Modal

1.3.3 · active · verified Sun Mar 01

Serverless cloud infrastructure for AI workloads. Run Python functions on GPUs with sub-second cold starts. v1.0 released May 2025 with multiple breaking API changes. Rapid release cadence — multiple versions per week.

Warnings

breaking modal.Stub renamed to modal.App. Any code using modal.Stub() raises AttributeError on current versions.
Fix: Replace all modal.Stub() with modal.App().
breaking modal.Mount removed from public API in v1.0. Passing mount= to @app.function or @app.cls raises TypeError.
Fix: Use Image.add_local_file() or Image.add_local_dir() instead of modal.Mount.
breaking Automounting of local Python modules removed in v1.0. Remote containers no longer automatically include local imports.
Fix: Explicitly add local dependencies with Image.add_local_python_source('your_module').
breaking Autoscaler parameters renamed in v1.0: keep_warm → min_containers, concurrency_limit → max_containers, container_idle_timeout → scaledown_window.
Fix: Find-and-replace all three parameter names in function decorators.
breaking @modal.build decorator deprecated in v1.0. Using it for model weight downloads is no longer the recommended pattern.
Fix: Use modal.Volume to store model weights instead. Mount the Volume to the framework's cache directory.
deprecated Image.copy_local_dir and Image.copy_local_file deprecated. Replaced by Image.add_local_dir and Image.add_local_file.
Fix: Use Image.add_local_dir() / Image.add_local_file(). Pass copy=True if subsequent build steps need the files.
deprecated modal.web_endpoint renamed to modal.fastapi_endpoint. Old name still works but will be removed in a future v1.x release.
Fix: Replace @modal.web_endpoint() with @modal.fastapi_endpoint().
gotcha Modal releases multiple versions per week including pre-release .devN builds. Unpinned installs in CI can pick up pre-release versions unexpectedly.
Fix: Pin to a stable release in production: pip install modal==1.3.3. Avoid pip install modal --pre in automated environments.
gotcha modal.Dict values expire after 7 days of inactivity on the new backend (introduced May 2025). Previously created Dicts use the old backend with no expiry.
Fix: Do not use modal.Dict for persistent storage. Use modal.Volume for data that must survive beyond 7 days of inactivity.

Install

pip install modal Python
modal setup CLI authentication (run after install)

Imports

App
```
import modal
app = modal.App()
```
modal.Stub was renamed to modal.App in v0.x. All tutorials using Stub() are outdated and will raise AttributeError.
fastapi_endpoint
```
@modal.fastapi_endpoint()
```
modal.web_endpoint renamed to modal.fastapi_endpoint in v0.73.89 / v1.0. Old decorator still works but is deprecated.
concurrent
```
@modal.concurrent(max_inputs=10)
```
allow_concurrent_inputs parameter replaced by @modal.concurrent decorator in v1.0.

Quickstart

Deploy a GPU function to Modal. Run with: modal run script.py

import modal

app = modal.App()

@app.function(
    gpu="A10G",
    image=modal.Image.debian_slim().pip_install("torch")
)
def run_inference(prompt: str) -> str:
    import torch
    # your model code here
    return "result"

@app.local_entrypoint()
def main():
    result = run_inference.remote("hello")
    print(result)

view raw JSON →