Allow mainstream reverse-proxy path mounts to keep their X-Forwarded-Prefix when Home Assistant Supervisor ingress already consumes nearly the old 64-character budget. Keep validation bounded and keep rejected non-empty prefixes diagnosable with a deduplicated warning.
Constraint: HA Supervisor ingress prefixes are 63 chars before add-on subpaths, so the old 64-char cap dropped valid dashboard deployments.
Rejected: remove the length cap entirely | a bounded header budget is still a conservative validation guard.
Confidence: high
Scope-risk: narrow
Directive: Keep prefix validation centralized in hermes_cli.dashboard_auth.prefix so auth routes, cookies, and SPA asset rewriting agree.
Tested: python probe for the 73-char HA ingress prefix; scripts/run_tests.sh tests/hermes_cli/test_dashboard_auth_prefix.py -q; .venv/bin/python -m pytest tests/hermes_cli/test_web_server.py -k 'spa_assets_are_read_as_utf8' -q; python -m ruff check hermes_cli/dashboard_auth/prefix.py tests/hermes_cli/test_dashboard_auth_prefix.py; git diff --check
Not-tested: full test suite
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