Pre-registered: flow-noise stable-triple reseed (ideas #18.2)

Amendment (2026-08-06 ~05:5xZ): empirical σ_draw finalized at 0.0159 from the draws chain’s pooled mean-of-N curve (finalization amendment) — below the 0.045 naive floor, so the re-bank band is 6.6232 ± 0.135 = [6.4882, 6.7582]. The fairness probe’s direct per-draw measurement supersedes if larger (lands before the flip eval).

2026-08-05, work session ~20:45–21:20Z. Design + implementation landed behind a flag; the instrument break itself is pre-registered here and executes at the next anchor boundary. Source: the bijou deep-dive, finding 1 — the top item of the fix queue.

The defect being fixed

Flow-policy eval noise is keyed to the corpus-relative concat index: frame i’s draw is sample_noise(seed + i) where i is recomputed from the current eval data dir (bijou/eval/policies.py, eval/plan.py). Sealed plans pin frame identity (repo_id, episode, frame) but not noise identity — adding, removing, or regrowing any dataset shifts the index of everything after it, silently redrawing every flow prediction downstream of the edit while state-copy stays bitwise identical. Across-noise-draw std is ~5.9° per frame, orders of magnitude above the 1e-4 anchor bands, so the drift would read as model change. Until this executes, flow anchors are valid only at frozen corpus composition (that caveat has been quoted with them since the deep-dive).

The design (landed today, default OFF)

--noise-key {index,stable} on bijou.eval, default index:

• index — the legacy scheme, byte-identical to every banked flow number (draw 0 ≡ sample_noise(seed + i); draws at + draw·2²⁶). Retained permanently so any historical report can be reproduced.
• stable — noise keyed to the frame’s identity triple: blake2b("{repo_id}\x1f{episode_index}\x1f{frame_index}", 16 bytes) → four 32-bit words fed with (seed, draw) into a numpy SeedSequence → PCG64 standard_normal(float32) on CPU. Properties, each load-bearing:
  • Corpus-composition-invariant (the point): the key is a pure function of frame identity + run seed + draw. Same frame, same noise, forever — plans and anchors survive corpus edits.
  • 128-bit keying, no torch manual_seed: torch’s CPU generator ignores seed bits ≥32 (measured 2026-08-05 — it forced the draws stride down to 2²⁶ and nearly caused silent draw collapse). The SeedSequence route sidesteps that entire trap class and makes birthday collisions across a 25.8k-frame × 10-draw panel impossible in practice.
  • Draw number is entropy, not arithmetic: no stride, no stride-vs-corpus-size bound to police.
  • Deterministic across device, batch composition, and eval order (CPU generation, same as legacy).
• The report JSON records noise_key (extending the #18.1 scoring- semantics block) and the run banner prints it — a number can no longer be quoted without its keying.
• SmolVLA eval policy threads the same flag; the Q3 counterfactual pass shares noise with the scalar pass under both keyings (condition overrides don’t touch identity fields — verified).
• Out of scope: the in-run training probe (own generator, same-run comparisons only) and train-time τ/ε draws (not an instrument).

Gates on today’s landing

• Oracle (scoring path, bit-exact): the banked AR-100k panel report recomputed from its npz through the edited path — 4 policies × {chunk_mae, chunk_mse, first_mae} = 12 cells, all deltas 0.0e+00 (state-copy pair and both bijou policies, incl. the 5.8026 pooled anchor).
• Tests: 7 new (tests/test_stable_noise.py): default-path byte identity vs history; corpus-index invariance of stable; determinism + identity sensitivity per slot; 10-draw pairwise distinctness; separator anti-aliasing (episode/frame slot swap, repo_id digit bleed); N(0,1) distribution check; loud failure on an unknown key. Full suite + check.py green.

The pre-registered instrument break (executes at the anchor boundary)

When: after the box-batch 40k panel reads land and are posted (they run under index — mid-experiment keying flips are exactly what this amendment exists to prevent). First eval after that boundary.

What: re-bank the flow anchor under stable keying — one panel eval of flow-80k @ heun-30, N=1, panel curated_v0_k4l2 (~1.7 h GPU). From then on stable is the quoted keying for all new flow numbers; index numbers stay valid as-labeled at frozen corpus.

Predictions / decision rules:

1. Controls (hard): state-copy and any AR summaries must be bitwise identical across keyings — neither takes noise. Any delta ⇒ the change leaked outside the noise path ⇒ do not adopt, investigate.
2. Flow shift band (primary): re-keyed chunk_mae is a fresh draw of the same noise distribution. Naive band: per-frame across-draw std ~5.9° pooled over 17,204 core frames ⇒ σ ≈ 0.045. The draws chain landing tonight gives the empirical per-draw pooled spread from its per-draw dumps; the band is 6.6232 ± 3·max(0.045, empirical σ_draw). Inside ⇒ adopt and re-bank. Outside ⇒ something other than the draw moved — hold the flip, diagnose.
3. Secondary observables (quoted, not gated): first_mae and state-copy-relative margin under the new keying.

Why not flip now: zero live comparisons should change keying mid-flight, and re-banking costs a GPU eval that would contend with the draws chain. The flag landing today makes the flip a one-token change + one eval at a boundary we already have to visit.