Two coordinated changes that stop the engine from routinely rewriting the
prompt prefix and burning DeepSeek V4's prefix-cache discount:
1. `Settings::default().auto_compact` flips from `true` to `false`. The
`auto_compact = on` opt-in and the explicit `/compact` slash command
stay available for users / agents that decide their workload benefits
from compaction more than from cache stability. With V4's 1M-token
window the user has plenty of headroom to run long sessions without
auto-trimming, and aggressive compaction has been the dominant
cost-spike vector in long sessions (the rewritten prefix invalidates
~90% of the cache discount on every compaction event).
2. `DEFAULT_COMPACTION_TOKEN_THRESHOLD` raised from `50_000` to
`102_400` (80% of `DEFAULT_CONTEXT_WINDOW_TOKENS = 128_000`). This is
the last-resort threshold used when `context_window_for_model` returns
`None` — i.e. an unrecognised model id. Pre-v0.8.11 the fallback
compacted at ~5% of a V4 window when model detection silently fell
through. Now the fallback inherits the same late-trigger discipline as
the V4 path, so model-detection drift doesn't quietly burn cache.
Together: the two changes mean compaction never fires automatically by
default, and even when explicitly opted in (or when the runtime-thread /
capacity-flow paths invoke compaction with their own `enabled = true`
config), the threshold is anchored at 80% of the model's context window
(or 80% of the 128K default if the model is unknown), never below.
Tests
=====
- `default_settings_disable_auto_compact_to_protect_v4_prefix_cache` —
pins the new default and explains the rationale inline.
- `auto_compact_remains_explicitly_configurable` — unchanged; still
asserts the `set("auto_compact", "on" | "off")` round-trip works.
- `compaction_threshold_scales_with_context_window` — updated to assert
`compaction_threshold_for_model("unknown-model") == 102_400`.
- `v4_soft_caps_only_apply_to_v4_models` — updated to assert the
unknown-model + reasoning-effort path also lands on the new floor.
Verification
============
- `cargo fmt --all -- --check` clean.
- `cargo clippy -p deepseek-tui --bin deepseek-tui --all-features
--locked -- -D warnings` clean.
- `cargo test -p deepseek-tui --bin deepseek-tui --locked` →
2028 passed, 2 ignored.
Refs #664 (handoff-instead-of-compact pattern, full implementation
deferred). Behaviour-only change for v0.8.11; the larger
agent-aware-handoff mechanism is its own design surface.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Hunter Bown
fc4f1e6564