YAML Validator Guide — Validate and Fix YAML Online

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YAML is powerful and readable, but its whitespace-sensitivity makes it uniquely prone to subtle errors. A missing space, a tab character where spaces are expected, or a value that is accidentally parsed as a boolean can break your entire Kubernetes manifest, GitHub Actions workflow, or application configuration. A YAML validator catches these issues before they reach production.

Why YAML Validation Is Critical

Unlike JSON, which has a strict, unambiguous syntax, YAML's flexibility creates many ways to write incorrect YAML that looks correct to human eyes:

• A tab character looks identical to spaces in most editors
• NO looks like it could be a country code but parses as boolean false
• Inconsistent indentation may parse without error but produce unexpected nesting
• Colons without trailing spaces cause silent parse failures in some parsers

In a Kubernetes deployment, a single YAML error can prevent a service from deploying — often with a cryptic error message that points to the wrong location.

YAML Syntax Rules

Indentation — Spaces Only, Never Tabs

YAML uses indentation to express structure. The rules are strict:

• Use spaces only — tab characters are illegal in YAML
• Indentation must be consistent within a document (2 spaces or 4 spaces — pick one)
• Child elements must be indented further than their parent
• List items at the same level must have identical indentation

# Valid — consistent 2-space indentation
server:
  host: localhost
  port: 8080
  options:
    debug: true
    timeout: 30

# Invalid — tab character on "port" line
server:
  host: localhost
	port: 8080  # TAB character here — syntax error

Colons Must Have a Space After Them

# Invalid — no space after colon
server:
  host:localhost  # ERROR: missing space

# Valid
server:
  host: localhost

Exception: colons in quoted strings and URLs (https://example.com) do not need a space after them.

String Quoting

Most strings in YAML do not need quotes, but you must quote strings that:

• Start with special characters: :, -, #, {, }, [, ], &, *, !, |, >, ', ", %, @, `
• Look like other types: "true", "null", "123", "yes"
• Contain a colon followed by a space
• Are empty strings: value: ""

YAML Boolean Gotchas

YAML 1.1 (used by many parsers including PyYAML and some Kubernetes tools) treats several values as booleans:

# These are parsed as TRUE in YAML 1.1
yes, Yes, YES
true, True, TRUE
on, On, ON

# These are parsed as FALSE
no, No, NO
false, False, FALSE
off, Off, OFF

This causes real-world bugs. The Norwegian country code NO parses as boolean false. The word on in a configuration value parses as true. Always quote these values if you mean them as strings: country: "NO".

YAML 1.2 (used by newer parsers) only treats true and false as booleans, but parser behaviour varies. When in doubt, quote strings that look like booleans.

YAML Multiline Strings

YAML supports two multiline string styles:

# Literal block (|) — preserves newlines
description: |
  Line one.
  Line two.
  Line three.

# Folded block (>) — folds newlines into spaces
description: >
  This is a long sentence
  that will be joined into
  a single line.

Common YAML Errors and Their Fixes

Duplicate Keys

# Invalid — duplicate key
server:
  host: localhost
  host: 127.0.0.1  # Duplicate — second value silently overwrites first

Incorrect List Syntax

# Invalid — dash must be followed by a space
roles:
  -admin  # ERROR: no space after dash

# Valid
roles:
  - admin

Inconsistent Indentation

# Invalid — mixed indentation depths
config:
  host: localhost
    port: 8080  # Over-indented — makes port a child of host

How to Validate YAML Online

Use the YAML validator on DataConvertProTools:

1. Go to DataConvertProTools → Validator tab → select YAML
2. Paste your YAML document
3. Click Validate — every error appears with line number and description
4. Click Auto-Fix to automatically repair tab characters, missing spaces after colons, boolean normalisation, and trailing whitespace

You can also convert validated YAML to JSON using the YAML to JSON converter — useful for checking that the parsed structure matches your intent.