Invalid control character in JSON: How to Fix It in Python

Python raises "Invalid control character at" when a JSON string contains a raw control character, such as a real newline or tab, instead of its escaped form. Escape the character as \n or \t, or parse with strict=False if you cannot change the source.

What “Invalid control character in JSON” means

The message means the parser found a character between U+0000 and U+001F inside a string, unescaped. The JSON standard requires those characters to be escaped inside strings. Python's json module is strict by default, so it stops and reports the position as line, column and char offset. The position points at the offending character or just after it. Newline, tab and carriage return are the usual ones.

Causes and the fix for each

A raw newline sits inside a string value.

A line break typed or pasted into a string ends up as a real newline instead of the two characters \n.

Before (fails):

{"note": "a
b"}

After (valid):

{"note": "a\nb"}

A raw tab sits inside a string value.

Text copied from a spreadsheet or terminal often carries real tab characters.

Before (fails):

{"name": "Ann	Lee"}

After (valid):

{"name": "Ann\tLee"}

A carriage return from Windows line endings sits inside a string.

A multi-line value written on Windows can bring in \r characters that were never escaped.

Before (fails):

{"text": "first
second"}

After (valid):

{"text": "first\r\nsecond"}

The JSON was built by string formatting instead of json.dumps().

An f-string or concatenation puts user text into quotes without escaping it. json.dumps() escapes control characters for you.

Before (fails):

{"msg": "hello
world"}

After (valid):

{"msg": "hello\nworld"}

An LLM or log pipe emitted a multi-line string unescaped.

Model output and copied logs often break a long value across real lines inside the quotes.

Before (fails):

{"summary": "Step 1
Step 2"}

After (valid):

{"summary": "Step 1\nStep 2"}

How to fix it in Python

1. Read the line, column and char numbers in the error. Print text[pos-20:pos+20] with repr() to see the exact character.
2. If you build the JSON yourself, stop formatting strings by hand. Pass a dict to json.dumps(), which escapes newlines, tabs and other control characters.
3. If the source is fixed and you cannot change it, call json.loads(text, strict=False). This setting lets control characters appear inside strings.
4. If the text is a one-off file or API response, paste it into the Repair tool. The tool escapes the raw characters and returns valid JSON.
5. Run the result through the Validator to confirm it parses before you use it.
6. If the data comes from an LLM, ask for JSON through a tool or schema, and keep the strict=False fallback for the cases that still break.

import json

def load_json(text):
    try:
        return json.loads(text)
    except json.JSONDecodeError as err:
        if "control character" not in err.msg:
            raise
        # Retry while allowing raw control characters inside strings
        return json.loads(text, strict=False)

data = load_json('{"note": "line one\nline two"}')
print(data["note"])

Variants of the message

Related errors

• Fix Python JSON Errors: json.loads() Decode Errors and How to Repair Them
• Invalid JSON Meaning: What It Is and How to Fix It
• Single quotes are not valid JSON: Fixing json.loads() in Python
• Why Computer-Use Agents Break JSON: Multi-Step Agent JSON Error Handling (2026)

Key takeaways

• The error means a raw control character sits inside a JSON string.
• Newline, tab and carriage return are the usual causes.
• Build JSON with json.dumps() instead of string formatting.
• json.loads(text, strict=False) accepts the raw characters when you cannot fix the source.
• The the Repair tool tool escapes the characters and returns valid JSON.

FAQ

What does strict=False do in json.loads()?

It allows control characters, including raw newlines and tabs, inside strings. The rest of the syntax is still checked as usual.

Is a raw newline inside a JSON string valid?

No. The JSON standard requires characters from U+0000 to U+001F to be escaped inside strings.

Why does the error say char 12 when I see the problem on another line?

The char value is a zero-based offset into the whole text. Line and column count from the start of the document, so use them together to locate the character.

Does json.dumps() produce this error?

No. json.dumps() escapes control characters in strings, so its output does not trigger this message when parsed again.

Can I just strip newlines from the text before parsing?

You can, but it also removes the line breaks between keys and values and may change the data. Escaping them inside strings keeps the content intact.