What Is a YAML Formatter and Validator?
This yaml formatter and yaml validator online reformats YAML to consistent indentation, checks its syntax and structure, and can convert it directly to JSON. YAML is the configuration language behind Kubernetes manifests, Docker Compose files, GitHub Actions workflows, and Ansible playbooks, but its indentation-sensitive syntax is easy to get subtly wrong in a way that either fails outright or, worse, parses into the wrong structure silently. Use it as a yaml syntax checker before a config file reaches a real deployment, or as a straightforward yaml lint pass while editing.
How YAML Indentation and Structure Work
YAML is defined by the YAML 1.2 specification, and it is explicitly designed as a superset of JSON: any valid JSON document is also valid YAML. Unlike JSON, which uses braces and brackets to mark structure, YAML uses indentation level to represent nesting, the same significant-whitespace approach Python uses for code blocks. A mapping (YAML's term for a key-value object) nests its children one indentation level deeper; a sequence (YAML's term for a list) uses a leading dash for each item. Getting the indentation wrong does not always produce an error, sometimes it silently produces a different structure than intended, which is why a dedicated validator matters more here than it does for a stricter format.
Why "Tabs Not Allowed" Is the Most Common YAML Error
The YAML specification explicitly forbids tab characters for indentation, only spaces are permitted. A yaml tabs not allowed error almost always comes from an editor auto-indenting with a tab character, or from copy-pasting YAML through a tool that silently converts spaces to tabs. This is one of the most common yaml indentation error fix scenarios: replace every leading tab with spaces, and configure your editor to insert spaces for YAML files specifically to prevent it recurring. YAML anchors and aliases are a related feature worth knowing: an anchor (marked with &) tags a node for reuse elsewhere in the same document, and an alias (marked with *) references it back, effectively a copy-paste mechanism built into the format itself, commonly used in Docker Compose and Kubernetes YAML to avoid repeating identical configuration blocks across multiple services.
The YAML Duplicate Key Problem
A yaml duplicate key error happens when the same key appears twice within one mapping. Different YAML implementations resolve this inconsistently: some error immediately, some silently keep only the last value, some keep the first. This tool's parser rejects a duplicate key outright as a syntax error, reporting the exact line and column, rather than silently keeping one value and discarding the other.
What This Tool Does Not Do
This tool validates YAML syntax and structure; it does not check a document against a schema, so it cannot confirm a Kubernetes manifest has the correct fields for its resource kind. To validate kubernetes yaml against the cluster's actual API schema, a dedicated Kubernetes linter is still the right tool after this one confirms plain syntax correctness. It also does not resolve custom tags or language-specific type extensions some YAML implementations support beyond the core 1.2 specification. Nothing pasted here is uploaded or stored; formatting and validation happen entirely in your browser.