The Anthropic token-mode handler walked past prefix_tracker.frozen_message_count
whenever an upcoming tool_result's content-hash matched comp_cache._stable_hashes
or should_defer_compression returned True. That conflated content equality with
positional cache membership.
Anthropic's prefix cache is POSITIONAL: bytes 0..K cached, anything past K is
fresh. _stable_hashes is content-keyed and grows unbounded. In long Claude Code
sessions where tool_result content rhymes across turns (repeated system prompts,
repeated file reads, repeated tool descriptions), the walker advanced
frozen_message_count to len(messages) on every turn and the pipeline produced
transforms_applied=[] on 73% of requests in user SvenMeyer's reported session
(headroom-stats-2026-05-01.json: 74 of 101 eligible requests "prefix_frozen") —
even after the prior fix in 44944fb. The 15 requests that did compress averaged
21%, proving compression itself works when reached.
Fix: delete the walker. The freeze boundary is now
frozen_message_count = min(
prefix_tracker.frozen_message_count, # positional ground truth
comp_cache.compute_frozen_count(messages), # local cache lower bound
)
compute_frozen_count's use of _stable_hashes can only LOWER the freeze via the
min clamp, never raise it past prefix_tracker's value. For any position in the
gap [compute_frozen_count, prefix_tracker.frozen_count], recompressing produces
byte-stable output (compression is deterministic on input content), so
Anthropic's prefix cache stays valid.
Cross-handler verification:
* OpenAI handler (proxy/handlers/openai.py:358-382) does not have this walker
— uses only compute_frozen_count. Codex routes through OpenAI handler. Both
unaffected.
* Streaming and non-streaming both invoke anthropic_pipeline.apply() before the
upstream call. One fix covers both paths.
* Cache mode (is_cache_mode) takes the _extract_cache_stable_delta path and is
independent of the walker. Unaffected.
Tests: six new regression tests lock down the post-fix invariants — clamp to
min(prefix_tracker, compute_frozen_count); fresh tool_result whose hash matches
old _stable_hashes entry is not frozen; frozen prefix byte-stable across the
pipeline; 10-turn session produces non-empty compression suffix every turn;
streaming and non-streaming compute identical frozen_message_count; OpenAI
handler never calls the walker functions. Plus scripts/smoke_issue_327.py
(gated by RUN_LIVE_API=1) drives a 10-turn conversation against
api.anthropic.com in both shapes (string + list-of-blocks) and both modes
(streaming + non-streaming).
ci-precheck clean. 191 tests pass.
Follow-ups (separate PRs):
* Fix _cache perpetually empty (anthropic.py result.messages != working_messages
comparison rarely fires in token mode).
* Cap _stable_hashes with bounded LRU + 1h TTL — hygiene only after the freeze
gate is removed.
* List-shape tool_result content gates at content_router.py:1975 and
intelligent_context.py:657 (cluster A from the audit).
