How to Check YAML Syntax

Two kinds of "wrong"

YAML problems come in two flavors. The first is a parse error — the document is malformed and won't load at all. The second, more insidious, is a silent type surprise — the document parses fine, but YAML's automatic type inference turned a value into something you didn't intend. A good check catches both, which is why "does it parse?" alone isn't enough for YAML.

Errors that break parsing

• Tabs for indentation. Forbidden by the spec. A single tab where spaces are expected is an immediate error, and it's invisible in most editors — the #1 cause of broken YAML.
• Inconsistent indentation. Siblings in the same block must align at the same column. Mixing 2 and 4 spaces, or under-indenting a nested key, fails to parse or nests under the wrong parent.
• Missing space after the colon. A mapping needs key: value. Without the space, key:value is read as one plain scalar string.
• Unquoted special characters. A value containing a colon-space, a leading @, {, [, or # often needs quoting. time: 12:30 is ambiguous; time: "12:30" is safe.
• Duplicate keys. The same key twice in one mapping is invalid YAML (many parsers accept it and silently keep the last — strict ones reject it).

Parsers report the first error with a line and column, then stop.

The silent one: the Norway problem

YAML infers types from bare values, and that's where data quietly changes. The classic example: a list of country codes.

countries:
  - GB
  - US
  - NO   # parsed as the boolean false, not "Norway"

NO becomes false because YAML treats yes/no/on/off/true/false as booleans. Similarly, a ZIP code 01234 can lose its leading zero or be read as octal, and a version like 1.20 becomes the number 1.2. The fix is always the same: quote it. - "NO" stays the string "NO". A syntax check that shows you the parsed types — not just "valid/invalid" — is what surfaces this.

Checking in the browser

In JavaScript, the js-yaml library parses and throws a descriptive error on malformed input — the same approach an online checker uses, running entirely client-side:

import yaml from "js-yaml";
try {
  const data = yaml.load(yamlString);
  console.log("Valid ✓", data);
} catch (e) {
  console.log("Invalid:", e.message); // includes line & column
}

Checking on the command line

# Quick parse check with Python (no output = valid)
python -c "import yaml,sys; yaml.safe_load(open(sys.argv[1]))" file.yaml

# yamllint — also flags indentation and style issues
yamllint file.yaml

The Python one-liner answers "does it parse?" yamllint goes further, catching things a bare parser accepts — inconsistent indentation, trailing spaces, lines that are too long, and missing document markers — making it the better choice in CI.

Checking in Python

import yaml

try:
    data = yaml.safe_load(open("file.yaml"))
    print("Valid ✓")
except yaml.YAMLError as e:
    print(f"Invalid: {e}")  # mark.line / mark.column on the error

Always use safe_load, not load — the latter can construct arbitrary Python objects from untrusted YAML and is a security risk.

Check your YAML syntax now

Paste your YAML and get an instant validity check with the exact line of any error. Runs in your browser — nothing is uploaded.

Frequently Asked Questions