Malformed Percent Encoding in Query String – Why URLSearchParams Still Parses

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Show how malformed percent-encoded UTF-8 can parse with replacement characters rather than raising an exception.

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Malformed query encoding

Why malformed percent encoding may not throw a query parse error

URLSearchParams is forgiving. An incomplete UTF-8 sequence can be decoded with the Unicode replacement character while an incomplete percent escape remains literal text. This tool is free to use. No account or payment is required.

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What you can do here

  • Browser-native URLSearchParams behavior
  • Full URL and raw query parsing
  • Repeated parameter preservation
  • JSON object to query construction
  • Browser-local processing

Steps

  1. Paste the malformed query into Parse to JSON.
  2. Look for the replacement character � or leftover percent text.
  3. Correct the original UTF-8 percent sequence rather than relying on forgiving decoding.

Forgiving parsing does not mean the source encoding is correct

The browser URLSearchParams implementation does not necessarily reject malformed percent-encoded UTF-8 as a syntax error.

A replacement character in the decoded result is a signal to inspect the original bytes or upstream URL construction.

Incomplete UTF-8 percent sequence

The parser returns a replacement character and leftover %A text.

Query string

?value=%E0%A4%A

Parsed JSON

{
  "value": "�%A"
}

Common questions

Frequently asked questions

Why is there no parser error?

URLSearchParams uses forgiving form-url-encoded decoding rather than strict UTF-8 validation.

What does � mean?

It is the Unicode replacement character, commonly used when bytes cannot be decoded as valid text.

Should I accept this value as valid?

Usually not without understanding the upstream encoding. The page describes parser behavior, not application-level validity.

Recommended next steps

Percent-Encoded Unicode Query ValueDecode UTF-8 percent escapes such as check marks and CJK text in query parameters.URL Encoder & Decoder OnlineEncode or decode one URL component when percent escapes are the issue.Query String Parser & Builder OnlineParse a URL or raw query into JSON, or build query parameters from JSON.

Debugging a specific Query String issue? Browse Query String troubleshooting.