Routes/caddy-upstreams.js had 4 callsites with argument-order swapped: errorResponse(res, 'message', 503) instead of errorResponse(res, 503, 'message'). The canonical signature from src/utils/responses.js:66 takes statusCode FIRST; the swapped call passed a STRING where Express expected a status code. res.status('Caddy upstream watcher not initialized') throws RangeError [ERR_HTTP_INVALID_STATUS_CODE], Express's error middleware catches it, and the response is 500 with an HTML stack trace instead of the intended 503 JSON. Four `!caddyUpstreamWatcher` defensive guards had this exact pattern; all fixed.
Defense-in-depth (responses.js): errorResponse() now validates that statusCode is an integer in 100..599 and that message is a string BEFORE calling res.status(). Future arg-order mistakes fail fast with a clear TypeError naming the wrong arg and the message — instead of writing a 500 HTML panic to the wire. Legacy error(res, message, statusCode) helper (used by ~7 files that import as 'error: errorResponse' alias) is intentionally untouched.
Tests (__tests__/utils-responses-dc-062.test.js, NEW, 21 tests pass):
- correct (res, 503, msg) order: 503 JSON
- swapped (res, msg, statusCode) order: TypeError (was: silent 500 HTML panic)
- 10 invalid-statusCode cases: NaN, Infinity, '503', null, undefined, underflow, overflow, float, object, array — all rejected
- non-string message rejected
- DC-086 extras.code propagation preserved
- legacy error() helper regression: still works
- pre-fix Express server proves the bug class (500 HTML when statusCode is a string)
- all 4 caddy-upstreams routes with null watcher now return 503 JSON
- static source scan: 0 swapped patterns, 4 canonical (statusCode, 'message') occurrences
Full suite: 92 suites / 2039 tests / all green pre and post fix.
[glm-grade=A] from deleg_45e44614 (3 tool calls, 82s, MiniMax-M3 stand-in per Sami's 2026-08-17 authorization)
DashCaddy Polish Loop