@swc/cli seems to reuse config somewhere under the hood

Describe the bug

When invoking swcDir() programmatically in the same process for two separate builds (ESM then CommonJS), the second call seems to reuse some parts of the config. As a result, even though I pass swcOptions.module.outFileExtension: 'cjs' on the second run, all import-path rewrites still use .mjs. If I comment out the ESM build, the CJS build works correctly—but running both back-to-back, the first invocation “wins” and contaminates the second somehow.

Input code

import path from "node:path";
import { swcDir } from "@swc/cli";
import { toMerged } from "es-toolkit";

export async function compileSwc() {
  const defaultOptions = {
    cliOptions: {
      outDir: path.join("dist", "es"),
      filenames: ["src"],
      extensions: [".js", ".jsx", ".cjs", ".mjs"],
      stripLeadingPaths: true,
    },
    swcOptions: {
      jsc: {
        parser: {
          syntax: "ecmascript",
        },
        baseUrl: path.resolve("src"),
      },
      module: { type: "es6", resolveFully: true, outFileExtension: "mjs" },
    },
  };

  // Config for ES6 modules output
  const swcModulesConfig = defaultOptions;

  // Merge options for CommonJS output
  const swcCommonJSConfig = toMerged(defaultOptions, {
    cliOptions: {
      outDir: path.join("dist", "cjs"),
      outFileExtension: "cjs",
    },
    swcOptions: {
      module: { type: "commonjs", outFileExtension: "cjs" },
    },
  });

  console.log("Compiling esm.");
  await swcDir(swcModulesConfig);

  console.log("Compiling CommonJS.");
  await swcDir(swcCommonJSConfig);
}

Config

Link to the code that reproduces this issue

https://codesandbox.io/p/devbox/w9lwrg?file=%2Fdist%2Fcjs%2Findex.cjs

SWC Info output

No response

Expected behavior

In the dist/cjs folder the import paths should be rewritten to use '.cjs' extensions

Actual behavior

In the dist/cjs folder the import paths are rewritten with '.mjs' extensions

Version

"@swc/cli": "^0.7.3", "@swc/core": "^1.11.21"

Additional context

No response