feat(scheduler): add cache_ttl "never" sentinel for always-warm lanes - #245

Problem

cache_ttl assumes idle > TTL means the provider evicted the prompt cache, so the next prefix rebuild is free. Two consumers act on that assumption:

1. the scheduler converts a defer pass into an execute pass (scheduler.ts), and
2. mustMaterialize fires the ttl_idle HARD fold (via hardCacheExpired in transform.ts).

On lanes kept warm by an external keepwarm mechanism (prewarm proxies, dedicated cache-keep tools that re-warm the provider cache out-of-band), the assumption is wrong: the cache never goes cold, MC's lastResponseTime goes stale anyway, and both consumers false-positive. MC then initiates a rebuild it believes is free that is actually a full paid cache-write — measured 450-560K tokens per fold on large sessions. There was no way to express "this lane never goes cold": the config requires a duration.

Fix

cache_ttl: "never" (case-insensitive, works as the string form or any per-model value):

• parseCacheTtl("never") returns Infinity; both consumers go inert through the existing comparisons (elapsed > Infinity / elapsed >= Infinity are never true) — no new branches in the hot path.
• The Rust scheduler mirrors the sentinel with u64::MAX (its predicates already use saturating_sub, so no overflow path).
• Status surfaces render honestly instead of a bogus countdown: /ctx-status shows never expires (always-warm lane); the TUI sidebar gets a JSON-safe cacheNeverExpires flag on StatusDetail (Infinity does not survive JSON-RPC); Pi's status dialog now uses the shared parseCacheTtl (it previously used a private fallback parser that would have shown a 5m countdown).
• hardCacheExpired is extracted into a pure computeHardCacheExpired helper so the "never" -> Infinity -> false chain has direct unit coverage instead of only flag-consumption coverage.

Docs: CONFIGURATION.md describes the sentinel, what it disables, and the tradeoff — after a genuinely cold start (e.g. the keepwarm process died), the free-fold window is not detected on such lanes; mutations then apply at the execute threshold.

Verification

• plugin: 2995 pass / 0 fail; pi-plugin: 666 pass / 0 fail; tsc --noEmit clean on both
• new tests red-verified: reverting the sentinel fails the scheduler tests and the computeHardCacheExpired test
• Rust: sentinel + predicate + scheduler-decision tests added in scheduler.rs's test module (note: I could not run cargo test locally — the crates workspace has path deps outside this repo; the change is a 3-line early return mirroring the TS logic)
• schema + generated docs regenerated (build-schema / build-config-docs drift tests green); check:tui-compiled green

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Summary by cubic

Adds cache_ttl: "never" to disable the idle-TTL heuristic on always-warm lanes, preventing false executes and paid cache rebuilds. Status surfaces now show “never expires” and keep RPC fields JSON-safe; invalid-TTL diagnostics are restored.

• New Features

  • parseCacheTtl("never") returns Infinity (TS) / u64::MAX (Rust) so idle-execute and ttl_idle never fire; computeHardCacheExpired exported for consistent checks with onInvalid.
  • Status UIs (/ctx-status, TUI, Pi) render “never expires” via a JSON-safe cacheNeverExpires flag; Pi uses the shared parser; docs and schema updated.

• Bug Fixes

  • Status RPC avoids Infinity by keeping cacheRemainingMs numeric (0) when TTL is “never” and keying on cacheNeverExpires; auto-execute messaging switches to threshold-only.
  • Transform restores invalid-TTL pass outcomes and session logs through the onInvalid callback.

^{Written for commit 16315e4. Summary will update on new commits.}

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failures remain in the fixes associated with the previous review threads.

_{Reviews (5): Last reviewed commit: "fix(scheduler): keep cacheRemainingMs JS..." | Re-trigger Greptile}