The usage anchor (real usage.prompt_tokens + delta estimate of what was appended since)
identified the priced transcript by id() of the last message, so it was None on EVERY
gateway turn (history is re-read from the DB each turn) and in every fresh process
(--resume, desktop per-turn serve). Those are exactly the surfaces where the bytes/4
estimate then fired local compression against payloads the provider priced far under
threshold (#99421, #104462).
- agent/usage_anchor.py owns the anchor: content fingerprint instead of id(), persisted on
the session row (model_config._usage_anchor) via set_usage_anchor(), restored on the first
resumed turn while the durable transcript still matches, cleared with the row on
compaction / codex-native rewrite / session reset.
- Callers repointed from model_metadata (the compat table follows).
Design and persistence slot from #99585 by @686f6c61; re-authored against the Sep 2026
layout (the branch predates the model_metadata / agent_init split).
The codex_app_server runtime bypasses the conversation loop, so the
usage-anchored context accounting captured there never ran: agent._usage_anchor
stayed None forever. Every preflight estimate therefore fell back to the rough
mirror-transcript heuristic, which is deliberately never compacted on this
runtime (_record_codex_app_server_compaction preserves the mirror), so it grows
monotonically while the real thread may be tiny or freshly compacted. With
compression.codex_app_server_auto=hermes that estimate alone tripped the
threshold and fired thread/compact/start on nearly every turn of a long-lived
session (#100381).
Mirror the main loop's post-response capture: _record_codex_app_server_usage
now snapshots the reported thread usage into agent._usage_anchor (base prompt
+ completion exactly as the provider counted them, with estimation confined to
messages appended since). A usage-less turn keeps the previous anchor, and the
post-compaction invalidation site is unchanged.
Every provider response carries usage.prompt_tokens — exact ground truth
for the full request (system prompt + tool schemas + history). Context-size
checks now anchor on the last main-loop response's usage and estimate only
the messages appended since, instead of re-estimating the whole history
with chars/4 heuristics and flat 1500-token image costs. The estimate error
window shrinks from the entire conversation to one turn and self-corrects
at every response.
- agent/model_metadata.py: capture_usage_anchor() / anchored_context_tokens()
with a structural base-message identity check that fails closed on any
transcript rewrite.
- agent/conversation_loop.py: anchor captured at the single main-loop usage
site (MoA uses pre-fold aggregator usage; advisor/aux calls never anchor);
pre-API pressure check prefers the anchor.
- agent/turn_context.py: preflight compression estimate prefers the anchor.
- agent/context_breakdown.py: /context display prefers the anchor.
- Invalidation: compaction rewrite (conversation_compression), codex native
compaction (codex_runtime), session reset/switch (run_agent), plus the
fail-closed structural check for splices/micro-compaction.
- Usage-less responses keep the previous anchor; no anchor -> pure
estimation fallback (first request of a session).
686f6c61