acdream/docs/research/2026-05-23-a6-p3-issue98-comparison-harness-findings.md

A6.P3 #98 — Comparison harness shipped, root cause identified

Session: 2026-05-23 evening (continuation of full-day session) Worktree: C:\Users\erikn\source\repos\acdream\.claude\worktrees\strange-albattani-3fc83c Branch: claude/strange-albattani-3fc83c

Read this AFTER the morning's handoff doc (2026-05-23-a6-p3-issue98-harness-handoff.md) — this picks up from "Option A: build the side-by-side comparison harness" and documents the FIRST evidence-driven step in the saga.

---

TL;DR

Updated 2026-05-23 evening v3: NEW root-cause hypothesis identified — STALE RAMP CONTACT PLANE causes per-tick Z drift, which is what makes the cottage-floor cap reachable in the first place.

• Player position at cap: world (141.5, 7.2, 92.7). The cellar ramp's actual world XY is X=[129.7, 131.3] — the player is 10 meters away from the ramp in cell-local space.
• Body's contact plane: ramp's plane (n=(0, 0.719, 0.695), d=-69.5035). Stale; should be the flat cellar floor (n=(0,0,1)).
• AdjustOffset projects forward motion along that stale ramp plane. Mathematically: requested delta (+0.0266, -0.4022, 0) → projected (+0.0266, -0.1943, +0.2010). +0.2010 m of Z lift per tick.
• After enough horizontal-walking ticks, the head sphere rises to Z=94 and hits the cottage floor's downward-facing back-face polygon. Cap fires.
• The cap is a SYMPTOM. The root cause is the contact plane not refreshing when the player walks off the ramp onto the flat cellar floor. Retail must re-find the walkable plane each tick; we're keeping the stale ramp seed.

This explains why six prior fix attempts missed. Step-up, AdjustOffset projection, SidesType, edge-slide, +X residual — all were investigating the cap event mechanics, not the upstream Z drift that made the cap reachable. The harness convergence (Section "What shipped 2026-05-23 evening v2") is still valuable as the deterministic reproduction infrastructure; the new hypothesis is the next thing to verify against that infrastructure.

(Sections below preserve the evening-v2 arc for context: apparatus + cap-event reproduction.)

• Evidence-driven apparatus shipped. PhysicsResolveCapture writes one JSON Lines record per player ResolveWithTransition call when ACDREAM_CAPTURE_RESOLVE=<path> is set. 41,228 records from a single cellar-walk session.
• Comparison test reproduces the cap divergence on the first try. The new LiveCompare_* tests in CellarUpTrajectoryReplayTests.cs load three representative records (spawn, on-ramp, first-cap) and replay them through the harness engine. Spawn + on-ramp PASS bit-perfect; first-cap FAILED with a clear divergence — the right divergence.
• Root cause identified: the cottage GfxObj was missing from the harness. Live cap attributes the blocking entity to obj=0xA9B47900 — a landblock-baked static building. The cottage's floor polygons live in this GfxObj's polygon table (registered as a ShadowEntry), NOT in any cottage CELL.
• Apparatus convergence (v2 update). With the cottage GfxObj 0x01000A2B extracted via the new ACDREAM_DUMP_GFXOBJS infrastructure and registered as a ShadowEntry in BuildEngineWithCellarFixtures, the harness now reproduces the live cn=(0,0,-1) cap exactly. The full per-field round-trip reveals one residual: live preserves +0.0266 m of +X motion through the cap; harness blocks all motion. That's the next investigation target — see the "Residual divergence" section below.
• Not a step-up / AdjustOffset bug. The head sphere (top at Z=foot+1.2) hits the cottage floor at Z=94.0 from BELOW. Math: cap at foot Z=92.74 matches 94.0 − 1.2 = 92.80. Confirmed by user reporting same cap when JUMPING in the cellar (purely vertical motion). The retail comparison question is now sharpened to "how does live's post-cap edge-slide preserve the +X component that the harness drops?"

---

What ran this session (chronological, 3 commits)

Commit	What
fb5fba6	Apparatus: PhysicsResolveCapture static class + JSON Lines writer + body snapshot record + capture probe in ResolveWithTransition + smoke tests (capture writes when IsPlayer + enabled, skips otherwise)
44614ab	Comparison test: 3 fixture records sampled from live capture + 3 LiveCompare_* tests + diagnostic dump that prints cell polygons in world frame
0f2db62	Converted FirstCap test to documents-the-bug pattern (passes while harness lacks cottage GfxObj; fails when added)

Live capture launches:

• launch-a6-issue98-capture.ps1 — first capture run (no probes beyond cell-transit). Produced a6-issue98-resolve-capture.jsonl (12 MB, 5789 records when checked mid-session, finished at 91 MB / 41,228 records).
• launch-a6-issue98-polydump.ps1 — second capture with ACDREAM_PROBE_POLY_DUMP, ACDREAM_PROBE_PUSH_BACK, ACDREAM_PROBE_RESOLVE, ACDREAM_PROBE_INDOOR_BSP, and ACDREAM_DUMP_CELLS covering 0xA9B40140-0xA9B4014F. Produced a6-issue98-resolve-capture-2.jsonl (135 MB, 70,572 records) plus 16 cell-dump JSON fixtures and a launch log with 214 [poly-dump] entries.

---

The apparatus (committed code)

PhysicsResolveCapture (src/AcDream.Core/Physics/PhysicsResolveCapture.cs)

Static module. When ACDREAM_CAPTURE_RESOLVE=<path> is set, every player-side PhysicsEngine.ResolveWithTransition call appends one JSON Lines record:

{
  "tick": 0,
  "timestampMs": 40919993,
  "input": { ... full inputs ... },
  "bodyBefore": { ... full PhysicsBody snapshot ... },
  "result": { ... full ResolveResult ... },
  "bodyAfter": { ... full PhysicsBody snapshot ... }
}

Filtered to IsPlayer mover flag so NPC / remote DR calls don't pollute. Thread-safe writer with per-record flush. Process-exit hook for clean shutdown.

Comparison harness (tests/AcDream.Core.Tests/Physics/CellarUpTrajectoryReplayTests.cs)

Three LiveCompare_* tests + one diagnostic dump:

Test	Outcome	Meaning
LiveCompare_Tick0_Spawn	PASSES	Spawn at Z=92.5333; engine matches live bit-perfect
LiveCompare_Tick376_OnRamp	PASSES	Player on ramp at Z=91.49; ramp walkable polygon hydrates correctly, engine reproduces live
LiveCompare_FirstCap_HarnessMissesCottageFloorBecauseCottageGfxObjNotRegistered	PASSES (documents the bug)	Live cap at Z=92.74 with cn=(0,0,-1); harness does NOT reproduce because cottage GfxObj isn't registered
LiveCompare_FirstCap_DiagnosticDump	PASSES (probe-only)	Prints cell polygons in world frame + enables every probe — captured stdout shows harness BSP query path

The diagnostic dump test runs the cap replay with [poly-dump], [push-back], [indoor-bsp], [step-walk] probes ALL enabled. The captured stdout shows:

[cell-dump] 0xA9B40147 resolved-poly-count=37
  poly id=0x0018 ... worldVerts=[(140.12,11.50,90.95),...(142.10,11.50,90.95)]   ← cellar floor
  poly id=0x0001 ... worldVerts=[(142.10,11.50,93.80),...(140.50,8.70,93.80)]   ← cellar ceiling

[cell-dump] 0xA9B40143 resolved-poly-count=14
  poly id=0x0004 ... worldVerts=[(136.70,3.90,94.00),(140.50,3.90,94.00),(140.50,8.70,94.00)]   ← cottage floor (triangle)
  ... more cottage floor triangles, all at world Z=94.00 ...

[other-cells] primary=0xA9B40147 iter=0xA9B40143 wpos=(141.605,7.097,93.351) result=OK poly=n/a
[other-cells] primary=0xA9B40147 iter=0xA9B40146 wpos=(141.605,7.097,93.351) result=OK poly=n/a

Both other-cells iterations return OK — the cottage floor polys in 0xA9B40143 don't extend to the sphere's XY (X=141.39 > rightmost-vertex X=140.50). So the harness sees no collision, even though the live engine does.

---

How we identified the missing object (it's NOT a cell)

The second capture pass enabled ACDREAM_PROBE_RESOLVE=1, which logs each call's hit details including the entity guid of the blocking object. The cap event prints:

[resolve] ent=0x000F4240 in=(141.605,7.304,92.656) tgt=(141.624,6.875,92.656)
  out=(141.605,7.304,92.656) ok=True groundedIn=True cp=valid
  hit=yes n=(0.00,0.00,-1.00) obj=0xA9B47900 walkable=True

obj=0xA9B47900 is in the landblock-baked static range (0xA9B47XXX guids belong to landblock 0xA9B4's static objects). This is the cottage BUILDING as a GfxObj registered as a ShadowEntry on the landblock — NOT a cottage cell.

The harness's BuildEngineWithCellarFixtures loads three CELL fixtures (0xA9B40143, 0xA9B40146, 0xA9B40147) but does not register any landblock-baked static. There IS a RegisterStairRampGfxObj helper that constructs ONE polygon (the ramp), but it's commented out today.

So the missing apparatus is: register the cottage GfxObj as a ShadowEntry with its FULL polygon table — ramp + walls + floor + ceiling. Once registered, the harness's multi-cell BSP iteration's FindObjCollisions will query the GfxObj's BSP and find the cottage floor polygon's downward-facing plane just like live.

---

The cap geometry (math)

Live capture analysis confirmed the sphere physics:

• Foot sphere center at world Z = foot_z, radius 0.48m
• Head sphere center at world Z = foot_z + sphereHeight = foot_z + 1.2m
• Head sphere top at Z = foot_z + 1.2 + 0.48 = foot_z + 1.68m

Cap point in live capture: foot_z = 92.7390 (from tick 1183). Predicted head sphere position: head center = 93.9390, head top = 94.4190.

The cottage floor is at world Z = 94.0 (from cell 0xA9B40143's poly 0x04 worldVerts: (136.70,3.90,94.00), etc.).

Head sphere center at Z=93.94 is BELOW the cottage floor at Z=94.0 by 0.06. Head sphere top at Z=94.42 is ABOVE the cottage floor by 0.42.

The head sphere PENETRATES the cottage floor. BSP push-back direction is the negative of the polygon's outward normal (which is +Z facing UP), so push-back direction is −Z (pushes sphere DOWN). That matches the live cn=(0,0,-1).

The "exact" cap position: foot_z when head center is at Z=94.0 (just touching). foot_z = 94.0 − 1.2 = 92.80. The observed cap at foot_z=92.74 is ~0.06 below the predicted (push-back includes epsilon and walk-interp adjustments).

---

User's confirming observation

"I noticed a thing. When I jump in the cellar, I'm getting blocked at the same height (I think) as I am when running up the stairs."

This is the key observation that nailed the diagnosis. Jumping is pure vertical motion — no ramp slope, no AdjustOffset projection. If the cap fires on a pure jump, the obstruction must be a horizontal geometric obstacle at the cap height. That immediately rules out every step-up / AdjustOffset hypothesis from the prior 6+6 saga and pinpoints the bug as a head-sphere head-on collision with a cottage-floor polygon facing DOWN.

---

What's NOT yet known

1. Why retail doesn't have this cap. Either:

   • (a) Retail's cottage GfxObj has a HOLE in the floor above the ramp (cottage floor polygons stop at the ramp opening; our dat-read produces a contiguous floor)
   • (b) Retail's BSP query treats single-sided polygons correctly (cottage floor's SidesType allows collision from +Z side only, not from −Z side; we treat it as both-sided)
   • (c) Retail uses portal-aware collision: when the sphere is inside the cellar EnvCell, queries skip polygons that belong to the cottage portal's "other side"

   Need a retail cdb trace at the ramp-top to disambiguate.

2. The cottage GfxObj's full polygon list. We have the ramp polygon (poly 0x0008 in the cottage GfxObj, normal (0,-0.719,0.695)) and we know the floor polygon is at Z=94.0 with normal (0,0,-1) or (0,0,+1). We do NOT have:

   • the full polygon list of GfxObj 0xA9B47900
   • the cottage GfxObj's id, BSP root, or scale/rotation

   These can all be extracted by enabling ACDREAM_PROBE_BUILDING=1 for a future capture — the [resolve-bldg] probe dumps per-poly geometry when a building shadow entry is hit.

3. ACDREAM_PROBE_POLY_DUMP doesn't fire for the cottage hit. The [poly-dump] probe is wired into AdjustSphereToPlane, but the cottage-floor collision goes through FindObjCollisions → BSPQuery.FindCollisions on the GfxObj's internal BSP — a different code path. Future probing should use ACDREAM_PROBE_BUILDING instead to capture the per-object collision details.

---

Next-session pickup

What shipped 2026-05-23 evening v2 (post-prior-section)

Three commits land apparatus convergence on the cap event:

Commit	What
cc3afbc	GfxObj dump infrastructure. Mirrors ACDREAM_DUMP_CELLS: new env var ACDREAM_DUMP_GFXOBJS triggers PhysicsDataCache.CacheGfxObj to write the full resolved polygon table as JSON, suffix .gfxobj.json so dumps don't collide with cell dumps in the same dir. New GfxObjDump DTO + GfxObjDumpSerializer parallel to CellDump; round-trip tests cover Capture / Write / Read / Hydrate; the Hydrate path constructs a synthetic single-leaf BSP for query coverage.
97fec19	Harness reproduces the cottage-floor cap event. BuildEngineWithCellarFixtures now registers a stub landblock 0xA9B40000 (TerrainSurface at z=-1000) so TryGetLandblockContext succeeds at the cellar XY, plus a new RegisterCottageGfxObj helper that loads the dumped cottage GfxObj fixture, hydrates it with synthetic BSP, and registers as a ShadowEntry at world (130.5, 11.5, 94.0) with 180° Z rotation — matching production's GameWindow.cs:5893 registration shape for landblock-baked statics. The cottage fixture (74 polys, 6 downward-facing floor triangles, BSP radius 13.989 m) lives at tests/.../Fixtures/issue98/0x01000A2B.gfxobj.json; capture launch script is launch-a6-issue98-cottage-gfxobj-dump.ps1.

Test outcome at apparatus convergence:

Test	Outcome	Meaning
LiveCompare_Tick0_Spawn	PASS	Spawn round-trip preserved by the new landblock + cottage state
LiveCompare_Tick376_OnRamp	PASS	On-ramp round-trip preserved
LiveCompare_FirstCap_HarnessReproducesCottageFloorCapNormal	PASS (NEW)	Harness reproduces the live cn=(0,0,-1) cap-event normal exactly
LiveCompare_FirstCap_ResidualXMotionDivergence_DocumentsNextInvestigation	PASS (documents-the-bug)	Captures the ONE remaining post-cap divergence: live preserves +0.0266 m of +X motion through the cap (edge-slide along the cottage floor in XY); harness blocks ALL motion. Y and Z agree.

The residual divergence (next investigation target)

After registering the cottage GfxObj:

Live:    cn=(0,0,-1), position=(141.3865, 7.2243, 92.7390)   ← +X motion preserved
Harness: cn=(0,0,-1), position=(141.3599, 7.2243, 92.7390)   ← X stuck at input
Input:   currentPos=(141.3599, 7.2243, 92.7390)
         targetPos =(141.3865, 6.8221, 92.7390)
         requestedDelta=(+0.0266, -0.4022, 0)

The cap-event collision normal matches bit-perfect. Position diverges in X only. Working hypothesis: live's response to a cn=(0,0,-1) head-bump treats it as a Z-only constraint and edge-slides the remaining XY component along the cottage floor; harness's BSP path is rejecting the entire move vector instead of computing a slid offset.

That hypothesis is the next-session investigation target — work the slide path in Transition.transitional_insert / AdjustOffset against the production cap-event call. The new LiveCompare_FirstCap_ResidualXMotionDivergence_DocumentsNextInvestigation test PASSES today (asserting the current residual) and FAILS when the divergence closes — that's the signal to flip it into AssertCallMatchesCapture form.

Alternative pickup move: retail cdb trace at the cottage ramp-top

If apparatus polish is enough and the user wants to widen the question to "how does retail differ?", attach cdb to a running retail acclient (see CLAUDE.md "Retail debugger toolchain"), set breakpoints on BSPTREE::find_collisions and CGfxObj::shadow_find_obj_collisions, walk up the cottage ramp, and log every BSP query against the cottage GfxObj. Compare which polygons retail finds vs which polygons our acdream engine finds. Retail's trace is the ultimate oracle for the "how does retail differ?" question — but the apparatus-side X residual investigation is the more focused, faster-feedback next step.

Pre-existing test flakiness (out of scope but documented)

While verifying the cottage helper, the full dotnet test serial run produced 8–19 failures across 1192 tests depending on order — the suite has static-state leakage between test classes (likely from PhysicsResolveCapture.CapturePath, PhysicsDiagnostics.Probe*Enabled, and similar global mutators). The flakiness is independent of A6.P3: stashing the cottage helper out and rerunning produces the same flaky range. All 21 issue-#98-relevant tests (12 harness + 4 GfxObjDumpRoundTripTests + 1 new PhysicsDiagnosticsTests + 4 CellDumpRoundTripTests) pass deterministically in isolation.

---

Apparatus that exists to use

Tool	Location	Status
PhysicsResolveCapture	src/AcDream.Core/Physics/	Production-ready; env-var gated; off by default
LiveCompare_* tests	tests/.../CellarUpTrajectoryReplayTests.cs	4 tests; 1 documents the bug, 3 are matches
live-capture.jsonl fixture	tests/.../Fixtures/issue98/	3 representative records (spawn, on-ramp, first-cap)
launch-a6-issue98-capture.ps1	worktree root	Capture-enabled launch (no diagnostic probes)
launch-a6-issue98-polydump.ps1	worktree root	Capture + poly-dump + push-back + dump-cells launch
16 cell-dump fixtures	tests/.../Fixtures/issue98/0xA9B4014X.json	All cells in 0xA9B4014X range from second capture
41K-record live capture	a6-issue98-resolve-capture.jsonl (gitignored size)	First capture — full session of cellar movement
70K-record live capture w/ probes	a6-issue98-resolve-capture-2.jsonl	Second capture — included poly-dump events
a6-issue98-polydump-launch.log	worktree root	56K+ line log with [resolve], [poly-dump], [other-cells], [indoor-bsp] events

---

The stale-contact-plane finding — full evidence (2026-05-23 evening v3)

How the question led to the answer

User asked: "We know how retail OPENs it from above, how hard can it be to know how to open it from below?" — the implicit question being "if walking on the cottage floor from above works fine, why doesn't walking up from below?"

That reframed the investigation. The cottage floor is the SAME polygon set whether viewed from above (walking on it) or below (head-bumping it from the cellar). Retail handles both. If our cap fires from below, what's different about our state?

Tracing the harness's LiveCompare_FirstCap_DiagnosticDump output revealed:

1. The contact plane the engine started with: ramp's plane n=(0, 0.7190, 0.6950), d=-69.5035. From the live capture's bodyBefore.contactPlane.

2. Cellar ramp's actual world position: vertices computed from the cellar cell's fixture put the ramp at world X∈[129.7, 131.3], Y∈[10.19, 13.09], Z∈[92.5, 95.5]. The ramp is in the +Y corner of the cellar, ~1.6 m wide.

3. Player position at cap: world (141.5, 7.22, 92.74). 10+ m away from the ramp in X.

4. The +Z drift math: AdjustOffset projects the requested motion onto the plane perpendicular to the contact-plane normal:

   • requested = (+0.0266, -0.4022, 0)
   • dot(requested, ramp normal) = 0·0.0266 + 0.719·(-0.4022) + 0.695·0 = -0.2892
   • projected = requested - (-0.2892)·rampNormal = (+0.0266, -0.1943, +0.2010)
   • +0.2010 m of Z gain per tick, applied because the contact plane the engine believes the player is on is the slope.

5. The cap math: foot Z at cap = 92.74. Head sphere center at foot Z + sphereHeight 1.2 = 93.94. Head sphere top at foot Z + 1.68 = 94.42. Cottage floor at world Z=94.00. Head sphere top exceeds cottage floor by 0.42 m → cap fires from below.

If the contact plane were the flat cellar floor (n=(0,0,1) at Z=90.95) instead of the ramp, AdjustOffset's projection would produce zero Z gain (requested motion has no Z component, projection onto flat-floor plane preserves XY). No drift, no cap.

Why this fits the user-facing bug

• "Stuck climbing cellar" — the player walks forward, accumulates Z, bumps cottage floor, can't progress. Matches what the user sees.
• "Pure jump in cellar caps at same Z" — jumping doesn't refresh the contact plane either. Drift continues. Matches.
• "Six prior fix attempts failed" — all attempted to fix the CAP mechanics (step-up, slope projection at the cap, edge-slide). None questioned why the contact plane was the ramp at all.

What still needs verification (next session's task)

1. Chronological evidence: walk the live capture from the start of the cellar session. When did the player last stand on the actual ramp? Does bodyBefore.contactPlane persist as the ramp's plane across many ticks of horizontal walking? Quantify the cumulative Z drift.

2. The walkable-refresh gap: where in Transition.FindEnvCollisions / SpherePath.SetWalkable / related is the contact plane supposed to be refreshed when the sphere is over a different walkable polygon? Retail's CObjCell::find_env_collisions is the decomp anchor — find the path that detects a NEW walkable and overwrites the contact plane, and find where our engine skips that.

3. Retail cdb cross-check (optional, definitive): attach cdb to a running retail acclient, walk to a cottage cellar, log the contact plane each tick. If retail's contact plane refreshes to (0,0,1) when the player walks off the ramp, hypothesis confirmed.

---

Pickup prompt for next session

A6.P3 #98 — apparatus convergence landed, NEW root-cause hypothesis
(stale ramp contact plane) needs verification.

Read FIRST (in order, ~15 min):
  1. docs/research/2026-05-23-a6-p3-issue98-comparison-harness-findings.md
     — start with TL;DR (evening v3 update at top), then the section
     "The stale-contact-plane finding — full evidence" near the bottom.
     Skip the middle sections (evening v1 + v2 arcs) unless context is
     needed.
  2. CLAUDE.md "Current A6 phase" block — look for the "Evening v3
     finding" paragraph.
  3. tests/AcDream.Core.Tests/Physics/CellarUpTrajectoryReplayTests.cs
     — the RegisterCottageGfxObj helper + 2 LiveCompare_FirstCap_*
     tests are what you'll iterate against.

State both altitudes (one sentence each):
  Currently working toward: M1.5 — Indoor world feels right
  Current phase: A6.P3 — apparatus convergence shipped (cap event
  reproduces bit-perfect). New root-cause hypothesis: stale ramp
  contact plane causes per-tick Z drift that makes the cap reachable.
  Needs verification.

What was shipped today (3 commits — DO NOT REDO):
  - cc3afbc: GfxObj dump infrastructure (ACDREAM_DUMP_GFXOBJS)
  - 97fec19: Harness reproduces cottage-floor cap (RegisterCottageGfxObj)
  - 7729bdc + (this commit): findings doc + CLAUDE.md updates

The hypothesis with full math:
  - Body's contact plane = ramp's plane (n=(0,0.719,0.695), d=-69.5035)
  - Player position at cap = world (141.5, 7.22, 92.74)
  - Cellar ramp's actual world XY = X∈[129.7, 131.3] — 10m from player
  - AdjustOffset projects requested move along contact-plane perpendicular
  - Per-tick Z gain ≈ 0.201m from slope projection on STALE ramp plane
  - Accumulates over ticks → head sphere reaches Z=94 → bumps cottage
    floor → cap fires
  - If contact plane refreshed to flat cellar floor (n=(0,0,1)) when
    player walks off ramp, no Z drift, no cap

Concrete next moves (in order):

(1) **Verify the hypothesis chronologically.** Walk
    a6-issue98-resolve-capture-2.jsonl (or the cottage capture
    fixture's full file) from the start. Find when the player last
    stood on the actual ramp (within world X∈[129.7, 131.3], Y∈[10.19,
    13.09]). Quantify: how many ticks does the body's contact plane
    persist as the ramp's plane while the player walks horizontally
    away? Compute the cumulative Z drift. Should match observed Z=92.74
    at cap if the hypothesis holds. (Probably 30 min PowerShell jq.)

(2) **Locate the walkable-refresh code path.** In
    src/AcDream.Core/Physics/TransitionTypes.cs, search for where
    Transition.FindEnvCollisions or SpherePath.SetWalkable is supposed
    to detect a new walkable polygon under the sphere and overwrite
    the contact plane. The fix likely lives at the call site that
    EITHER fails to fire OR fires but doesn't replace the existing
    contact plane.

(3) **Cross-ref retail decomp.** acclient_2013_pseudo_c.txt's
    CObjCell::find_env_collisions + the walkable-detection chain.
    Find the path where retail unconditionally replaces
    contact_plane when a new walkable is found. Quote the line
    numbers in the fix commit.

(4) **Implement the fix + verify against harness.** The harness's
    LiveCompare_FirstCap_HarnessReproducesCottageFloorCapNormal test
    currently PASSES asserting the cap reproduces. After the fix,
    if the contact plane refreshes correctly, the cap should NOT fire
    (no Z drift to make it reachable). The test should start FAILING
    — that's the signal the fix works.

(5) **Visual verification (user-side).** Launch acdream live, walk
    into a Holtburg cottage, down to the cellar, then back up. The
    user-facing bug should resolve if the hypothesis is correct.

Decomp grep targets:
  - CObjCell::find_env_collisions
  - CPhysicsObj::find_object_collisions
  - CTransition::find_walkable
  - CSpherePath::set_walkable / walkable_hits_sphere
  - OBJECTINFO::object → contact_plane writes

CLAUDE.md rules apply throughout:
  - NO speculative fixes — the saga's converted to evidence-driven.
    Verify hypothesis with chronological capture BEFORE coding.
  - Visual verification belongs to the user.
  - If the chronological verification (step 1) shows the contact
    plane is NOT actually stale across many ticks, the hypothesis is
    wrong — pivot to retail cdb trace (definitive oracle).

Out-of-scope but observed: pre-existing test suite has 8–19 failures
across runs of the same code due to static-state leakage between test
classes (PhysicsResolveCapture, PhysicsDiagnostics statics). Targeted
issue-#98 tests pass deterministically in isolation. Don't touch the
flakiness this session; it's a separate investigation.

Test baseline: harness's 12 CellarUpTrajectoryReplayTests + 4
GfxObjDumpRoundTripTests + 1 new PhysicsDiagnosticsTests + 4
CellDumpRoundTripTests all pass in isolation. Maintain.

Test baseline: 1178 + 8 pre-existing failures (serial run).
Maintain throughout. The previously-failing
LiveCompare_FirstCap_HarnessMissesCottageFloorBecauseCottageGfxObjNotRegistered
test is now in documents-the-bug form (PASSES while bug exists; FAILS
when fix lands) — flip it when the cottage GfxObj is registered.

---

Resolution 2026-05-24

What was wrong with the evening-v3 hypothesis

The v3 "stale ramp contact plane" hypothesis (top of this doc) was FALSIFIED by chronological walk of a6-issue98-resolve-capture-2.jsonl:

• Player position at the first cap event (tick 55101, line 55102 of the JSONL): world (141.605, 7.304, 92.656)
• bodyBefore.walkableVertices: the ramp polygon at world X∈[140.5, 142.1], Y∈[5.80, 8.70], Z∈[90.99, 93.99]
• Player XY is inside the ramp polygon's footprint
• bodyBefore.contactPlane.normal = (0, 0.7189884, 0.69502217) — the ramp's plane

The v3 doc claimed "ramp at world X∈[129.7, 131.3], 10m away from player." That geometry was computed from a wrong source (not the actual ramp polygon). The live capture's walkableVertices are the ground truth and show the player IS on the ramp at the cap event. The contact plane is the ramp's plane because the player is on the ramp — correct, not stale.

Tick 55020 (line 55021) shows the contact plane refreshing in real time as the player crossed onto the ramp: bodyBefore had the previous polygon's plane, bodyAfter had the ramp's plane. The walkable-refresh chain works. No drift mechanism exists in the way v3 described.

What the actual mechanism was

The evening-v2 finding was correct: head-sphere bumps the cottage GfxObj's downward-facing floor poly (poly 0 in the GfxObj fixture, a triangle covering world X∈[136.3, 142.5], Y∈[3.5, 19.5], Z=94) from below. Player at (141.605, 7.304) is inside that triangle. Head sphere top at Z=foot+1.68=94.336 penetrates the cottage floor at Z=94 by 0.336m → cn=(0,0,-1) push-back → stuck.

Why retail doesn't have this cap: decomp grep of CObjCell::find_obj_collisions (line 308916) shows retail iterates this->shadow_object_list — a per-cell list. CObjCell::find_cell_list (line 308742) branches indoor/outdoor at registration time: indoor adds only the indoor cell + portal-visible neighbors; outdoor adds all overlapping outdoor cells via add_all_outside_cells. So a landblock- baked static like the cottage gets added to outdoor cells' shadow_object_list only — never to indoor EnvCells like the cellar. CEnvCell::find_collisions therefore never tests the sphere against the cottage when sphere is inside the cellar.

sides_type (the polygon flag the v2 finding option (b) speculated about) does NOT affect retail's BSP collision code — it only appears in rendering/mesh-batch code. The collision-path divergence is purely architectural: per-cell list vs spatial-radius registry.

What shipped (commit b3ce505)

Smallest behavioral patch matching retail's effect at the query level:

• ShadowObjectRegistry.GetNearbyObjects gained an optional primaryCellId parameter. When indoor (≥ 0x0100), the outdoor radial sweep is skipped — only indoor-scoped shadows from indoorCellIds are returned.
• Transition.FindObjCollisions passes sp.CheckCellId.
• Harness LiveCompare_FirstCap_HarnessReproducesCottageFloorCapNormal flipped to LiveCompare_FirstCap_FixClosesCottageFloorCap — asserts the downward-facing cottage-floor cap does NOT fire after the fix.
• Residual-X-motion test deleted — it documented post-cap edge-slide, irrelevant once the cap is gone.

Verified: 11/11 cellar harness tests pass. 55 directly-affected physics tests pass. Pre-existing static-state leakage failures (8–19 across serial runs) unchanged. Full dotnet build clean.

Visual verification: user confirmed "Finally I can go up!" in the Holtburg cottage cellar.

Known regression caused by b3ce505 + next phase

Doorway edge case (flagged in the commit message): doors are server- spawned entities with their own cylinder collision, registered via UpdatePosition to whichever cell their position resolves to. Doors at building thresholds typically resolve to outdoor cells. With the indoor-primary radial-sweep gate, a sphere inside an indoor doorway- adjacent cell doesn't see the outdoor door → can walk through.

User reported this: "I can also run through doors."

This regression is the direct consequence of NOT doing retail's full portal-aware shadow propagation at registration time. Retail's find_cell_list indoor branch recurses through VisibleCellIds and adds the object to all portal-visible cells. Our Register doesn't do this; the b3ce505 stopgap covers cottage-cellar but not doorways.

Next phase: A6.P4 — port retail's per-cell shadow_object_list architecture in full. Design spec at docs/superpowers/specs/2026-05-24-phase-a6-p4-retail-shadow-architecture.md (this session). Approach: refactor ShadowObjectRegistry.Register to compute the cell set via the retail-faithful indoor/outdoor branch + portal-visible recursion (using CellPhysics.VisibleCellIds). Eliminate the cellScope=0 spatial approximation. GetNearbyObjects becomes pure per-cell list iteration. Removes the b3ce505 stopgap. Closes the door regression as a side effect.

Also-likely-closed by A6.P4: #97 (phantom collisions on 2nd floor), indoor sling-out (Finding 3 family), other indoor/outdoor seam bugs.

Memory updates (this resolution)

• feedback_retail_per_cell_shadow_list.md — the architectural lesson
• feedback_apparatus_for_physics_bugs.md — the apparatus pattern that finally cracked this saga (template for future physics bugs)