Test Flash apps locally

Use flash run to test your Flash application locally before deploying.

The flash run command starts a local development server that lets you test your Flash application before deploying to production. The development server runs locally and updates automatically as you edit files.

When you call a @Endpoint function, Flash sends the latest function code to Serverless workers on Runpod, so your changes are reflected immediately.

Start the development server

From inside your project directory, run:

flash run

# If using uv:
uv run flash run

The server starts at http://localhost:8888 by default. Your endpoints are available immediately for testing, and @Endpoint functions provision Serverless endpoints on first call.

Using a custom host and port

# Change port
flash run --port 3000

# Make accessible on network
flash run --host 0.0.0.0

# If using uv:
uv run flash run --port 3000
uv run flash run --host 0.0.0.0

Test your endpoints

Using curl

# Call a queue-based endpoint (gpu_worker.py)
curl -X POST http://localhost:8888/gpu_worker/runsync \
  -H "Content-Type: application/json" \
  -d '{"input": {"input_data": {"message": "Hello from the GPU"}}}'

# Call a load-balanced endpoint (lb_worker.py)
curl -X POST http://localhost:8888/lb_worker/process \
  -H "Content-Type: application/json" \
  -d '{"input_data": {"message": "Hello from Flash"}}'

Using the API explorer

Open http://localhost:8888/docs in your browser to access the interactive Swagger UI. You can test all endpoints directly from the browser.

Using Python

import requests

# Call queue-based endpoint
response = requests.post(
    "http://localhost:8888/gpu_worker/runsync",
    json={"input": {"input_data": {"message": "Hello from the GPU"}}}
)
print(response.json())

# Call load-balanced endpoint
response = requests.post(
    "http://localhost:8888/lb_worker/process",
    json={"input_data": {"message": "Hello from Flash"}}
)
print(response.json())

Reduce cold-start delays

The first call to a @Endpoint function provisions a Serverless endpoint, which takes 30-60 seconds. Use --auto-provision to provision all endpoints at startup:

flash run --auto-provision

# If using uv:
uv run flash run --auto-provision

This scans your project for @Endpoint functions and deploys them before the server starts accepting requests. Endpoints are cached in .runpod/resources.pkl and reused across server restarts.

How it works

With flash run, Flash starts a local development server alongside remote Serverless endpoints:

What runs where:

Component	Location
Development server	Your machine (localhost:8888)
@Endpoint function code	Runpod Serverless
Endpoint storage	Runpod Serverless

Your code updates automatically as you edit files. Endpoints created by flash run are prefixed with live- to distinguish them from production endpoints.

Clean up after testing

Endpoints created by flash run persist until you delete them. To clean up:

# List all endpoints
flash undeploy list

# Remove a specific endpoint
flash undeploy ENDPOINT_NAME

# Remove all endpoints
flash undeploy --all

# If using uv:
uv run flash undeploy list
uv run flash undeploy ENDPOINT_NAME
uv run flash undeploy --all

Troubleshooting

Port already in use

Flash automatically selects the next available port if your specified port is in use. You’ll see a message like Port 8888 is in use, using 8889 instead. If you need a specific port, stop the process using it or specify a different starting port with --port.

Slow first request

Use --auto-provision to eliminate cold-start delays:

flash run --auto-provision

# If using uv:
uv run flash run --auto-provision

Authentication errors

Run flash login to authenticate:

flash login

# If using uv:
uv run flash login

Alternatively, set RUNPOD_API_KEY as an environment variable or in your .env file:

export RUNPOD_API_KEY="your_api_key_here"

Values in your .env file are only available locally for CLI commands. They are not passed to deployed endpoints.

Next steps

• Deploy to production when your app is ready.
• Clean up endpoints after testing.
• View the flash run reference for all options.