PolicyEngine Modal deployment

How to deploy PolicyEngine backend APIs (custom Modal backends for dashboards and interactive tools) to Modal under the PolicyEngine organizational workspace.

This skill applies only when a dashboard or tool uses the custom-backend data pattern. If the project uses api-v2-alpha (stub data or direct API calls), no Modal deployment is needed.

Workspace

PolicyEngine uses a shared Modal workspace called policyengine. All backend deployments MUST target this workspace — never a personal workspace.

Environments

The policyengine workspace has three environments:

Environment	Web suffix	URL pattern	Purpose
main	(empty)	policyengine--<app>-<func>.modal.run	Production
staging	staging	policyengine-staging--<app>-<func>.modal.run	Pre-production testing
testing	testing	policyengine-testing--<app>-<func>.modal.run	Development/CI

Default to main for production deployments.

Authentication

Prerequisites

1. A Modal account linked to the PolicyEngine workspace (ask a workspace owner for an invite)
2. The Modal CLI installed: pip install modal
3. A token for the policyengine workspace stored in a local profile

Setting up authentication

# Create a token for the policyengine workspace (opens browser)
modal token new --profile policyengine

# Activate the profile
modal profile activate policyengine

# Verify — must show "Workspace: policyengine"
modal token info

Verifying authentication before deploy

HUMAN GATE: Before any deployment, verify the active workspace:

modal token info
modal profile list

Expected output from modal profile list:

┏━━━┳━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━┓
┃   ┃ Profile      ┃ Workspace    ┃
┡━━━╇━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━┩
│ • │ policyengine │ policyengine │
└───┴──────────────┴──────────────┘

The • indicates the active profile. If policyengine is not active or not present:

Authentication required. Your Modal CLI is not configured for the policyengine workspace.

Run:

modal token new --profile policyengine
modal profile activate policyengine

If you don't have access to the PolicyEngine workspace, ask a workspace owner to invite you at https://modal.com/settings/policyengine

Do NOT proceed with deployment until modal token info shows Workspace: policyengine.

Critical: environment variable override

Modal CLI respects MODAL_TOKEN_ID and MODAL_TOKEN_SECRET environment variables, which override the profile. If these are set (e.g., from a CI environment), the CLI will deploy to whatever workspace those tokens belong to — potentially a personal workspace.

Always unset before deploying:

unset MODAL_TOKEN_ID MODAL_TOKEN_SECRET

App structure

Naming convention

The Modal app name MUST match the dashboard/tool repo name in kebab-case:

app = modal.App("my-dashboard-name")

This keeps the app name consistent with the GitHub repo and Vercel project.

Image and dependencies

Build a container image with the required Python packages:

image = (
    modal.Image.debian_slim(python_version="3.12")
    .pip_install(
        "policyengine-us>=1.155.0",  # Pin minimum version
        "fastapi[standard]",
        "numpy",
        "pandas",
    )
    .env({"NUMEXPR_MAX_THREADS": "4"})
    .add_local_dir("api", "/root/api")          # Local source code
    .add_local_file("config.yaml", "/root/config.yaml")  # Config files
)

Guidelines:

• Use debian_slim with Python 3.12 or 3.13
• Pin minimum versions for policyengine-us / policyengine-uk
• Use .add_local_dir() / .add_local_file() for project source code
• Use .env() for non-secret environment variables
• Set memory and timeout on the function, not the image

Web endpoint patterns

Simple endpoint (single function)

For tools with one calculation endpoint:

@app.function(image=image, timeout=300, memory=2048)
@modal.web_endpoint(method="POST")
def calculate(data: dict) -> dict:
    from policyengine_us import Simulation
    # Build simulation from data, return results
    return {"result": value}

URL: https://policyengine--<app-name>-calculate.modal.run

Full FastAPI app (multiple endpoints)

For dashboards with multiple API routes:

@app.function(image=image, timeout=300, memory=2048)
@modal.concurrent(max_inputs=100)
@modal.asgi_app()
def fastapi_app():
    from api.main import app as api
    return api

URL: https://policyengine--<app-name>-fastapi-app.modal.run

Health check endpoint

Every Modal backend SHOULD include a health check:

@app.function(image=image)
@modal.web_endpoint(method="GET")
def health():
    return {"status": "ok"}

Function configuration

Parameter	Default	Recommended for PE	Purpose
timeout	60s	300	PolicyEngine simulations can take minutes
memory	128MB	2048	PE models are memory-intensive
@modal.concurrent(max_inputs=N)	1	100	Handle concurrent requests without cold starts

Secrets

Store sensitive values as Modal Secrets (not in code or .env files):

# Create a secret
modal secret create my-secret API_KEY=abc123

# List secrets
modal secret list

Reference in code:

@app.function(
    image=image,
    secrets=[modal.Secret.from_name("my-secret")],
)
def my_function():
    import os
    api_key = os.environ["API_KEY"]  # Injected by Modal

Existing secrets in the policyengine workspace:

• policyengine-logfire — logging/observability
• gcp-credentials — Google Cloud access
• huggingface-token — HuggingFace model access
• anthropic-api-key — Anthropic API access

Deployment

Deploy command

# 1. Ensure correct workspace
unset MODAL_TOKEN_ID MODAL_TOKEN_SECRET
modal token info  # Verify "Workspace: policyengine"

# 2. Deploy to production
modal deploy modal_app.py --env main

# 3. Deploy to staging (for testing)
modal deploy modal_app.py --env staging

Flags:

• --env main / --env staging / --env testing — target environment
• --name TEXT — override the deployment name (rarely needed)
• --tag TEXT — tag the deployment with a version string
• --stream-logs — stream container logs during deployment

Verify deployment

# List deployed apps
modal app list --env main

# Health check
curl -s -w "\n%{http_code}" https://policyengine--DASHBOARD_NAME-health.modal.run

# Test the endpoint
curl -X POST https://policyengine--DASHBOARD_NAME-calculate.modal.run \
  -H "Content-Type: application/json" \
  -d '{"test": true}'

Connecting to Vercel frontend

After Modal deployment, set the API URL as an environment variable in the Vercel project:

vercel env add NEXT_PUBLIC_API_URL production
# Enter: https://policyengine--DASHBOARD_NAME-calculate.modal.run
vercel --prod --force --yes --scope policy-engine

The --force flag is required to rebuild with the new environment variable.

Redeployment

Redeploying an existing app is the same command — Modal handles zero-downtime transitions:

1. New containers build while old ones handle requests
2. New containers start accepting requests
3. Old containers finish in-flight requests then terminate

modal deploy modal_app.py --env main

Stopping an app

WARNING: This is destructive and irreversible. A stopped app cannot be restarted; you must redeploy.

modal app stop <app-name>

Monitoring

# View logs for a deployed app
modal app logs <app-name>

# List all apps and their status
modal app list --env main

App states:

• deployed — running and accepting requests
• ephemeral — temporary (from modal serve)
• stopped — permanently stopped

Troubleshooting

Issue	Cause	Fix
modal token info shows wrong workspace	Wrong profile active	modal profile activate policyengine
Deploy goes to personal workspace	MODAL_TOKEN_ID env var set	unset MODAL_TOKEN_ID MODAL_TOKEN_SECRET
404 on endpoint URL	App stopped or never deployed	modal deploy modal_app.py --env main
Cold start latency (5-15s)	No warm containers	Add @modal.concurrent(max_inputs=100)
MemoryError during simulation	Container memory too low	Increase memory= (try 4096)
Timeout error	Simulation exceeds limit	Increase timeout= (try 600)
Python dependency conflict	Version mismatch	Pin exact versions in .pip_install()
Missing local files in container	Forgot .add_local_dir()	Add source dirs to image definition
Modal app silently disappeared	Unknown — can happen	curl the URL; if 404, redeploy

What NOT to do

• MUST NOT deploy to a personal workspace — always use the policyengine workspace
• MUST NOT leave MODAL_TOKEN_ID / MODAL_TOKEN_SECRET env vars set when deploying via profile
• MUST NOT use modal run for production deployments — use modal deploy
• MUST NOT use modal serve for production — it creates ephemeral apps that stop when you close your terminal
• MUST NOT skip health check verification after deployment
• MUST NOT use generic app names — always match the dashboard/tool repo name
• MUST NOT hardcode secrets in Python code — use Modal Secrets
• MUST NOT deploy without confirming the target workspace with modal token info

Related skills

• policyengine-vercel-deployment-skill — Frontend deployment to Vercel
• policyengine-interactive-tools-skill — Pattern C (custom Modal API) for interactive tools
• policyengine-dashboard-workflow-skill — Full dashboard creation workflow