🚨 Docker Error Guide: 'daemon.json: invalid character looking for beginning of object key string' — Fix the JSON

Overview

This error appears when the Docker daemon reads its configuration file and cannot parse it as valid JSON. The daemon refuses to start:

unable to configure the Docker daemon with file /etc/docker/daemon.json: invalid character '}' looking for beginning of object key string

The message comes straight from Go’s JSON parser. invalid character '}' looking for beginning of object key string almost always means a trailing comma — the parser saw a comma promising another "key": value pair, then hit the closing } instead of a new key. Because daemon.json is read at startup, a single stray comma or brace takes the whole Docker daemon down until the syntax is fixed.

Symptoms

• sudo systemctl start docker fails and systemctl status docker shows the daemon in a failed/restart-loop state.
• docker commands all fail with Cannot connect to the Docker daemon at unix:///var/run/docker.sock.
• journalctl -u docker shows the invalid character ... looking for beginning of object key string line at each start attempt.
• The failure started immediately after someone edited /etc/docker/daemon.json (added a registry mirror, log opts, DNS, a runtime, etc.).
• The exact character in the message varies — '}', ']', or ',' — but always points at a JSON syntax slip.

Common Root Causes

• A trailing comma after the last key/value pair in an object or array — the single most common cause of this exact message.
• A missing comma between two pairs, or a stray/duplicate closing } or ].
• Comments in the file — JSON has no comments; a // or # line makes the parser fail.
• Unquoted keys or values, or single quotes instead of double quotes.
• Two config sources fighting — options set both in daemon.json and on the dockerd command line, or a merge that produced malformed JSON.

Diagnostic Workflow

First read the exact parser error and the file it names from the daemon log:

journalctl -u docker --no-pager --since '10 min ago' | grep -i 'daemon.json\|invalid character'
systemctl status docker --no-pager

Validate the file with a real JSON parser — this points at the offending line far more precisely than the daemon does:

jq . /etc/docker/daemon.json
python3 -m json.tool /etc/docker/daemon.json

jq will print something like parse error: ... at line 6, column 1, taking you straight to the trailing comma. A malformed file typically looks like this, with the comma after the last pair being the culprit:

{
  "log-driver": "json-file",
  "log-opts": { "max-size": "10m" },
}

The corrected version simply drops the trailing comma:

{
  "log-driver": "json-file",
  "log-opts": { "max-size": "10m" }
}

Docker can also validate its own configuration without fully starting, which confirms the fix before you restart the service:

sudo dockerd --validate --config-file /etc/docker/daemon.json

Once the file parses cleanly, start the daemon and confirm it is healthy:

sudo systemctl restart docker
docker info | head

Example Root Cause Analysis

An operator added a registry mirror to /etc/docker/daemon.json, restarted Docker, and the daemon would not come back — every docker command reported it could not reach the socket.

journalctl -u docker showed invalid character '}' looking for beginning of object key string. Running jq . /etc/docker/daemon.json reported parse error at line 5, column 1. Line 4 was the new "registry-mirrors": [...] entry — and the operator had appended it with a comma while it was the last key in the object:

{
  "log-driver": "json-file",
  "registry-mirrors": ["https://mirror.corp.internal"],
}

The trailing comma after the array told the parser to expect another key, but line 5 was the closing }. Removing the comma made jq pass, dockerd --validate succeeded, and sudo systemctl restart docker brought the daemon up. The root cause was purely syntactic — a leftover comma from editing — which jq’s line-and-column output identified in seconds.

Prevention Best Practices

• Always run jq . /etc/docker/daemon.json (or python3 -m json.tool) before restarting Docker after any edit — never restart on faith.
• Use dockerd --validate --config-file ... as a pre-flight check in provisioning and change scripts.
• Keep daemon.json under configuration management (Ansible/Chef/Terraform) that renders valid JSON from templates, eliminating hand-edited commas.
• Remember JSON has no comments and requires double quotes — do not paste in // notes or single-quoted values.
• Back up the working file before editing so you can restore instantly if a change breaks daemon startup.

Quick Command Reference

# Read the exact parser error
journalctl -u docker --no-pager --since '10 min ago' | grep 'invalid character'
systemctl status docker --no-pager

# Validate the JSON (pinpoints the bad line/column)
jq . /etc/docker/daemon.json
python3 -m json.tool /etc/docker/daemon.json

# Let Docker validate its own config without starting
sudo dockerd --validate --config-file /etc/docker/daemon.json

# Fix, then restart and confirm
sudo systemctl restart docker
docker info | head

Conclusion

daemon.json: invalid character ... looking for beginning of object key string is a JSON syntax error — nearly always a trailing comma — that stops the Docker daemon from starting at all. The path out is quick and deterministic: read the parser message in journalctl, run jq to get the exact line and column, remove the stray comma or brace, confirm with dockerd --validate, then restart. To keep it from recurring, validate every daemon.json edit before restarting and render the file from configuration management rather than editing it by hand.