Tricky `BigInt` values displayed incorrectly

[dnechay]

Hey 👋🏻
Thank you for this amazing tool!

We use pino and pino-pretty in everyday dev env and constantly work with logging different BigInt values. Recently found out that by some reason pino-pretty distorts the value that is passed to pino.info and correctly serialized by it.

Env:

• Node.js version: v22.14.0
• pino version: 9.7.0
• pino-pretty version: 13.0.0

Sample script:

import pino from 'pino';

const MESSAGE_KEY = 'message';

const logger = pino({
  name: 'PlaygorundLogger',
  base: null,
  messageKey: MESSAGE_KEY,
  timestamp: () => `,"timestamp":${Date.now()}`,
  formatters: {
    level: (label) => ({ level: label })
  },
  // transport: {
  //   target: 'pino-pretty',
  //   options: {
  //     messageKey: MESSAGE_KEY,
  //     // translateTime: "UTC:yyyy-mm-dd'T'HH:MM:ss'Z'",
  //     translateTime: "SYS:yyyy-mm-dd, HH:MM:ss'Z'",
  //   },
  // },
});

const wierdBigintStr = '1666666666666666700';
const weirdBigint = BigInt(wierdBigintStr);

(async () => {
  try {
    logger.info({
      weirdBigint,
      wierdBigintStr,
      asString: weirdBigint.toString(),
      asValueOf: weirdBigint.valueOf(),
    }, 'weirdBigint');
    process.exit(0);
  } catch (error) {
    logger.error('Failed', error);
    process.exit(1);
  }
})();

Result w/o pino-pretty transport:

{
  "level": "info",
  "timestamp": 1762865459988,
  "weirdBigint": 1666666666666666700,
  "wierdBigintStr": "1666666666666666700",
  "asString": "1666666666666666700",
  "asValueOf": 1666666666666666700,
  "message": "weirdBigint"
}

Result w/ pino-pretty transport:

[2025-11-11, 15:55:42Z] INFO: weirdBigint
    weirdBigint: 1666666666666666800
    wierdBigintStr: "1666666666666666700"
    asString: "1666666666666666700"
    asValueOf: 1666666666666666800

As you can see it prints a value with 8 instead of 7, but not sure why.

Hope you can help with it 🙏🏻

[dnechay]

From what I found so far, under the hood pino uses safe-stable-stringify that is configured with default bigint: true (ref). As a result - in output JSON it puts "number-like" value for bigint and when it gets parsed by pino-pretty later (here) it's casted to number with loss of precision.
Taking into account that pino-pretty is supposed to be used mostly in dev environments, I would suggest to add a reviver fn that will return bigint for the case where pino writes it as a number

return {
  value: sjs.parse(input, (key, value, context) => {
    if (
      typeof value === 'number'
      && value > Number.MAX_SAFE_INTEGER
      && context.source.indexOf('.') === -1
    ) {
      return BigInt(context.source);
    }
    return value;
  }, { protoAction: 'remove' }),
}

wdyt?

As an alternative - configure safe-stable-stringify in pino to not serialize bigint by default and let users do that