journal: erase arm cin_t/cin_s crossover (teacher->student hack-grad handoff)

n=1 live obs from pueue 29: cin_t +0.27->~0, cin_s ~0->+0.15, crossover
~step 10-14. Mechanism inference (advantage-variance collapse on the
all-hacking teacher group + student becoming the hack-grad source) held at
0.6 with the 3 competing failure modes (erase-does-it / refresh-artifact /
noise-floor), each with a falsifier against the queued vanilla+route arms.

Co-Authored-By: Claudypoo <288921227+claudypoo@users.noreply.github.com>

RESEARCH_JOURNAL.md

@@ -1766,4 +1766,42 @@ shorter table... it has a few hacks but doesn't look like it's learning at all ~
   projection stopping hacking or learning... I guess we will see. anything else you notice? i might be clearer with ema showing it goes up, or even just groupby step                                                               
                                                                                                                                                                                                                                     
   open questions: do we need 500 steps? is this experiment even worth running or can be disprove it? are we aplpying steering vectors in wrong domain (gradient vs activation vs SVD activaiton), should we just be dettecting hack 
-   samples and blocking those, idk. is it worth the $10 an experiment self funded. hmm lets see   is it a valid setup?
+   samples and blocking those, idk. is it worth the $10 an experiment self funded. hmm lets see   is it a valid setup?
+
+## 2026-05-31 — erase arm: cin_t/cin_s crossover (teacher->student hack-gradient handoff)
+
+**Context:** commit `d781b56` on `probe/distill-cosine`; pueue id 29 (`just run-substrate erase 41 60 5`, 4-mode substrate run_tests/stdout_marker/sentinel/file_marker, prog_wide v_hack, refresh-every-5); observed live at step 26/60. cin_t = signed cos of teacher-only delta_S grad with v_hack (pre-projection); cin_s = same for student-only grad.
+
+**Hypothesis:** on a working extraction, cin_t > cin_s throughout (v_hack lights up more on the hacky cached-teacher gradient than the not-yet-hacking student). Expected cin_t to stay high.
+
+### Observations
+
+- [obs] cin_t decays from +0.27 (step 0) to ~0.00 / slightly negative (steps 19-26); cin_s drifts from ~-0.02 (step 0) to +0.10..+0.16 (steps 19-26). They cross around step 10-14. (pueue log 29, per-step table; rows below.)
+- [obs] Per-refresh the move is not monotone: at refresh@9 BOTH dropped (step 8 cin_s=+0.12/cin_t=+0.24 -> step 9 cin_s=+0.06/cin_t=+0.11); the clean "student up, teacher down" only shows at refresh@14 and @19. The global trend is the robust signal, not the per-refresh delta.
+- [obs] hack_s climbs 0 -> 14/28 (~50%) by step 26 under ERASE; per-mode student hacks hk_rt 53/175, hk_fm 38/168 climbing hard, hk_so just starting 2/189, hk_se pinned 0/224. cout stays ~-0.10..-0.16 throughout (erase removes the in-subspace alignment) yet hack_s rises anyway.
+- [obs] refresh basis_overlap_with_prev stays healthy 0.74-0.85 through the whole run (no basis rotation).
+
+| step | cin_t (teacher) | cin_s (student) | hack_s |
+|---:|---:|---:|---:|
+| 0  | +0.27 | -0.02 | 0/28 |
+| 6  | +0.53 | +0.07 | 0/28 |
+| 9  | +0.11 | +0.06 | 0/28 |
+| 14 | +0.07 | +0.11 | 4/28 |
+| 19 | +0.00 | +0.16 | 10/28 |
+| 26 | -0.00 | +0.12 | 14/28 |
+
+### Inferences
+
+- [inf] The crossover is a teacher->student handoff of the hack-gradient source, not "the student learning away from the teacher". {reason: "student is learning the SAME modes the teacher pool has (run_tests + file_marker, the dominant teacher hacks); as it does, its own advantage-bearing rollouts carry the hack-ward gradient (cin_s up) while the all-hacking cached teacher group loses within-group advantage variance -> teacher grad shrinks/randomizes -> signed cosine -> noise floor (cin_t -> 0). Cosine is scale-invariant so this is the advantage-variance collapse, not magnitude.", credence: 0.6}
+- [inf] Erase is REDUCING but not PREVENTING here: hack_s reaches ~50% under active projection (cout~-0.12). {reason: "cout negative means the in-subspace component is removed each step, yet hk_rt/hk_fm climb monotonically; the hack capability is reaching delta_S through directions v_hack does not span, or faster than one-sided removal suppresses.", credence: 0.55}
+- [inf] sentinel (hk_se=0) is most likely just the hardest mode to discover, not erase-suppressed. {reason: "erase isn't suppressing the other three, so a mode-specific suppression is implausible; needs the vanilla baseline to confirm.", credence: 0.5}
+
+### Failure modes considered
+
+- **Likely:** cin_t decay is erase doing its job (removing the in-subspace teacher gradient), not advantage-variance collapse. Prior: 0.35. Check: the VANILLA arm (pueue 30, no erase) — if cin_t also collapses there, it isn't erase.
+- **Subtle:** refresh artifact. Each refresh re-extracts v_hack at current weights, so the basis tracks whatever direction is currently active; once the student hacks, v_hack rotates toward the student's gradient, mechanically lifting cin_s and dropping cin_t regardless of advantage. Prior: 0.30. Check: a frozen-V run (no refresh) — if the crossover still happens, it's not a refresh artifact.
+- **Null:** end-state cin_t~0 is just the signed-cosine noise floor (~K/N axes) and the cin_s rise is within run-to-run/seed noise (n=1). Prior: 0.20. Check: compute the null band sqrt(K/N) for the kept-axis count and see if step-19+ cin_t/cin_s sit inside it; replicate at seed 42/43.
+
+### Next action
+
+Compare against vanilla (pueue 30) and route (pueue 28) when they land: (a) does vanilla learn MORE/faster modes than erase (the headline #137 "K classes learned" cut), (b) is cin_t decay erase-specific or universal, (c) 4-arm per-mode overlay plot. Hold the mechanism inference at 0.6 until the vanilla cin_t trajectory is in.