Your app built successfully but never started listening on its port How to fix it.

What it means

The build worked. Antideploy then started your app and waited for it to answer on its port, and it never did. The message tells you the port, the command it ran, and, when it can, what the app printed before it stopped. This is the most common failure on Antideploy.

The three usual causes

• The app listens on a fixed port, or on 127.0.0.1. The platform tells your app the port in the PORT environment variable, and the app must listen on 0.0.0.0.
• The start command needs a package that is not in your dependencies. Development dependencies are removed from the image after the build.
• The app crashed on boot. A missing config file, a missing environment variable, or a database query before the tables exist.

How to fix it

1. Read what the app printed

   The message includes the last lines the app printed before it exited. They usually name the cause. Your agent can also read the app's logs.

2. Read the port and listen on all addresses

   Use the PORT variable, and bind to 0.0.0.0.

   Node (Express)

   const port = process.env.PORT || 3000;
   app.listen(port, "0.0.0.0");
   
   # Python (Flask)
   import os
   app.run(host="0.0.0.0", port=int(os.environ.get("PORT", 5000)))
   
   # Python (Gradio)
   demo.launch(server_name="0.0.0.0", server_port=int(os.environ.get("PORT", 7860)))

3. Check the start command

   The message shows the command that was run. If your project has no start script, add one, or add a Procfile with a web: line. A start command that needs a package must find it in dependencies.

4. Deploy again

   Redeploying the same code unchanged fails the same way, so change something first. A failed deploy never counts against your plan.

Frameworks that are started for you

When Antideploy chooses the start command, FastAPI, Flask, Django and Streamlit already get the port and the address. If you wrote your own start command or a Procfile, yours is used instead. Gradio is started with plain python, so your code must pass them. A Go or Java app must listen on the port in PORT, which is 8080 for these languages.