CP shared KV and HiCache need a stable contract where physical cache coverage is page-aligned, while scheduler/radix-visible hit length remains the valid token length. This records the contract, adds page-aligned extent metadata, keeps owner assignment on actual tail pages instead of short-prefix fallback, and updates partial current reuse tests around tail-page masking.
Constraint: CP owner lanes operate on page units while scheduler and radix hit accounting must remain token-valid.
Rejected: Pad short suffixes to cp_size or 2*cp_size pages | wastes KV capacity and can turn a small tail into a much larger physical span.
Rejected: Silent direct-write or prefetch fallback | production fallback must be warning-visible for diagnosis.
Confidence: medium
Scope-risk: moderate
Directive: Do not reintroduce replicated short-radix fallback without checking docs/advanced_features/nsa_prefill_cp_page_aligned_cache_contract.md.
Tested: local py_compile for touched runtime, utility, owner, and unit-test files.
Tested: remote g0034 container three-file suite: 122 passed, 5 warnings.
Not-tested: full local pytest, blocked by missing runtime dependencies such as orjson.
Not-tested: CUDA E2E runtime for this commit.
Co-authored-by: OmX <omx@oh-my-codex.dev>
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