Developer Tools
Validate XML and Find Errors
XML Validator for formatting, validating, or converting XML data.
Direct Answer
Use the XML Validator when a parser, API, or import process rejects your XML and you need to find exactly which tag, attribute, or character is causing the failure. It checks well-formedness, the baseline syntax rules every XML document must follow, and flags where the structure breaks.
Well-Formed vs Valid Against a Schema
Well-formed and valid are not the same thing, and most XML errors people hit day to day are well-formedness problems rather than schema mismatches.
- Well-formed means basic XML syntax rules are followed: one root, matched tags, quoted attributes, proper nesting.
- Valid means the document is well-formed and also matches a specific XSD or DTD's rules for allowed elements and structure.
- This validator checks well-formedness; full schema validation needs the schema file loaded into a schema-aware tool.
- A document can be perfectly well-formed and still be rejected by an application expecting a specific schema.
Common Reasons XML Fails Validation
Most well-formedness errors trace back to a small set of recurring mistakes.
- Tag name case mismatch, such as opening with a capitalized tag and closing with a lowercase one, since XML is case-sensitive.
- An unescaped ampersand inside text or an attribute value.
- Unquoted or mismatched-quote attribute values.
- More than one root-level element, or content after the closing root tag.
- A DOCTYPE declaring an external entity that references a file or URL, a potential security risk.
How to Use XML Validator
- Paste or type your content into the input box.
- The transformed result appears instantly in the result panel below.
- Click Copy result to copy the output to your clipboard.
- Download .txt saves the transformed text as a file.
- Reset clears everything and lets you start over.
Reference
| Feature | Details |
|---|---|
| Purpose | Transform pasted text into a cleaner or different format. |
| Input | Text, lines, lists, or copied content. |
| Result | Transformed text ready to copy or download. |
| Best for | Cleanup, formatting, list preparation, and copy-paste workflows. |
| Category | Developer Tools |
| Works on | Desktop, tablet, and mobile browsers |
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A Focused XML Validator Alternative
People looking for alternatives to JSONLint, Code Beautify, FreeFormatter, Browserling often want a simpler workflow. This xml validator focuses on the task first: clear inputs, a visible result, useful related tools, free access, and no account requirement.
- Puts the tool input and result near the top of the page
- Free to use with no account or payment step
- Includes focused explanations and related tools
- Designed to avoid misleading download buttons and forced interstitials
Useful Related Tools
Frequently Asked Questions
Why does it say unclosed tag when I did close it?
This is usually a case-sensitivity problem: if you opened a tag written one way and closed it with different capitalization, XML treats those as two different tag names because tag matching is case-sensitive, unlike HTML which many browsers tolerate mismatches in. Check that the opening and closing tag names match exactly, including case.
What does not well-formed mean versus invalid?
Not well-formed means the document breaks XML's own baseline syntax rules, such as an unclosed tag, a missing root, or an unescaped special character, and a parser cannot even build a structure from it. Invalid means the document is well-formed but fails to meet the rules of a specific schema or DTD, which is a separate, later check that assumes well-formedness already passed.
What is XXE and why do validators warn about DOCTYPE declarations?
A DOCTYPE can declare an external entity that points to a local file or a network URL, and if a parser resolves that entity automatically, an attacker-controlled document could read local files or reach internal network endpoints, an attack known as XML External Entity injection. Safe validators disable external entity resolution by default and flag DOCTYPE declarations rather than silently processing them.
Why does it flag an attribute value as not quoted?
XML requires every attribute value to be wrapped in single or double quotes; an unquoted value like a bare number or word after an equals sign is not legal XML syntax even though many other formats allow it. Add matching quotes around the value to resolve the error.
Why does an unescaped ampersand fail validation?
An ampersand has special meaning in XML because it starts an entity reference, so a raw ampersand appearing in text or an attribute value confuses the parser about where an entity name should follow. Replace it with the escaped form, or use a CDATA section if the surrounding text has many special characters to escape.
Can validating XML check it against my XSD or DTD?
A baseline validator checks well-formedness only, meaning the document follows XML's own syntax rules. Full schema validation, checking that specific elements, attributes, and structures match a given XSD or DTD, requires a schema-aware validator that has your schema file loaded alongside the document.
Why do I see extra content at the end of the document?
This error means there is content after what the parser identified as the closing tag of the root element, commonly caused by a duplicate root element, stray text, or an extra closing tag left over from editing. Remove or properly nest anything that appears after the single root element closes.
Why does whitespace before the XML declaration break my file?
The XML declaration must be the very first thing in the file with nothing, not even a blank line or leading space, before it; a byte order mark or accidental whitespace at the top of the file causes an error about the declaration only being allowed at the start of the document. Check the very first bytes of the file, not just the visible content.
Why does case sensitivity trip people up coming from HTML?
HTML parsers in browsers are forgiving about tag name case and will often render a mismatched-case tag pair without complaint, which trains many developers to not think about it. XML has no such leniency, since tag names are treated as literal strings that must match exactly, including capitalization, for open and close tags to pair correctly.
Can the same attribute appear twice on one tag?
No, a duplicate attribute name on the same element, such as an id attribute listed twice with different values, is not well-formed XML and will be rejected. Each attribute name can only appear once per element.
Can I mix single and double quotes for attribute values?
Yes, XML allows either single or double quotes for an attribute value, but the opening and closing quote character for that specific attribute must match. Mixing is fine across different attributes in the same document, just not within a single value.
Why does it say undefined namespace prefix?
This happens when a tag uses a namespace prefix, such as one commonly seen in SOAP documents, but the document never declares that prefix with a matching xmlns attribute anywhere in scope. Add the namespace declaration on the root element or the specific element using the prefix.
Can an XML comment contain a double hyphen?
No, XML comments cannot contain two consecutive hyphens anywhere in their content, since that sequence is reserved for the comment's own delimiters, so a comment like one describing a range with double dashes inside it will fail validation. Rewrite the comment text to avoid consecutive hyphens.