YAML Validator Online - Fast, Secure & Private
Updated
Validate YAML data instantly. 100% client-side processing. Free online YAML validation.
What it does with tricky input
Each row is an automated test: the tool's own code runs on the input before every release, and the release stops if the result changes. Error text is what Chrome prints; Firefox and Safari word some errors differently. More on how we test.
| Case | Input | Result |
|---|---|---|
| A tab used for indenting | Inputa:\n\tb: 1 | ResultError at line 2, column 1: Tabs are not allowed as indentation. YAML indents with spaces only. Replace the tab with spaces. |
| A duplicate key | Inputa: 1\na: 2 | ResultError at line 2, column 1: Map keys must be unique. This key appears twice in the same block. Delete or rename one of them. |
| A colon inside an unquoted value | Inputtitle: Note: read this | ResultError at line 1, column 8: Nested mappings are not allowed in compact mappings. A value with ": " in it needs quotes around it, like "Note: read this". |
| An alias that was never defined | Inputa: *nope | ResultError at line 1, column 4: The alias *nope has no matching anchor. Add &nope to the value it should repeat, above this line. |
| country: NOYAML 1.2 reads NO as the text "NO", not false. | Inputcountry: NO | ResultValid |
| Several documents split by ---Like a Kubernetes manifest. | Inputkind: Service\n---\nkind: Deployment | ResultValid |
| Anchors and aliases | Inputbase: &b {x: 1}\nother: *b | ResultValid |
Key Features
- ✅ Instant YAML validation
- ✅ Real-time error highlighting
- ✅ 100% client-side - your data never leaves your browser
How to Use
- Paste your YAML code into the editor
- Click "Validate"
- View validation results instantly
Expert FAQ
- Two sibling keys are indented by a different number of spaces — is that always an error?
It depends on whether the mismatch is consistent. YAML only requires that all keys at the same nesting level share the same indentation as each other — the absolute number of spaces doesn't matter (2 vs 4 is fine), but if sibling keys under the same parent use different indentation from one another, the parser can no longer tell they're siblings, and that is a hard error. The validator points at exactly which line broke the alignment. - My file has the same key twice in one mapping, like name: appearing twice under the same object — is that flagged?
Yes, as a YAML spec violation rather than just a style warning: YAML 1.2 explicitly defines duplicate keys within a single mapping as an error, unlike JSON, where the grammar is silent on duplicates and behavior is parser-dependent. This is a stricter rule than JSON validation applies, and it's one of the more common errors in hand-merged YAML config files. - It says my YAML is syntactically valid, but a value I expected to be a string came out as a boolean — is that a validator bug?
That's expected, and it's a different category of issue than a syntax error — it's the well-known "Norway problem," where bare yes/no/on/off/true/false (and y/n in YAML 1.1) parse as booleans rather than strings. The document is valid YAML; the value just isn't the type you intended. The fix is in your source: quote the value ("no" instead of no) wherever it's meant to be a literal string rather than a boolean. - Does it validate against a JSON Schema, or only check YAML syntax?
Only syntax (and YAML-spec-level structural rules like duplicate-key and indentation-consistency) — it confirms the document parses correctly per the YAML grammar, not whether it satisfies any particular schema. For Kubernetes manifests or Helm values files, syntactic validity doesn't guarantee the API server will accept the resource; that requires a schema-aware tool like kubectl --dry-run or a Helm/Kustomize linter.
Technical Details
YAML validation checks several things at once: that indentation is internally consistent (sibling keys at the same level must share the same indentation, though the absolute width is flexible), that the document doesn't contain duplicate keys within the same mapping (a hard error per the YAML 1.2 spec, stricter than JSON's silence on the matter), that block scalar markers and flow collections are properly closed, and that any anchor reference (*name) actually points to a previously defined anchor (&name). A YAML document can be 100% syntactically valid and still not mean what you intended, because YAML's scalar-typing rules are looser than JSON's. The canonical example is "the Norway problem": an unquoted no, yes, on, or off is interpreted as a boolean under YAML 1.1 conventions (still followed by much of the YAML tooling ecosystem), so a country-code field containing NO silently becomes the boolean false. This is not something a syntax validator can or should flag as an error — it's valid YAML — which is why this validator focuses strictly on grammar and structural correctness, while the YAML Formatter's quoting behavior on conversion is the relevant tool for preventing this class of issue when generating YAML from typed source data. Syntactic validity is also a narrower guarantee than schema validity: a Kubernetes manifest or Helm values.yaml can parse perfectly as YAML while still being rejected by the Kubernetes API server for using a field that doesn't exist in that resource's schema. This validator answers "does this parse," not "will my specific consumer accept it" — for that, a schema-aware linter for your specific target (kubectl, Helm, Ansible-lint) is the right next step after confirming basic YAML validity here.