[heydryft]

Issues are disabled on this repo (gh repo view aeonmindai/arc --json hasIssuesEnabled -> false), so this is the issue, filed the way the project actually tracks work: a record in memory/mission/, alongside wave64-CP and its 60-odd siblings.

Docs only. No code changes.

The defect

V4 decode re-applies compress-theta RoPE to the entire compressed KV cache on every step, on every one of the 41 compressed layers — deepseek4.rs:1409-1422 -> layers.rs:1826-1843. It recomputes a rotation that compressed_kv_from_rows's own doc comment says "depends on nothing but j". The rows are already cached pre-RoPE by the rolling compressor; the rotation on top of them is not.

Derived from tensor shapes at cc5487ad3, not measured — no GPU was used:

• ~22 MB/token of redundant traffic at 2 048 ctx (21 CSA layers at ~1.05 MB each)
• 123 launches/token in copies (narrow(..).contiguous() + a two-input cat, x41)
• 82 launches/token in the cos/sin gathers
• CSA re-rotates an unchanged row set on 3 of every 4 steps; HCA on 127 of 128

Why it is registered rather than fixed

The record names both hazards, because they are the reason a blind fix is worse than none:

1. The row set arrives through two accessors with different aliasing — state.advance(...) on the eager path, state.compressed_rows_fixed(rows) under graph decode across pin_comp_capacity. A "the rows did not change" cache has to be right for both, and for the advance -> None case.
2. A stale entry is a wrong answer, not a slow one. Coherent output, degraded long-context recall, nothing downstream that catches it — the RUN-161 long-context collapse and the ARC_V4_FP8_KV default-on-without-a-GPU shape.

The file prescribes the treatment: identity proof written down, an engagement counter that reports rather than assumes, a poison control observed going red, and an on-GPU A/B before the default flips. Default-off behind a flag first; flip second. It also flags that the cos/sin memo is independent of the cache and much cheaper, so it should land first.

Related

Surfaced by the census in #206, which removes 467 launches/token from the mask block on this same path and names this as the single worst remaining item it did not fix.

Note for triage: CAPTURE_LANE retired op count as a lever, so the ~205 launches here are worth only ~0.2 ms. The ~22 MB/token is the bigger half, and bytes are not a retired lever.

🤖 Generated with Claude Code

https://claude.ai/code/session_01UMmjFy8TvsgypxVWNVhhC7

[github-actions]

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 Markdown                210        47456            0        36902        10554
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 |- C                      3           17           17            0            0
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 |- Rust                  66         2051         1716           77          258
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 (Total)                            53299         4759        37444        11096
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 Rust                    679       338548       291468        17592        29488
 |- Markdown             502        31477          471        27132         3874
 (Total)                           370025       291939        44724        33362
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