JSON vs YAML vs TOML: A Comprehensive Comparison for Developers

February 11, 2026

Every software project needs configuration. Whether you are setting up a web server, defining a CI/CD pipeline, or publishing a package, you are choosing a format to store structured settings. The three most common options in 2026 are JSON, YAML, and TOML, and each one makes fundamentally different tradeoffs between simplicity, readability, and expressiveness.

This guide provides a thorough, side-by-side comparison to help you make an informed choice. We will look at syntax, data types, real-world use cases, known pitfalls, and ecosystem support. By the end, you will know exactly which format fits your project and why.

⚙ Try it yourself: Validate your config files with our JSON Formatter, YAML Validator, or TOML Validator — all free, all running in your browser.

The Same Config in Three Formats

Before diving into theory, let us look at the same configuration expressed in all three formats. This is a realistic application config with a server section, database settings, and a list of allowed origins.

JSON

{
  "app_name": "my-web-app",
  "version": "2.1.0",
  "debug": false,
  "server": {
    "host": "0.0.0.0",
    "port": 8080,
    "workers": 4,
    "tls": {
      "enabled": true,
      "cert": "/etc/ssl/cert.pem",
      "key": "/etc/ssl/key.pem"
    }
  },
  "database": {
    "host": "localhost",
    "port": 5432,
    "name": "myapp_production",
    "pool_size": 20,
    "timeout_ms": 5000
  },
  "allowed_origins": [
    "https://example.com",
    "https://app.example.com",
    "https://staging.example.com"
  ],
  "logging": {
    "level": "info",
    "format": "json",
    "file": "/var/log/myapp/app.log"
  }
}

YAML

# Application configuration
app_name: my-web-app
version: "2.1.0"  # Must quote to prevent float interpretation
debug: false

server:
  host: 0.0.0.0
  port: 8080
  workers: 4
  tls:
    enabled: true
    cert: /etc/ssl/cert.pem
    key: /etc/ssl/key.pem

database:
  host: localhost
  port: 5432
  name: myapp_production
  pool_size: 20
  timeout_ms: 5000

allowed_origins:
  - https://example.com
  - https://app.example.com
  - https://staging.example.com

logging:
  level: info
  format: json
  file: /var/log/myapp/app.log

TOML

# Application configuration
app_name = "my-web-app"
version = "2.1.0"
debug = false

[server]
host = "0.0.0.0"
port = 8080
workers = 4

[server.tls]
enabled = true
cert = "/etc/ssl/cert.pem"
key = "/etc/ssl/key.pem"

[database]
host = "localhost"
port = 5432
name = "myapp_production"
pool_size = 20
timeout_ms = 5000

allowed_origins = [
    "https://example.com",
    "https://app.example.com",
    "https://staging.example.com",
]

[logging]
level = "info"
format = "json"
file = "/var/log/myapp/app.log"

At a glance, all three are readable. But the differences become meaningful at scale and under pressure, when a production deployment depends on getting the syntax exactly right.

Feature-by-Feature Comparison

The following table compares the three formats across every dimension that matters for configuration files. Green indicates an advantage, red indicates a disadvantage or missing feature, and yellow indicates partial support.

Feature JSON YAML TOML
Comments ✗ No ✓ Yes (#) ✓ Yes (#)
Native date/time type ✗ No ~ Implicit ✓ RFC 3339
Trailing commas ✗ No N/A ✓ Yes
Indentation-sensitive ✓ No ✗ Yes ✓ No
Implicit type coercion ✓ None ✗ Extensive ✓ None
Multi-document support ✗ No ✓ Yes (---) ✗ No
Anchors & references ✗ No ✓ Yes (& / *) ✗ No
Deep nesting (4+ levels) ✓ Natural ✓ Natural ~ Verbose
Spec complexity Short (RFC 8259) Very long (80+ pages) Short (~3K words)
Multiline strings ✗ No (use \n) ✓ Yes (| and >) ✓ Yes (""" and ''')
Null value ✓ null ✓ null / ~ ✗ No
API data interchange ✓ Standard ~ Possible ✗ Not designed for it
Duplicate key handling ~ Undefined (impl varies) ~ Last value wins ✓ Error (rejected)
Primary use case Data exchange, APIs Infrastructure config, CI/CD App config, manifests

JSON: The Universal Data Format

What It Is

JSON (JavaScript Object Notation) was formalized by Douglas Crockford in the early 2000s, based on a subset of JavaScript syntax. It is defined in RFC 8259 and has become the lingua franca of data interchange on the web. Every modern programming language includes a JSON parser in its standard library.

Syntax Overview

JSON has six data types: strings, numbers, booleans, null, arrays, and objects. Every string must be double-quoted. Every key must be a string. There are no comments, no trailing commas, and no date type.

{
  "string": "hello world",
  "number": 42,
  "float": 3.14,
  "boolean": true,
  "null_value": null,
  "array": [1, 2, 3],
  "nested": {
    "key": "value"
  }
}

Strengths

Weaknesses

Where JSON Dominates

🔨 Tool: Paste your JSON into our JSON Formatter to validate, pretty-print, and minify it instantly.

YAML: The Human-Friendly Heavyweight

What It Is

YAML (YAML Ain't Markup Language) was first released in 2001 and is currently at version 1.2.2 (October 2021). It was designed to be a human-readable data serialization format, and it is technically a superset of JSON: every valid JSON document is also valid YAML.

Syntax Overview

YAML uses indentation (spaces, never tabs) to represent nesting. Colons separate keys from values. Dashes denote list items. It supports a rich set of data types including strings, numbers, booleans, null, dates, arrays, and mappings.

# Scalars
string: hello world
quoted_string: "contains: colon"
number: 42
float: 3.14
boolean: true
null_value: null
date: 2026-02-11

# Sequence (array)
fruits:
  - apple
  - banana
  - cherry

# Mapping (object)
server:
  host: 0.0.0.0
  port: 8080

# Multiline strings
description: |
  This is a block scalar.
  Line breaks are preserved.
  Indentation is stripped.

summary: >
  This is a folded scalar.
  Line breaks become spaces.
  Good for long paragraphs.

# Anchors and references
defaults: &default_settings
  timeout: 30
  retries: 3

production:
  <<: *default_settings
  timeout: 60

Strengths

Weaknesses

# These are all booleans in YAML 1.1, not strings:
country: NO        # false (Norway disappears)
answer: yes        # true
enabled: on        # true
disabled: off      # false

# This is a float, not a version string:
version: 1.0       # 1.0 (float)

# This is an integer, not a string:
zipcode: 010       # 8 (octal interpretation)

Where YAML Dominates

🔨 Tool: Catch YAML indentation errors before they reach production with our YAML Validator.

TOML: The Configuration Specialist

What It Is

TOML (Tom's Obvious, Minimal Language) was created by Tom Preston-Werner (co-founder of GitHub) in 2013, with version 1.0.0 released in January 2021. It was designed specifically for configuration files, not general data serialization. The goal: a format that is easy for humans, unambiguous for parsers, and maps cleanly to a hash table.

Syntax Overview

TOML uses key = value pairs organized into [sections] (tables). It supports strings, integers, floats, booleans, dates, arrays, and tables. All types are explicit and unambiguous.

# Scalars
string = "hello world"
number = 42
float = 3.14
boolean = true
date = 2026-02-11
datetime = 2026-02-11T09:30:00Z

# Arrays
fruits = ["apple", "banana", "cherry"]

# Tables (sections)
[server]
host = "0.0.0.0"
port = 8080

# Nested tables
[server.tls]
enabled = true
cert = "/etc/ssl/cert.pem"

# Inline tables
point = { x = 1, y = 2 }

# Array of tables
[[routes]]
path = "/api"
handler = "api_handler"

[[routes]]
path = "/health"
handler = "health_check"

# Multi-line strings
description = """
This is a multi-line string.
Quotes and backslashes work as expected."""

# Literal string (no escaping)
regex = '\d+\.\d+'

Strengths

Weaknesses

Where TOML Dominates

🔨 Tool: Validate TOML files and catch syntax errors with our TOML Validator. Read more in our complete TOML guide.

Head-to-Head: Readability

Readability depends on context. Here is the same nested structure in all three formats to illustrate how each handles increasing complexity.

Shallow config (1-2 levels) — All three are fine

At shallow nesting depths, all three formats are readable and the choice is largely a matter of preference. YAML is the most concise, TOML is explicit, and JSON is familiar.

Deep config (4+ levels) — YAML wins

# YAML handles deep nesting naturally
kubernetes:
  deployment:
    spec:
      template:
        spec:
          containers:
            - name: api
              image: myapp:latest
              ports:
                - containerPort: 8080
              env:
                - name: DB_HOST
                  value: postgres.default.svc
# TOML becomes verbose at this depth
[[kubernetes.deployment.spec.template.spec.containers]]
name = "api"
image = "myapp:latest"

[[kubernetes.deployment.spec.template.spec.containers.ports]]
containerPort = 8080

[[kubernetes.deployment.spec.template.spec.containers.env]]
name = "DB_HOST"
value = "postgres.default.svc"

The YAML version is clearly more readable for deeply nested structures. The TOML version repeats the full path for each section, which becomes tedious. This is by design: TOML optimizes for flat-to-moderate configs, not deeply hierarchical data.

Head-to-Head: Safety and Correctness

Safety in configuration means: "Does the parser produce the data structure I expect?" This is where the three formats diverge most dramatically.

The YAML Trap: Implicit Types

# What a developer writes:
countries:
  - GB
  - IE
  - NO
  - FR

# What YAML 1.1 parsers produce:
# ["GB", "IE", false, "FR"]
# Norway just became a boolean.
# More YAML surprises:
version: 1.0      # float, not string "1.0"
port: 0800        # integer 512 (octal), not 800
value: .inf       # infinity, not the string ".inf"
answer: yes       # boolean true, not string "yes"

YAML 1.2 fixed some of these issues (only true and false are booleans, not yes/no), but many popular parsers still default to YAML 1.1 behavior. Python's PyYAML, one of the most widely used YAML libraries, defaults to YAML 1.1 semantics.

JSON and TOML: What You See Is What You Get

In JSON, "NO" is always a string, 42 is always a number, and true is always a boolean. There is no ambiguity. In TOML, the same is true: "NO" is a string, 42 is an integer, and true is a boolean. Both formats trade convenience for safety.

The practical impact is significant. A YAML config file that works correctly in development can silently produce different data in production if the values happen to match one of YAML's implicit type patterns. JSON and TOML eliminate this entire category of bugs.

Ecosystem and Tooling Comparison

JSON Ecosystem

YAML Ecosystem

TOML Ecosystem

When to Use Each Format: A Decision Guide

Choosing a configuration format is not about which is "best." It is about which one fits your specific constraints. Use this decision framework.

Use JSON when:

Use YAML when:

Use TOML when:

Quick Reference Cheat Sheet

Scenario Recommended Format
REST API response JSON
Package lock file JSON
TypeScript / ESLint / Prettier config JSON
Kubernetes manifest YAML
CI/CD pipeline (GitHub Actions, GitLab CI) YAML
Docker Compose YAML
Ansible playbook YAML
Rust package manifest TOML
Python project config (pyproject.toml) TOML
Application settings file TOML
Static site generator config TOML

Common Mistakes to Avoid

Here are the most common mistakes developers make with each format, compiled from years of Stack Overflow questions and production incidents.

JSON Mistakes

// WRONG: trailing comma (parse error)
{
  "name": "myapp",
  "version": "1.0",  <-- trailing comma
}

// WRONG: single quotes (parse error)
{ 'name': 'myapp' }

// WRONG: comments (parse error)
{
  // This is not valid JSON
  "name": "myapp"
}

// WRONG: unquoted keys (parse error)
{ name: "myapp" }

YAML Mistakes

# WRONG: using tabs for indentation (parse error)
server:
	host: localhost    # Tab character = parse error

# WRONG: forgetting to quote special values
country: NO          # Becomes boolean false
version: 1.0         # Becomes float 1.0
port: 08080          # Becomes integer 4160 (octal)

# WRONG: inconsistent indentation
server:
  host: localhost
   port: 8080        # 3 spaces instead of 2 = different structure

# FIX: always quote ambiguous values
country: "NO"
version: "1.0"
port: 8080           # No leading zero

TOML Mistakes

# WRONG: bare string values (parse error)
name = hello         # Must be "hello"

# WRONG: redefining a key (parse error)
port = 8080
port = 9090          # Duplicate key rejected

# WRONG: extending inline tables (parse error)
point = { x = 1, y = 2 }
point.z = 3          # Cannot add to inline table

# WRONG: mixing table and array-of-tables
[server]
host = "alpha"
[[server]]           # Error: already defined as table
host = "beta"

Migration Tips: Converting Between Formats

Sometimes you need to convert configuration files from one format to another. Here are practical approaches.

JSON to YAML

# Using yq (the YAML equivalent of jq)
yq -P config.json > config.yaml

# Using Python
python3 -c "
import json, yaml, sys
data = json.load(open('config.json'))
yaml.dump(data, sys.stdout, default_flow_style=False)
"

YAML to JSON

# Using yq
yq -o=json config.yaml > config.json

# Using Python
python3 -c "
import json, yaml, sys
data = yaml.safe_load(open('config.yaml'))
json.dump(data, sys.stdout, indent=2)
"

JSON to TOML / YAML to TOML

# Using Python (requires tomli_w: pip install tomli-w)
python3 -c "
import json, tomli_w, sys
data = json.load(open('config.json'))
tomli_w.dump(data, sys.stdout.buffer)
"

# Using taplo for formatting after conversion
taplo format config.toml

Important caveat: automatic conversion does not preserve comments (JSON has none to begin with), and YAML-to-TOML conversion may fail if the YAML contains deeply nested structures or types that TOML does not support (like null values).

Frequently Asked Questions

What is the difference between JSON, YAML, and TOML?

JSON is a strict data interchange format with no comments, designed for machine-to-machine communication. YAML is a human-friendly superset of JSON that uses indentation for nesting and supports comments, anchors, and multi-document files. TOML is a minimal configuration format that uses explicit key-value pairs with section headers, supports comments and native date types, and avoids indentation-based nesting. Each format excels in different contexts: JSON for APIs, YAML for complex infrastructure config, and TOML for application configuration files.

Should I use YAML or JSON for configuration files?

If humans will regularly read and edit the file, YAML is generally better than JSON because it supports comments and is less verbose. However, YAML's indentation sensitivity and implicit type coercion can cause subtle bugs. If the configuration is simple (1-2 levels of nesting), consider TOML instead. If the file is machine-generated and machine-consumed (like API responses or package-lock.json), JSON is the better choice due to its strict, unambiguous parsing.

Why does YAML have a reputation for being dangerous?

YAML's implicit type coercion is the primary source of problems. Values like "yes", "no", "on", "off", "true", and "false" are automatically converted to booleans. Country codes like "NO" (Norway) become boolean false. Unquoted version strings like 1.0 become floats. Octal numbers like 0777 may be interpreted unexpectedly. The indentation-sensitive syntax also means a single misplaced space can completely change the data structure without causing a parse error. These issues have caused real production incidents and security vulnerabilities.

When should I use TOML instead of YAML?

Use TOML when your configuration has moderate nesting (1-3 levels), you want explicit typing without surprises, and the file will be hand-edited by developers. TOML is ideal for application settings, package manifests (like Cargo.toml and pyproject.toml), and tool configuration. Use YAML instead when you need deep nesting (4+ levels), anchors and references to reduce repetition, or when your ecosystem mandates it (Kubernetes, GitHub Actions, Ansible).

Conclusion

There is no single "best" configuration format. JSON, YAML, and TOML each solve different problems, and the right choice depends on your specific context.

JSON is the right choice when you need universal interoperability and the file is primarily consumed by machines. Its strict syntax and ubiquitous parser support make it the default for APIs, data interchange, and toolchain configuration. Accept its limitations (no comments, verbose syntax) when those limitations do not matter because humans rarely edit the file.

YAML is the right choice when you need expressive power for deeply nested, complex configurations. Kubernetes manifests, CI/CD pipelines, and infrastructure-as-code tools chose YAML because its indentation-based nesting handles 5-10 levels of depth elegantly. Accept the complexity tradeoff (implicit typing, indentation sensitivity) and mitigate it with linting, quoting discipline, and YAML 1.2-compliant parsers.

TOML is the right choice when humans are the primary audience and the configuration is moderately complex. Package manifests, application settings, and tool configuration benefit from TOML's explicit typing, comment support, and short specification. Its adoption by Rust, Python, Hugo, and Netlify validates this positioning. Accept that deep nesting becomes verbose and that you will need external tooling for value reuse.

In practice, most developers use all three formats across different projects. The key insight is not to pick a favorite, but to understand the strengths and limitations of each so you can choose deliberately rather than by default.

Related Tools & Resources

JSON Formatter
Validate, format, and minify JSON data
YAML Validator
Validate and parse YAML documents
TOML Validator
Validate and format TOML files
TOML Configuration Guide
Deep dive into TOML syntax and real-world usage
JSON Schema Validator
Validate JSON against schemas
JSON Debugging Tips
10 practical tips for debugging JSON in APIs