OpenAI sunset the legacy unary /responses/compact endpoint (404, #5598,
#5624), so the account "compact probe" in the admin UI kept failing even for
healthy accounts, and the beta-feature negotiation header was only attached
to compaction turns.
Beta features (codex-rs session/mod.rs build_model_client_beta_features_header
+ client.rs build_responses_headers): the header is a session-level constant
attached to every /responses request, the WS handshake and /responses/compact.
Enumerating FEATURES shows no Experimental feature is enabled by default, so a
default install sends exactly "remote_compaction_v2". Mirror that:
- OAuth requests without a client-declared header get the default shape, so we
no longer produce a "header only on compaction turns" pattern real Codex
never emits (#5586 chains that strip the header)
- a client-declared header is preserved as-is: non-empty without v2 means the
user disabled the feature and the gateway must not rewrite that
- native v2 turns (compaction_trigger in body) always ensure v2 is present
- non-OAuth upstreams keep the compaction-turn-only behaviour
- the WS injection sits outside the client-header copy block so prewarm and
turn handshakes cannot land in different pool compatibility buckets
Compact probe now exercises native v2 (streaming /responses +
compaction_trigger) instead of the dead endpoint. Success requires an actual
compaction output item — scanning output_item.done/added, the terminal
response.output[] and the whole-JSON fallback — so a 2xx that silently drops
the trigger is reported as unsupported (the "got 0 items" class, #5478,
#5648). Probe identity is now UUID-shaped and applies the account's
convergence, matching real traffic on the same endpoint.
shaw