TOML Formatter and Validator

TOML in practice

TOML (Tom’s Obvious Minimal Language) is the configuration format of Rust’s Cargo.toml, Python’s pyproject.toml (used by pip, Poetry, uv, Ruff, Black and pytest), Hugo and Netlify sites, rust-toolchain.toml, Starship, Alacritty and many more. It maps directly to a hash table: key = value pairs grouped under [table] headers, with [[array.of.tables]] for repeated sections such as Cargo’s [[bin]] targets.

Files written by hand drift over time: inconsistent spacing around =, inline tables that grew too long, arrays squeezed onto one line. This page fixes the layout with taplo, the formatter behind the Even Better TOML extension for VS Code, compiled to WebAssembly so it runs in your browser.

Validation first, then formatting

taplo is deliberately lenient: it will happily format a file that Cargo or pip would reject. To avoid giving you a pretty but broken file, the input is first parsed with smol-toml, a strict TOML 1.0 parser. Any error stops the run and is shown with its line and column. After formatting, the output goes through the strict parser a second time, so you never receive a file that is less valid than the one you pasted.

Workflow: paste the file or drop a .toml file, press Ctrl/Cmd+Enter to format, and Ctrl/Cmd+Shift+C to copy. Ctrl/Cmd+K opens the command palette, where the Tree view and the TOML to JSON and TOML to YAML conversions live. TOML has no minified form, so there is no minify step for this format. Your config, including any registry tokens in .cargo/config.toml, is processed locally and not transmitted.

Options

• Align = signs pads keys within a block so the = signs form a column. It reads nicely in short, stable sections like [package], but every rename re-aligns its neighbours and inflates diffs.
• Trailing comma in multi-line arrays adds a comma after the last element of an array that spans several lines. Adding a dependency feature then touches one line in version control instead of two.
• Expand long arrays breaks arrays that exceed the line width onto one element per line. Turn it off to keep long arrays on one line regardless of width; short arrays are collapsed back to one line either way.
• Indent sub-tables indents [tool.ruff.lint] under [tool.ruff] according to its depth, which some people find easier to scan in long pyproject.toml files. TOML ignores indentation, so this is purely visual.

Indent size and line width apply too. Sort keys orders keys alphabetically within each run of consecutive lines; a blank line or comment starts a new run, and table headers keep their positions. Avoid sorting [package] in Cargo.toml, where name before version is the convention.

Rules that surprise people

• A table can only be defined once. Writing [dependencies] twice, or creating [server] after server.port = 8080 already implied it, is an error, not a merge.
• Inline tables belong on one line in TOML 1.0, with no trailing comma. The validator accepts the relaxed TOML 1.1 form, but older parsers do not, so a long { version = "1", features = [...] } entry is safer as a [dependencies.serde] table.
• Leading zeros are illegal in integers (port = 08080) to avoid octal confusion; use 0o for octal.
• Dates are real types. release = 2026-09-14 is a local date, not a string, and 2026-13-01 fails validation. Integers are 64-bit and are kept exactly, even beyond JavaScript’s safe range.

Examples

[package]
name="order-service"
version  =  "0.4.2"
edition="2021"
rust-version = "1.80"
[dependencies]
axum={version="0.7",features=["macros"]}
tokio = { version = "1", features = ["rt-multi-thread","macros","signal"] }
serde={version="1.0",features=["derive"]}
sqlx = { version = "0.8", default-features = false, features = ["postgres","runtime-tokio","tls-rustls","uuid","chrono"] }
[[bin]]
name="order-service"
path="src/main.rs"

[package]
name = "order-service"
version = "0.4.2"
edition = "2021"
rust-version = "1.80"
[dependencies]
axum = { version = "0.7", features = ["macros"] }
tokio = { version = "1", features = ["rt-multi-thread", "macros", "signal"] }
serde = { version = "1.0", features = ["derive"] }
sqlx = { version = "0.8", default-features = false, features = [
  "postgres",
  "runtime-tokio",
  "tls-rustls",
  "uuid",
  "chrono",
] }
[[bin]]
name = "order-service"
path = "src/main.rs"

[project]
name = "invoice-parser"
version = "1.3.0"
requires-python = ">=3.11"
dependencies = ["httpx>=0.27", "pydantic>=2.7", "python-dateutil"]
[tool.ruff]
line-length = 100
target-version = "py311"
[tool.ruff.lint]
select = ["E", "F", "I", "UP", "B"]
ignore = ["E501"]
[tool.pytest.ini_options]
addopts = "-ra --strict-markers"
testpaths = ["tests"]

[project]
name            = "invoice-parser"
version         = "1.3.0"
requires-python = ">=3.11"
dependencies    = ["httpx>=0.27", "pydantic>=2.7", "python-dateutil"]
[tool.ruff]
line-length    = 100
target-version = "py311"
  [tool.ruff.lint]
  select = ["E", "F", "I", "UP", "B"]
  ignore = ["E501"]
[tool.pytest.ini_options]
addopts   = "-ra --strict-markers"
testpaths = ["tests"]

[build]
command="npm run build"
publish="dist"
[[redirects]]
from="/api/*"
to="https://api.example.com/:splat"
status=200
force=true
[[redirects]]
from="/old-pricing"
to="/pricing"
status=301
[[headers]]
for="/*"
[headers.values]
X-Frame-Options="DENY"
Content-Security-Policy="default-src 'self'"

[build]
command = "npm run build"
publish = "dist"
[[redirects]]
from = "/api/*"
to = "https://api.example.com/:splat"
status = 200
force = true
[[redirects]]
from = "/old-pricing"
to = "/pricing"
status = 301
[[headers]]
for = "/*"
[headers.values]
X-Frame-Options = "DENY"
Content-Security-Policy = "default-src 'self'"

Common errors and how to fix them

Related tools

• toml validator
• TOML to JSON
• TOML to YAML
• JSON to TOML
• YAML to TOML
• toml vs yaml
• toml duplicate key