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You have a production stack. You want an agent on it. But you can’t just point an agent at production—it’ll make real changes, hit real APIs, affect real users. And you can’t test at scale against a single live instance with shared state. HUD lets you mock your production environment so agents can run against it safely. Connect your services in a few lines, mock external dependencies, and run thousands of agents in parallel—each isolated, each reproducible, each generating useful data.

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Connecting Your Stack

HUD wraps your existing infrastructure without rewriting it: 
from hud import Environment

env = Environment("my-env")

# Connect what you already have
env.connect_fastapi(app)                                    # FastAPI → tools
env.connect_openapi("https://api.example.com/openapi.json") # OpenAPI spec → tools
env.connect_hub("hud-evals/browser")                        # HUD Hub environments
env.connect_image("my-service:v1")                          # Docker images

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Making Databases Safe

Agents need isolated state. Three patterns work: In-memory SQLite — fastest, resets automatically: 
import sqlite3
db = sqlite3.connect(":memory:")  # Fresh per eval

@env.scenario("update-order")
async def update_order(order_id: str):
    db.executescript(Path("fixtures/orders.sql").read_text())  # Seed
    answer = yield f"Update order {order_id} to shipped"
    row = db.execute("SELECT status FROM orders WHERE id=?", (order_id,)).fetchone()
    yield 1.0 if row and row[0] == "shipped" else 0.0
Transaction rollback — use your real DB, undo changes: 
@env.scenario("process-refund")
async def process_refund(order_id: str):
    conn = await asyncpg.connect(DATABASE_URL)
    tx = conn.transaction()
    await tx.start()
    try:
        answer = yield f"Process refund for order {order_id}"
        # Check result...
        yield reward
    finally:
        await tx.rollback()  # Always undo
        await conn.close()
Fixture seeding — deterministic starting state: 
await db.execute("TRUNCATE orders, users CASCADE")
await db.executemany("INSERT INTO users ...", fixtures["users"])

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Mocking External Services

env.mock() intercepts at the tool layer. Agents only see tools, so this is usually all you need: 
env.mock()  # All tools return schema-based fake responses
env.mock_tool("send_email", {"status": "sent", "id": "mock-123"})
env.mock_tool("charge_card", {"success": True, "transaction_id": "tx-mock"})
For stateful mocking (tracking what happened for assertions): 
class MockPaymentService:
    def __init__(self):
        self.charges = []
    
    async def charge(self, amount: int, card_token: str) -> dict:
        self.charges.append({"amount": amount, "token": card_token})
        return {"success": True, "id": f"ch-{len(self.charges)}"}

payments = MockPaymentService()

@env.scenario("checkout")
async def checkout(cart_total: int):
    _ = yield f"Complete checkout for ${cart_total}"
    yield 1.0 if any(c["amount"] == cart_total for c in payments.charges) else 0.0

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Docker vs No Docker

PatternWhen to UseExamples
No DockerPure Python, API integrationsWeb research, LLM grading
DockerSystem dependencies, persistent servicesVNC, PostgreSQL, browsers

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Pattern 1: No Docker

Import and test directly: 
# local_test.py
from env import env

async def test():
    async with env:
        result = await env.call_tool("search", query="test")

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Pattern 2: Docker

Connect to the running container instead of importing. Same API, different transport—because your tools now run inside the container where dependencies live: 
# local_test.py
env = Environment("browser-env")
env.connect_url("http://localhost:8765/mcp")  # Connect instead of import

async def test():
    async with env:  # Same API from here
        result = await env.call_tool("navigate", url="https://example.com")

hud build                                 # Build image
hud dev -w scenarios -w tools --port 8765 # Start with hot-reload
python local_test.py                      # Connects to container

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Hot-Reload

hud dev -w path reloads Python on save. System services (postgres, VNC) persist. Rebuild (hud build) when: Dockerfile, system packages, or dependencies change.

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Environment Structure

Start simple, add structure as needed: 
# Simple                      # Organized
my-env/                       my-env/
├── env.py                    ├── env.py
├── local_test.py             ├── scenarios/
└── Dockerfile.hud            ├── setup/
                              ├── evaluate/
                              └── Dockerfile.hud
Most environments fall somewhere between. Split when files get hard to navigate.

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What’s Next

Test locally. See Testing Environments for debugging and scenario testing. Deploy. Push to GitHub, connect on hud.ai. See Deploy.