# #345 mechanism-session contract — who eats angled input on the steep-slope approach?

**Date:** 2026-08-08. **Order:** after #344 lands. **This is an
instrumentation session, not a fix session** — the fix contract gets written
from the capture, not before.

## Evidence in hand (do not re-derive)

- `uphill-45deg-capture.jsonl`: 838 uniform stuck ticks — request ~0.23 m/tick
  at 45° to the face, **result byte-identical to input position**,
  `collisionNormalValid=True` with **`collN=(0,0,1)`** (straight up),
  `slidingNormal=(0,0,0)`, carried contact = the ~6.8° approach terrain
  `(-0.083,0.083,0.993)`, `transient=0x3` (Contact|OnWalkable).
- `uphill-slide-capture.log`: the steep face itself was reached only 18
  times; every reach took branch2 cliff-slide with a healthy applied
  constraint. The eater is UPSTREAM of the face.
- `uphill-AB-precampaign.jsonl`: identical 0.0%-yield signature on the
  pre-campaign binary — **pre-existing**, Campaign S exonerated.
- Ruled out by measurement: the #331 sliding absorb (no latch), cliff-slide
  degeneracy (#32 fixed, visibly healthy), all Campaign S landings.

## The one probe this session builds

A per-tick TRANSITION PHASE TRACE, `ACDREAM_DUMP_TRANSIT_FAIL=1`, firing ONLY
when a resolve returns zero XY displacement against a nonzero XY request
(the stuck-tick predicate — self-selecting, so a normal session prints
nothing). For each such tick, one line each for:
1. every `TransitionalInsert` attempt: attempt #, phase reached
   (env/building/objects), each phase's TransitionState, and on Collided the
   colliding polygon's plane normal + which BSP/terrain source produced it;
2. every step-up entry and its step-down verdict (reuse/extend
   `ACDREAM_DUMP_STEPUP`'s existing lines — they already print the input
   normal and the landing verdict);
3. every `ValidateWalkable` outcome on the tick: dist, waterDepth, which
   branch (resting / below-push / CheckWalkable-fail), and BOTH flag guards
   (`oi.Contact`, `sp.StepDown`) at its `SetCollisionNormal` site — the
   `collN=(0,0,1)` fingerprint most plausibly comes from a
   `SetCollisionNormal(groundNormal)` *somewhere*, and this names which.
4. the final `AdjustOffset` input/output pair for the tick.

House rules: lives in `PhysicsDiagnostics` (rule 5), self-selecting trigger,
zero cost when off, mover id on every line
(`feedback_probe_identity_attribution`).

## The session

Build with the probe → the user repeats the exact 45° protocol (~1 min) →
read the trace → the failing phase + normal source IS the mechanism → write
the fix contract from it, citing the retail counterpart
(grep-named-first: `CTransition::transitional_insert` family, the step-down
refusal path, `validate_walkable`'s collision-normal writes — all already
mapped in this repo's pseudocode docs).

## Known trap inventory for the eventual fix (from this week's do-not-retry ledger)

The #331 absorb is retail-faithful — whatever the fix is, that
characterization pin must stay green. The #32 setter split must not be
re-latched. AD-65's snap arm is byte-anchored. The step-up chain's
`stepDownHeight = oi.StepUpHeight` oddity is retail-verbatim (#338's
research). Do not touch any of these on a theory.