JavaScript runtime incorrectly increments large integer literals (off-by-one due to float precision)

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

Entering large integer literals beyond the safe 64-bit floating-point range causes JavaScript to silently round the value to the nearest representable float. This produces incorrect output and leads to surprising equality comparisons.

Example:

348555896224571969 === 348555896224571970
// true (incorrect from user perspective)

Both numeric literals are parsed as IEEE-754 double-precision floats and rounded to the same stored value, resulting in incorrect comparisons and off-by-one errors.

What Should Happen?

JavaScript should either:

1. Preserve the integer exactly, OR

1. Throw a clear error or warning (e.g., “Integer literal exceeds safe precision”), OR

1. Force the number to be interpreted as a BigInt when it exceeds safe integer range.

Error Messages/Logs

There is no error or warning generated by JavaScript.

This is part of the problem:
JavaScript fails here ⚠️

Console Logs Examples
Input
console.log(348555896224571969);

Output
348555896224571970

Input
console.log(348555896224571969 === 348555896224571970);

Output
true

Steps to Reproduce

1. Open the JavaScript console (Chrome, Node.js, browser devtools, etc.)
2. Enter the following:

``348555896224571969``

1. Observe output:

``348555896224571970``

1. Then evaluate:

``348555896224571969 === 348555896224571970``

1. Output:

``true``

Claude Model

None

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

Latest

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_