[...] because we automatically parse numbers.

It depends on the option type wether we parse or not. The current behavior (without coerce functions) is:

• option is number: we parse numbers, configuration parse-numbers doesn't matter
• option is string: we don't parse numbers, configuration parse-numbers doesn't matter
• option is undefined: behavior depends on configuration parse-numbers

So if users define their option as string, they will get string "55".
It's clearly a bug when the coerce function returns a string into an option of type number.

The key question finally is whether the option type has precedence over the return type of the coerce function.

The key question finally is whether the option type has precedence over the return type of the coerce function.

Can you explain the bug to me that this solves for you in mocha, was coerce returning a number but it gets turned into a string?

const parse = require('yargs-parser');

const args = parse('--retries 2 --retries 3', {
    number: ['retries'],
    coerce: {
        'retries': v => Array.isArray(v) ? v.pop() : v
        // 'retries': v => Number(Array.isArray(v) ? v.pop() : v)
    },
    configuration: {
        'parse-numbers': true,               // default
        'duplicate-arguments-array': true    // default
    }
}
);

console.log(args);   // { _: [], retries: '3' }

In short: a number[] is passed to coerce which returns a string to a number option.
It also returns a string when just a number is passed to coerce.
The alternative with Number(...) correctly returns a number.

This is not a critical bug for Mocha, so don't wait with cutting your new release.