The kleague-results API previously accepted regex-shaped but impossible calendar dates and treated them like empty-result queries. This change adds a regression test first, then rejects invalid YYYY-MM-DD strings during normalization so bad input never reaches the fetch layer.
Constraint: Public API contract says date must be a valid Date or YYYY-MM-DD string
Rejected: Let upstream return zero matches | hides invalid input as a valid no-results response
Confidence: high
Scope-risk: narrow
Reversibility: clean
Directive: Keep string-date validation in normalizeDateInput before any schedule fetch
Tested: node --test packages/kleague-results/test/index.test.js; npm run ci; live smoke getKLeagueSummary(2026-03-22, K리그1, FC서울, standings); live smoke getMatchResults(2026-03-22, K리그2); invalid-date smoke getMatchResults(2026-13-40); LSP diagnostics on changed files; architect verification
Not-tested: Additional leap-year and month-end invalid-date permutations
The K League site already exposes JSON schedule and standings endpoints, so this change wraps those official surfaces in a reusable workspace package and wires the new skill/docs flow into the repo.
The implementation keeps the fetch/parse boundary small, locks normalization with fixtures and regression tests, and documents the publish follow-up needed for the new npm package. Korean-language request headers are pinned so live payloads keep the expected team names and result labels.
Constraint: Must use official K League surfaces instead of adding scraping or third-party dependencies
Rejected: HTML scraping from schedule pages | official JSON endpoints already provide schedule and standings data
Confidence: high
Scope-risk: moderate
Reversibility: clean
Directive: Keep the accept-language header pinned to Korean unless team alias normalization is expanded for English payloads
Tested: npm run ci; live getKLeagueSummary('2026-03-22', { leagueId: 'K리그1', team: 'FC서울', includeStandings: true }); live getMatchResults('2026-03-22', { leagueId: 'K리그2' })
Not-tested: Live in-progress/postponed match statuses beyond the fixture-covered finished-game path
Jeffrey (Dongkyu) Kim