@lovemilk2333 prettier doesn't have a config option for this, and the team has explicitly rejected adding one. their stance is "fewer options" by design.

the reason prettier defaults to _ is a CommonMark spec thing. underscores have stricter flanking rules (they don't work mid-word like foo_bar_baz), so they're "safer" as the default. prettier's printer source only switches to * when _ would break (adjacent words, nested emphasis, etc.).

practical workaround: disable prettier for markdown and use remark-stringify instead, which actually defaults to *:

// remark.config.mjs
export default {
  settings: { emphasis: '*' }
};

then in .prettierignore:

*.md

and format markdown with remark . --output instead of prettier.

alternatively if you just want linting without auto-fix, use remark-lint-emphasis-marker configured with "*".

ref: prettier options docs | rejected option request #3463 | remark-stringify settings