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YAML Validator

Warp Terminal

Paste your YAML content or upload a file to validate syntax. Scroll down to see the details on the errors, if any.

YAML Validator Tool

YAML Input

Paste your YAML content or upload a file to validate syntax.

Best practices to avoid common YAML syntax errors

Remember these points to avoid common syntax errors in your YAML file:

Indentation:

  • Using spaces consistently (not tabs)
  • Proper nesting levels
  • Correct alignment of list items

Syntax:

  • Colons followed by spaces
  • Proper quote matching
  • Correct array notation

Data Types:

  • Quoted special values that should remain strings
  • Proper boolean usage
  • Correct null representations

Structure:

  • No duplicate keys
  • Valid multi-line string syntax
  • Proper anchor/alias definitions

File Quality:

  • UTF-8 encoding
  • No invisible characters

Tips for writing better YAML

  1. Use consistent indentation: Stick to 2 or 4 spaces throughout your file
  2. Quote when in doubt: If a value might be interpreted incorrectly, quote it
  3. Validate frequently: Don't wait until your entire file is complete
  4. Use proper editors: Choose editors with YAML syntax highlighting
  5. Keep it simple: Avoid complex nested structures when possible
  6. Comment your code: Use # for explanatory comments
  7. Test with sample data: Validate with small examples before scaling up

Common troubleshooting tips for YAML validation

Let's go into details and let me share a few sample examples of common YAML syntax error and what you should use instead.

1. Fixing indentation problems

Error Message: "Invalid indentation" or "Mapping values are not allowed here"

Common Issues:

  • Mixing tabs and spaces
  • Inconsistent indentation levels
  • Missing indentation for nested items

Solutions:

# ❌ Wrong - Mixed tabs and spaces
user:
	name: John    # Tab used here
  age: 30       # Spaces used here

# ✅ Correct - Consistent spaces
user:
  name: John
  age: 30

Quick Fix: Use a text editor that shows whitespace characters, or configure it to convert tabs to spaces automatically.

2. Resolving syntax errors

Error Message: "Unexpected character" or "Could not find expected ':'"

Common Causes:

  • Missing colons after keys
  • Unescaped special characters
  • Incorrect quote usage

Solutions:

# ❌ Wrong - Missing colon
name "John Doe"
title: Developer

# ✅ Correct - Proper syntax
name: "John Doe"
title: Developer

# ❌ Wrong - Unescaped special characters
message: Hello: World! [Important]

# ✅ Correct - Quoted strings with special chars
message: "Hello: World! [Important]"

3. Handling quote and escape issues

Error Message: "Unterminated quoted scalar" or "Invalid escape sequence"

Common Problems:

  • Unmatched quotes
  • Backslash escape issues
  • Multi-line string problems

Solutions:

# ❌ Wrong - Unmatched quotes
description: "This is a test

# ✅ Correct - Properly closed quotes
description: "This is a test"

# ❌ Wrong - Invalid escape
path: C:\Users\John\Documents

# ✅ Correct - Escaped backslashes or quoted
path: "C:\\Users\\John\\Documents"
# OR
path: 'C:\Users\John\Documents'

4. Array and list formatting errors

Error Message: "Block sequence expected" or "Invalid array format"

Common Issues:

  • Mixing array notations
  • Incorrect list item indentation
  • Missing spaces after dashes

Solutions:

# ❌ Wrong - Missing space after dash
items:
  -apple
  -banana

# ✅ Correct - Space after dash
items:
  - apple
  - banana

# ❌ Wrong - Mixed array syntax
fruits:
  - apple
  - [banana, orange]  # Inline array mixed with block

# ✅ Correct - Consistent formatting
fruits:
  - apple
  - banana
  - orange

5. Boolean and data type conflicts

Error Message: "Invalid boolean value" or "Type mismatch"

Problem: YAML auto-converts certain values to booleans when you want strings.

YAML Boolean Keywords: true, false, yes, no, on, off, True, False, YES, NO

Solutions:

# ❌ Problematic - Becomes boolean
answer: yes
status: off
version: true

# ✅ Correct - Quoted to remain strings
answer: "yes"
status: "off"  
version: "true"

# ✅ Or use explicit booleans when intended
enabled: true
active: false

6. Duplicate key errors

Error Message: "Duplicate key found" or "Key already exists"

Problem: YAML doesn't allow duplicate keys in the same mapping.

Solutions:

# ❌ Wrong - Duplicate keys
user:
  name: John
  age: 30
  name: Jane  # Duplicate!

# ✅ Correct - Unique keys
user:
  name: John
  age: 30
  email: [email protected]

# ✅ Or use arrays for multiple similar items
users:
  - name: John
    age: 30
  - name: Jane
    age: 25

7. Multiline string issues

Error Message: "Invalid multi-line string" or "Unexpected line break"

Common Problems:

  • Incorrect multi-line operators
  • Indentation issues in multi-line content

Solutions:

# ❌ Wrong - Incorrect multi-line syntax
description: |
This is wrong indentation

# ✅ Correct - Proper indentation for literal block
description: |
  This preserves line breaks
  and maintains formatting
  exactly as written.

# ✅ Correct - Folded block for single line
summary: >
  This will fold multiple lines
  into a single line with spaces
  replacing the line breaks.

8. Null and empty value problems

Error Message: "Invalid null value" or parsing confusion

Understanding the differences:

# Different ways to represent empty/null values
name:           # null value
description: "" # empty string
title: null     # explicit null
count: 0        # zero (not null)
active:         # null value
missing_key:    # null value

# In some contexts, you might want:
optional_field: ~  # explicit null using tilde

9. Anchor and reference errors

Error Message: "Unknown alias" or "Circular reference detected"

Common Issues:

  • Using aliases before defining anchors
  • Circular references in anchors
  • Typos in alias names

Solutions:

# ❌ Wrong - Using alias before anchor definition
production:
  <<: *defaults  # Error: defaults not defined yet

default_settings: &defaults
  timeout: 30

# ✅ Correct - Define anchor before using
default_settings: &defaults
  timeout: 30
  retries: 3

production:
  <<: *defaults
  host: prod.example.com

development:
  <<: *defaults
  host: dev.example.com

10. Character encoding issues

Error Message: "Invalid character encoding" or "Unsupported character"

Solutions:

  • Ensure your file is saved in UTF-8 encoding
  • Check for invisible characters (copy-paste issues)
  • Remove or escape special Unicode characters
# ❌ Problematic - Special characters
name: Café  # May cause issues depending on encoding

# ✅ Safe approach
name: "Café"  # Quoted to handle special chars

I hope I helped you validate your YAML file for syntax errors here. The online YAML validator tool is good and I complemented it with some tips to avoid the common issues.