Found during the Phase 5B pass-3 architectural audit: sanitizeModuleResponse's
Vary merge used a plain Set, so an existing "Vary: *" from a module handler
became "*, Origin, Cookie" (RFC 9110 §12.5.5 forbids combining "*" with any
other value), and a differently-cased existing token (e.g. "origin") produced
a redundant duplicate alongside the forced "Origin"/"Cookie". mergedVaryHeader
now de-duplicates case-insensitively, always emits canonical Origin/Cookie
casing, and collapses to a bare "*" whenever one is present, per focused unit
tests. The full app's own pre-existing cors() middleware (apps/api/src/app.ts,
unchanged here, present since Phase 4B on every /api/v1/* route including
/api/v1/principal) still unconditionally appends its own "Vary: Origin"
outside this module's control — noted in the new tests, not addressed by this
narrow fix.
Co-Authored-By: Claude Sonnet 5 <[email protected]>
