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Overview

Migrate from the legacy .eslintrc configuration (with @rushstack/eslint-patch for module resolution) to ESLint 9's flat config format (eslint.config.mjs). This removes the rushstack workaround entirely, as flat config resolves plugins via direct imports — the exact problem rushstack was patching around.

Complexity: Medium
Target branch: develop

Context

The project currently uses @rushstack/eslint-patch/modern-module-resolution in the root .eslintrc.js to work around ESLint's legacy plugin resolution issues. The actual ESLint configuration lives in packages/kolibri-format/.eslintrc.js and is loaded programmatically by packages/kolibri-format/index.js using the ESLint Node.js API (new ESLint({ baseConfig, resolvePluginsRelativeTo })).

Key files involved:

• .eslintrc.js (root) — requires rushstack patch, delegates to kolibri-format
• packages/kolibri-format/.eslintrc.js — all rules, plugins, extends
• packages/kolibri-format/index.js — programmatic ESLint API usage
• packages/eslint-plugin-kolibri/ — custom lint rules (CJS, exports rules object)

kolibri-format is also consumed by projects outside this monorepo that may provide their own ESLint config, so the host-project config lookup pattern must be preserved.

The Change

Upgrade from ESLint 8 to ESLint 9 and convert the configuration from legacy .eslintrc format to flat config (eslint.config.mjs).

Key changes:

• Replace .eslintrc.js files with eslint.config.mjs files using ESM imports
• Replace eslint-plugin-import with eslint-plugin-import-x (the maintained fork with native flat config support)
• Update packages/kolibri-format/index.js to load the new config via await import() and use ESLint 9's API (which removes resolvePluginsRelativeTo — no longer needed since flat config resolves plugins via direct imports)
• Remove @rushstack/eslint-patch from root package.json
• Strict 1:1 rule parity — no rule additions, removals, or severity changes. The only rule name changes are the mechanical import/ → import-x/ rename.

eslint-plugin-kolibri and eslint-plugin-small-import need no changes — their CJS rules exports work as flat config plugins as-is.

Out of Scope

• Changing, adding, or removing any lint rules (beyond the import/ → import-x/ rename)
• Modernizing eslint-plugin-kolibri to ESM
• Converting kolibri-format/index.js to ESM
• Upgrading other linting tools (prettier, stylelint)

Acceptance Criteria

General

• @rushstack/eslint-patch is removed from root package.json
• Root .eslintrc.js is replaced by eslint.config.mjs
• packages/kolibri-format/.eslintrc.js is replaced by eslint.config.mjs
• packages/kolibri-format/index.js uses ESLint 9 API with await import() for config loading
• Host-project config lookup is preserved (check for eslint.config.mjs in host project, fall back to own)
• eslint-plugin-import is replaced by eslint-plugin-import-x
• All existing lint rules produce identical results — no new warnings or errors introduced

Testing

• pre-commit run lint-frontend --all-files output is identical before and after migration
• eslint-plugin-kolibri tests pass
• Manual smoke test: deliberate import-x/ rule violations (duplicate imports, wrong order, missing newline after imports) are caught

References

• ESLint flat config migration guide
• eslint-plugin-import-x
• ESLint 9 Node.js API changes

AI usage

This issue was restructured from a rough initial description using Claude Code. The design decisions (ESLint version, config format, plugin choices, migration approach) were made collaboratively through iterative Q&A, drawing on findings from a previous contributor's implementation attempt documented in the issue comments. The acceptance criteria and scope were reviewed and confirmed by a human maintainer.