Home Assistant Startup Fails — Complete Guide

In this tutorial, you'll learn about Home Assistant Startup Fails. We cover key concepts, practical examples, and best practices.

Hook

You restart Home Assistant Core or the system, and it never comes back. You check http://homeassistant.local:8123 — "Can't reach this page." The Docker container keeps restarting, or the service shows "failed" status.

The Wrong Way

Pulling the latest Docker image without checking for breaking changes often introduces incompatible configuration or database schema changes that cause startup loops.

# BAD: Pulling latest blindly
docker pull ghcr.io/home-assistant/home-assistant:latest
docker stop hass && docker rm hass && docker run ... ghcr.io/home-assistant/home-assistant:latest

[ERROR] Database migration from 2025.6 to 2026.1 failed
[ERROR] Configuration incompatible with version 2026.1
Container restarting in 5 seconds — infinite loop

Skipping version upgrades can break database compatibility.

The Right Way

Check the logs to identify the startup failure, then fix it without losing data.

# 1. Check Home Assistant logs
docker logs hass --tail 100
# Or for HA OS: journalctl -u hassio-apparmor -n 100

[ERROR] Invalid config for [sensor]:
  [platform] is an invalid option for [sensor]. Check: sensor -> platform.

The configuration has a syntax or schema error.

# 2. Validate the configuration
docker exec hass python -m homeassistant --config /config --script check_config

Invalid config for [sensor.test]:
  platform is required. Check: sensor -> test -> platform.

# 3. Fix the configuration.yaml file
nano /path/to/config/configuration.yaml

# 4. Restart Home Assistant
docker restart hass

Home Assistant started in 15 seconds
Web UI accessible at http://homeassistant.local:8123

If the issue is a database migration failure:

# Revert to the previous version temporarily
docker pull ghcr.io/home-assistant/home-assistant:2025.12
# Restore config from backup, then upgrade step-by-step

Prevention

• Read the breaking changes list before upgrading Home Assistant.
• Back up your configuration and database before every upgrade.
• Run ha config check (or the check_config script) after every config change.
• Pin your Home Assistant Docker tag to a specific version (not latest).
• Keep a copy of the previous working version's image.

Advanced Troubleshooting

Check the Logs

Most TOOL errors are logged to stdout or a dedicated log file. Check your logs first:

# Check system logs
journalctl -u tool --since "1 hour ago"

# Or check the application log
tail -50 ~/.tool/logs/error.log

Test with a Minimal Example

Create the simplest possible tool configuration to verify the base setup works:

tool --version
tool --help

If the minimal test passes, add configuration options one at a time until you find the breaking change.

Common Configuration Mistakes

• Using the wrong file path or URL in configuration
• Forgetting to restart TOOL after changing config files
• Mixing tabs and spaces in YAML configuration files
• Setting incorrect permissions on configuration directories

When to Reinstall

If none of the above resolves the issue, consider a clean reinstall:

# Backup your configuration
cp -r ~/.tool ~/.tool.bak

# Remove and reinstall
# Follow the official TOOL installation guide

This ensures you start from a known good state and can isolate the issue.

Common Mistakes with assistant startup

1. Placing the wildcard pattern first in case expressions, making all subsequent patterns unreachable
2. Using head and tail instead of pattern matching, causing runtime errors on empty lists
3. Forgetting that lazy evaluation defers computation until the value is forced, causing space leaks with unevaluated thunks

These mistakes appear frequently in real-world HOME code. DodaTech's contributors have identified these patterns through analysis of open-source projects and production systems.

Practice Exercise

Write a pure function that safely divides two integers using Maybe, then test it with edge cases like division by zero and negative numbers.

This exercise reinforces the concepts covered in this guide. Try implementing it before checking online solutions.

FAQ

DodaTech — start Home Assistant, every time.