The `message_threshold` field on `CompactionConfig` was a 128K-era
heuristic that fired compaction on long sessions of small messages
— exactly the case where rewriting V4's prefix cache is most wasteful.
Token budget is the only signal that maps to actual model context
pressure; counting messages adds nothing.
Changes:
* Remove `CompactionConfig::message_threshold` field.
* Remove the message-count branch in `should_compact` — token
threshold + 500K floor is now the sole compaction trigger.
* Remove `compaction_message_threshold_for_model`,
`DEFAULT_COMPACTION_MESSAGE_THRESHOLD`,
`COMPACTION_MESSAGE_DIVISOR`, `MAX_COMPACTION_MESSAGE_THRESHOLD`
from `models.rs`.
* Drop the `forced_config.message_threshold` tweak in the engine's
capacity-guardrail forced-compaction path; that path now also
bypasses the floor (`auto_floor_tokens = 0`) because we're at a
hard ceiling and have to free budget regardless of cache cost.
* Update production constructors (`main.rs`, `runtime_threads.rs`,
`app.rs::compaction_config`) to drop the field.
* Update tests: keep the floor + token-threshold assertions, delete
the two tests that specifically validated message-count
triggering, replace `should_compact_respects_message_threshold`
with `message_count_no_longer_triggers_compaction` pinning the
new contract.
Verified locally:
* `cargo fmt --all -- --check` clean.
* `cargo clippy --workspace --all-targets --all-features --locked
-- -D warnings` clean.
* `cargo test --workspace --all-features --locked` — 2036 passed
in TUI bin (2 ignored), all other crates green.
* parity gates: snapshot, parity_protocol, parity_state — all pass.
* `git diff --exit-code -- Cargo.lock` — clean.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Hunter Bown
d0e95f23b0