CUE Formatter & Validator

Paste CUE configuration and get back a consistently indented, correctly formatted version, a line-numbered syntax check, and a best-effort evaluation of every field that resolves to a concrete value — all in your browser, with nothing uploaded anywhere. The CUE Formatter & Validator also shows, field by field, which parts of your document are fixed values and which are still open constraints or schema.

CUE's constraint syntax — disjunctions (|), conjunctions (&), bound comparisons (>=0 & <=100), and structural typing — doesn't round-trip cleanly through a generic JSON/YAML formatter or an AI assistant's guess at "reasonable" spacing. This tool understands that syntax directly, without needing the cue CLI installed.

How to use it

1. Paste your CUE source into the input box.
2. Optionally switch Indent Size between 2 and 4 spaces.
3. Read the Validation table for any syntax error (with a line number), the Value vs. schema table to see which fields are already concrete, and the Evaluated JSON panel for the resolved output.

Everything reruns automatically as you type.

What it does

• Formats structs, lists, definitions (#Foo), hidden fields (_foo), optional fields (foo?:), pattern-constraint labels ([string]: int), and the package clause, re-indenting by actual nesting depth. Comments (//, leading and trailing) and blank-line spacing are preserved.
• Validates syntax and reports the first problem with an exact line number — an unbalanced bracket, an unterminated string, or a value where CUE expects one.
• Evaluates literal values (strings, numbers — including hex/octal/binary and the Ki/Mi/Gi/K/M/G… size suffixes — booleans, null, structs and lists of the same) and simple + - * / arithmetic to their concrete JSON equivalent.
• Distinguishes value from schema. A field like port: 8080 resolves to a real value; a field like replicas: >=1 & <=10 or env: "prod" | "staging" is a constraint with no single value yet — the Value vs. schema table calls that out per field (including inside nested structs), and the Evaluated JSON panel marks any field it can't resolve with a ⟨constraint: …⟩ placeholder instead of guessing.

What it doesn't do

This isn't a reimplementation of CUE's unification engine — it resolves literal values and simple arithmetic, not general constraint solving (#Point & {x: 0, y: 0} stays a constraint, since the tool doesn't merge struct definitions). import declarations, comprehensions (if/for) and let clauses, and hash-strings (#"…"#) aren't supported and are reported as a syntax error rather than silently mishandled. A triple-quoted string's common leading indentation isn't stripped the way real cue fmt strips it.

Privacy

Everything runs client-side in your browser — your configuration is never uploaded to a server.

Why not just use cue fmt?

If you have the CUE CLI installed, it's the authoritative formatter and does full type-checking. This tool is for when you don't — a snippet from a pull request, a config an AI assistant produced with mangled constraint syntax, or a quick check before you paste something into a Kubernetes manifest or a Dagger pipeline.

Why does my field show a ⟨constraint: …⟩ placeholder instead of a value?

That field isn't concrete yet — it's a disjunction, a bound, a reference to another field, or a type like string/int used as a schema. The Value vs. schema table shows the exact constraint text so you can see what's still open.

Related tools

• Kubernetes YAML Formatter — for the YAML manifests CUE configs are often compiled into.
• JSON Schema Validator — a different way to describe and check the shape of JSON/YAML data.
• TOML Formatter — the same kind of formatting for another config language.