_id}/sessions.
The middleware intercepts this request, reads preset.yaml, and merges its initial_state values into the request body's state before passing it to the application.
The key point: you don't need to touch your agent code at all — just add the middleware at server startup.
Setup
Directory Structure
.
├── .venv
├── hello_agent
│ ├── __init__.py
│ ├── agent.py # Agent definition
│ └── preset.yaml # Preset config
└── preset_server.py # Startup script
Target Agent
from google.adk.agents.llm_agent import Agent
root_agent = Agent(
model='gemini-2.5-flash',
name='root_agent',
instruction="""Greet the user in the following format:
Hello, it's currently {current_time} and the weather is {weather}.
""",
)
Without initial state, chatting with this agent in the Web UI will fail because current_time and weather don't exist in the session state.
Dependencies
You'll need pyyaml in addition to ADK's dependencies:
pip install pyyaml
Preset File (preset.yaml)
Define your initial state in YAML format:
initial_state:
current_time: "10:30 AM"
weather: "sunny"
Nested values are supported too.
Place the file in the same directory as agent.py, or inside an .adk/ directory. If both exist, .adk/preset.yaml takes priority.
Startup Script Implementation
The startup script has three main parts.
1. Loading preset.yaml
PRESET_FILENAME = "preset.yaml"
def find_preset_file(agent_dir: Path) -> Optional[Path]:
"""Search for preset.yaml in the agent directory."""
candidates = [
agent_dir / ".adk" / PRESET_FILENAME,
agent_dir / PRESET_FILENAME,
]
for p in candidates:
if p.is_file():
return p
return None
def load_preset(path: Path) -> dict:
with open(path, "r", encoding="utf-8") as f:
data = yaml.safe_load(f) or {}
return {
"state": data.get("initial_state", {}),
}
It checks .adk/preset.yaml first, then falls back to preset.yaml, and returns the initial_state as a dictionary.
2. The Middleware (PresetMiddleware)
This ASGI middleware detects session creation requests and rewrites the body:
SESSION_PATH = re.compile(r"^/apps/([^/]+)/users/[^/]+/sessions$")
class PresetMiddleware:
def __init__(self, app, agents_dir: Path):
self.app = app
self.agents_dir = agents_dir
async def __call__(self, scope, receive, send):
if scope["type"] != "http":
await self.app(scope, receive, send)
return
path = scope.get("path", "")
method = scope.get("method", "")
m = SESSION_PATH.match(path)
if method == "POST" and m:
app_name = m.group(1)
preset = self._load_preset(app_name)
if preset and preset.get("state"):
# Read request body
body_parts = []
while True:
message = await receive()
body_parts.append(message.get("body", b""))
if not message.get("more_body", False):
break
body = b"".join(body_parts)
# Merge state (preset as base, request values override)
data = json.loads(body) if body else {}
merged = {**preset["state"], **(data.get("state") or {})}
data["state"] = merged
new_body = json.dumps(data).encode()
# Update Content-Length and forward
# ... (header rewriting and receive replacement)
await self.app(scope, new_receive, send)
return
await self.app(scope, receive, send)
Here's what it does:
- Only targets
POST /apps/{app_name}/users/{user_id}/sessionsrequests - Reads the request body and merges
initial_statefrompreset.yamlintostate
- Updates
Content-Lengthwith the new body size and passes it to the application
Since request-side state takes priority, any values you manually set in the UI won't be overwritten by the preset.
3. CLI (adk web compatible)
The script supports all the same options as adk web, plus a --preset / --no-preset flag to toggle the middleware:
app = get_fast_api_app(
agents_dir=agents_dir,
# ... same options as adk web
)
if preset:
app.add_middleware(PresetMiddleware, agents_dir=agents_dir_path)
4. Full Startup Script
Full startup script
from __future__ import annotations
import sys
import json
import logging
import os
import re
from contextlib import asynccontextmanager
from pathlib import Path
from typing import Optional
import click
import uvicorn
from fastapi import FastAPI
from google.adk.cli.fast_api import get_fast_api_app
from google.adk.cli.utils import logs
try:
import yaml
except ImportError:
print("PyYAML is required. Install with: pip install pyyaml")
sys.exit(1)
logger = logging.getLogger(__name__)
LOG_LEVELS = click.Choice(
["DEBUG", "INFO", "WARNING", "ERROR", "CRITICAL"],
case_sensitive=False,
)
SESSION_PATH = re.compile(r"^/apps/([^/]+)/users/[^/]+/sessions$")
# ---------------------------------------------------------------------------
# Preset file handling
# ---------------------------------------------------------------------------
PRESET_FILENAME = "preset.yaml"
def find_preset_file(agent_dir: Path) -> Optional[Path]:
"""Search for preset.yaml in .adk/ directory of the agent."""
candidates = [
agent_dir / ".adk" / PRESET_FILENAME,
agent_dir / PRESET_FILENAME,
]
for p in candidates:
if p.is_file():
return p
return None
def load_preset(path: Path) -> dict:
with open(path, "r", encoding="utf-8") as f:
data = yaml.safe_load(f) or {}
return {
"state": data.get("initial_state", {}),
}
# ---------------------------------------------------------------------------
# PresetMiddleware (ASGI)
# ---------------------------------------------------------------------------
class PresetMiddleware:
"""ASGI middleware that injects preset state into POST /apps/{app_name}/users/{user_id}/sessions."""
def __init__(self, app, agents_dir: Path):
self.app = app
self.agents_dir = agents_dir
async def __call__(self, scope, receive, send):
if scope["type"] != "http":
await self.app(scope, receive, send)
return
path = scope.get("path", "")
method = scope.get("method", "")
m = SESSION_PATH.match(path)
if method == "POST" and m:
app_name = m.group(1)
preset = self._load_preset(app_name)
if preset and preset.get("state"):
body_parts = []
while True:
message = await receive()
body_parts.append(message.get("body", b""))
if not message.get("more_body", False):
break
body = b"".join(body_parts)
data = json.loads(body) if body else {}
merged = {**preset["state"], **(data.get("state") or {})}
data["state"] = merged
new_body = json.dumps(data).encode()
new_headers = []
seen_ct = False
seen_cl = False
for key, value in scope.get("headers", []):
lower_key = key.lower()
if lower_key == b"content-length":
new_headers.append((b"content-length", str(len(new_body)).encode()))
seen_cl = True
elif lower_key == b"content-type":
new_headers.append((b"content-type", b"application/json"))
seen_ct = True
else:
new_headers.append((key, value))
if not seen_ct:
new_headers.append((b"content-type", b"application/json"))
if not seen_cl:
new_headers.append((b"content-length", str(len(new_body)).encode()))
scope["headers"] = new_headers
body_sent = False
async def new_receive():
nonlocal body_sent
if not body_sent:
body_sent = True
return {"type": "http.request", "body": new_body, "more_body": False}
return await receive()
logger.info(
"PresetMiddleware: injected state for app=%s keys=%s",
app_name,
list(preset["state"].keys()),
)
await self.app(scope, new_receive, send)
return
await self.app(scope, receive, send)
def _load_preset(self, app_name: str) -> Optional[dict]:
agent_dir = self.agents_dir / app_name
preset_path = find_preset_file(agent_dir)
if preset_path is None:
return None
try:
return load_preset(preset_path)
except Exception as e:
logger.warning("PresetMiddleware: failed to load preset for %s: %s", app_name, e)
return None
# ---------------------------------------------------------------------------
# CLI (adk web compatible)
# ---------------------------------------------------------------------------
@click.command()
@click.argument(
"agents_dir",
type=click.Path(exists=True, dir_okay=True, file_okay=False, resolve_path=True),
default=os.getcwd,
)
@click.option("--host", type=str, default="127.0.0.1", show_default=True, help="The binding host of the server.")
@click.option("--port", type=int, default=8000, help="The port of the server.")
@click.option("--allow_origins", multiple=True, help="Origins to allow for CORS.")
@click.option("-v", "--verbose", is_flag=True, default=False, help="Enable verbose (DEBUG) logging.")
@click.option("--log_level", type=LOG_LEVELS, default="INFO", help="Set the logging level.")
@click.option("--trace_to_cloud", is_flag=True, default=False, help="Enable cloud trace for telemetry.")
@click.option("--otel_to_cloud", is_flag=True, default=False, help="Write OTel data to Google Cloud.")
@click.option("--reload/--no-reload", default=True, help="Enable auto reload for server.")
@click.option("--a2a", is_flag=True, default=False, help="Enable A2A endpoint.")
@click.option("--reload_agents", is_flag=True, default=False, help="Enable live reload for agents changes.")
@click.option("--eval_storage_uri", type=str, default=None, help="Evals storage URI (e.g. gs://bucket).")
@click.option("--extra_plugins", multiple=True, help="Extra plugin classes or instances.")
@click.option("--url_prefix", type=str, default=None, help="URL path prefix for reverse proxy.")
@click.option("--session_service_uri", default=None, help="Session service URI.")
@click.option("--artifact_service_uri", type=str, default=None, help="Artifact service URI.")
@click.option("--memory_service_uri", type=str, default=None, help="Memory service URI.")
@click.option("--use_local_storage/--no_use_local_storage", default=True, show_default=True, help="Use local .adk storage.")
@click.option("--logo-text", type=str, default=None, help="Logo text in web UI.")
@click.option("--logo-image-url", type=str, default=None, help="Logo image URL in web UI.")
# --- preset-specific option ---
@click.option("--preset/--no-preset", default=True, show_default=True, help="Enable PresetMiddleware for auto state injection.")
def cli_preset_web(
agents_dir: str,
host: str,
port: int,
allow_origins: Optional[tuple[str, ...]],
verbose: bool,
log_level: str,
trace_to_cloud: bool,
otel_to_cloud: bool,
reload: bool,
a2a: bool,
reload_agents: bool,
eval_storage_uri: Optional[str],
extra_plugins: Optional[tuple[str, ...]],
url_prefix: Optional[str],
session_service_uri: Optional[str],
artifact_service_uri: Optional[str],
memory_service_uri: Optional[str],
use_local_storage: bool,
logo_text: Optional[str],
logo_image_url: Optional[str],
preset: bool,
):
"""Starts a FastAPI server with Web UI for agents (with preset support).
AGENTS_DIR: The directory of agents, where each subdirectory is a single
agent, containing at least __init__.py and agent.py files.
Example:
python preset_server.py path/to/agents_dir
python preset_server.py . --port 8080 --no-preset
"""
if verbose and log_level == "INFO":
log_level = "DEBUG"
logs.setup_adk_logger(getattr(logging, log_level.upper()))
agents_dir_path = Path(agents_dir).resolve()
@asynccontextmanager
async def _lifespan(app: FastAPI):
preset_msg = " + PresetMiddleware" if preset else ""
click.secho(
f"""
+-----------------------------------------------------------------------------+
| ADK Web Server started{preset_msg:<53s}|
| |
| For local testing, access at http://{host}:{port}.{" " * (29 - len(str(port)))}|
+-----------------------------------------------------------------------------+
""",
fg="green",
)
yield
click.secho(
"""
+-----------------------------------------------------------------------------+
| ADK Web Server shutting down... |
+-----------------------------------------------------------------------------+
""",
fg="green",
)
app = get_fast_api_app(
agents_dir=agents_dir,
session_service_uri=session_service_uri,
artifact_service_uri=artifact_service_uri,
memory_service_uri=memory_service_uri,
use_local_storage=use_local_storage,
eval_storage_uri=eval_storage_uri,
allow_origins=list(allow_origins) if allow_origins else None,
web=True,
trace_to_cloud=trace_to_cloud,
otel_to_cloud=otel_to_cloud,
lifespan=_lifespan,
a2a=a2a,
host=host,
port=port,
url_prefix=url_prefix,
reload_agents=reload_agents,
extra_plugins=list(extra_plugins) if extra_plugins else None,
logo_text=logo_text,
logo_image_url=logo_image_url,
)
if preset:
app.add_middleware(PresetMiddleware, agents_dir=agents_dir_path)
click.secho(
"PresetMiddleware: auto-injecting preset state into POST /apps/*/users/*/sessions",
fg="cyan",
)
config = uvicorn.Config(
app,
host=host,
port=port,
reload=reload,
)
server = uvicorn.Server(config)
server.run()
if __name__ == "__main__":
cli_preset_web()
Testing It Out
Run the startup script:
python preset_server.py .
If the middleware is active, you'll see this in the startup log:
PresetMiddleware: auto-injecting preset state into POST /apps/*/users/*/sessions
Click "New Session" in the Web UI, and the state gets injected automatically:
Ask the agent for a greeting, and you'll see the values from preset.yaml reflected in the response:
Wrap-Up
- Define
initial_stateinpreset.yamland it gets auto-injected on session creation - Since it's implemented as ASGI middleware, no changes to your agent code are needed
- Fully compatible with all
adk weboptions
That's it — no more copy-pasting state values every time you start a new session.
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