Support undetermined-cipher-scenario-logic feature flag

[cad0p]

Summary

Vaultwarden's SUPPORTED_FEATURE_FLAGS whitelist (src/config.rs#L1429-L1432) does not include undetermined-cipher-scenario-logic, so self-hosted browser-extension users cannot opt in to Bitwarden's rewritten post-submit add/update login notification logic.

Context

The flag was introduced in bitwarden/clients#18395 (merged 2026-02-02, first shipped in Browser 2026.2.0) and gates the new notification triggering logic that defers cipher-type inference until the vault is unlocked, replacing the separate Change/Add scenarios with a unified Cipher scenario. The Bitwarden maintainer described the rewrite as making add/update notifications "more reliably/consistently" work (bitwarden/clients#17101, comment 2026-02-23, closed as duplicate of #17089).

The official Bitwarden server exposes the flag (src/Core/Constants.cs#L173-L176, Autofill Team), and the client-side default is FALSE (enum #L30, default #L143), so the new logic only runs when the server serves the flag as true in /api/config.

Vaultwarden serves only flags present in SUPPORTED_FEATURE_FLAGS (FeatureFlagFilter::ValidOnly), and treats unknown values in EXPERIMENTAL_CLIENT_FEATURE_FLAGS as unrecognized (src/config.rs#L1065-L1079). So today self-hosted users get the old logic and setting the env var only produces an "Unrecognized experimental client feature flags" warning.

Client-side only

bitwarden/clients#18395 touches only apps/browser and the shared flag enum; no request/response models change. The flag selects between notification candidates in overlay-notifications.background.ts#L450-L479 using existing decrypted ciphers. No server-side behavior is gated by it, so adding it to the whitelist does not require Vaultwarden API or data changes.

Request

Add undetermined-cipher-scenario-logic to the Autofill Team section of SUPPORTED_FEATURE_FLAGS and document it in .env.template, as done recently for enable-basic-auth-response (#7745) and pm-34171-card-scanner (#7477).

Related work

#7502 (closed 2026-08-03, unmerged) proposed this flag as part of a bulk 2026.7 flag sync; the maintainers rejected the bulk/untested approach and the author closed it committing to separate tested PRs. This request is the single-flag, client-side-only follow-up. #7797 proposes an EXPERIMENTAL_CLIENT_FEATURE_FLAGS_ALLOW_UNSUPPORTED bypass; the maintainers previously rejected the same idea in #6855 over data-safety concerns.

Note on enabling

After the flag is whitelisted, the new logic only activates when the admin also sets EXPERIMENTAL_CLIENT_FEATURE_FLAGS=undetermined-cipher-scenario-logic (comma-separated for multiple). The setting is env/.env-only (not admin-editable) and supports the usual _FILE variant (EXPERIMENTAL_CLIENT_FEATURE_FLAGS_FILE).

A small two-line PR is ready to follow.