Railway Deploy Fails - Nixpacks Build & Start Errors in CI

By Daniel Zoghalchali·Latchkey

Railway built your service (usually via Nixpacks) and the deploy failed - the builder could not detect or build the app, the start command crashed, or a required variable/service reference was missing.

What this error means

The Railway deploy log shows a Nixpacks build failure or the service crash-looping after deploy. A real build/start error reproduces each run; a missing variable or unset start command fails until configured. The build/runtime logs carry the detail.

Railway deploy log

Nixpacks build failed
╔════════ Nixpacks ════════╗
║ setup: nodejs            ║
╚══════════════════════════╝
error: no start command could be found
# or: ReferenceError: process.env.DATABASE_URL is undefined

Common causes

Nixpacks cannot detect or build the app

No recognizable build, or no start command, so Nixpacks does not know how to run the service. Common in monorepos or non-standard layouts.

Missing variable or service reference

A required variable is unset, or a reference variable (e.g. a database URL from a linked service) was not wired, so the app crashes on start.

How to fix it

Set explicit build/start commands

Pin the commands so Railway does not have to guess, and target the right directory in a monorepo.

railway.json

# railway.json
{
  "build": { "builder": "NIXPACKS", "buildCommand": "npm ci && npm run build" },
  "deploy": { "startCommand": "npm start" }
}

Wire variables and service references

Add required variables in the service’s Variables tab.
Use reference variables (e.g. ${{Postgres.DATABASE_URL}}) to pull values from linked services.
Bind the server to the injected PORT and redeploy.

How to prevent it

Set explicit build and start commands in railway.json rather than relying on detection.
Configure required variables and use service reference variables for linked services.
Listen on the injected PORT so the service stays healthy.

Frequently asked questions

What causes "Railway deploy failed"?