diff --git a/docs/ISSUES.md b/docs/ISSUES.md
index 91fe17b..a9eb292 100644
--- a/docs/ISSUES.md
+++ b/docs/ISSUES.md
@@ -48,6 +48,77 @@ Copy this block when adding a new issue:
 
 ---
 
+## #87 — Drop WB fork patch by switching to PrepareEnvCellGeomMeshDataAsync
+
+**Status:** OPEN
+**Severity:** MEDIUM (band-aid removal; not user-visible)
+**Filed:** 2026-05-19
+**Component:** rendering, WB integration
+
+**Description:** Phase 2 (2026-05-19) shipped a one-line patch in our
+WB fork at `references/WorldBuilder/Chorizite.OpenGLSDLBackend/Lib/ObjectMeshManager.cs:1230`
+(branch `acdream` on the fork, SHA `34460c4`) to guard a blind
+`TryGet<Setup>(stab.Id, ...)` call against GfxObj-prefixed ids. That
+patch fixes the symptom (missing floors) but is structurally a
+band-aid — per CLAUDE.md's no-workarounds rule we should retire it.
+
+The proper fix: switch our EnvCell rendering from
+`PrepareMeshDataAsync(envCellId, ...)` (general-purpose entry that
+also iterates static-object parts + emitters we don't need) to WB's
+narrower `PrepareEnvCellGeomMeshDataAsync(geomId, environmentId, cellStructure, surfaces)`
+at [`ObjectMeshManager.cs:386`](../references/WorldBuilder/Chorizite.OpenGLSDLBackend/Lib/ObjectMeshManager.cs:386).
+That function only builds the cell room mesh (floor / walls / ceiling),
+which is the only piece we actually use from WB for cells — we already
+hydrate static objects as separate `WorldEntity` instances in
+`BuildInteriorEntitiesForStreaming`, and we run particle scripts via
+our own `EntityScriptActivator` (Phase C.1.5b).
+
+**Root cause / status:** Misuse of WB's general-purpose API for a
+geometry-only need. The general-purpose path triggers static-object
+iteration that has a bug (TryGet<Setup> without type check) AND that
+does work we throw away. Both problems disappear if we use the
+geometry-only entry point WB already exposes for exactly this purpose
+(it's what WB's own `EnvCellRenderManager` uses internally).
+
+**Trade-offs:**
+
+| | Current (patched WB) | Switch to geom-only API |
+|---|---|---|
+| WB fork divergence | One-line patch | Zero |
+| Future WB upstream merges | Conflicts | Clean |
+| Performance | Slightly worse (wasted iteration) | Slightly better |
+| Risk to other functionality | None (working today) | Needs re-verification |
+
+**Files (the change):**
+
+- `src/AcDream.App/Rendering/GameWindow.cs` around line 5367-5378
+  (cell-entity hydration — change `MeshRefs[0].GfxObjId` from `envCellId`
+  to `envCellId | 0x100000000UL`, the synthetic geom id with bit 32 set).
+- `src/AcDream.App/Rendering/Wb/WbMeshAdapter.cs` — add a new method
+  `PrepareEnvCellGeomMesh(ulong geomId, uint environmentId, ushort cellStructure, List<ushort> surfaces)`
+  that forwards to `_meshManager.PrepareEnvCellGeomMeshDataAsync(...)`,
+  and call it from the streaming path instead of the bare
+  `IncrementRefCount(envCellId)`.
+- `references/WorldBuilder/Chorizite.OpenGLSDLBackend/Lib/ObjectMeshManager.cs:1230`
+  — revert the type-check guard we added. The function returns to
+  pristine WB state.
+
+**Acceptance:**
+
+- Floors still render in Holtburg Inn (regression check vs Phase 2).
+- `references/WorldBuilder` submodule pointer returns to upstream-clean
+  (no acdream-specific commits in the fork's `acdream` branch — or
+  rather, the `acdream` branch fast-forwards back to match upstream's
+  state for this file).
+- Probe re-capture at Holtburg confirms `[indoor-upload] completed` for
+  all cells previously failing.
+- No `[wb-error]` lines.
+
+**Research:** [`docs/research/2026-05-19-indoor-cell-rendering-cause.md`](research/2026-05-19-indoor-cell-rendering-cause.md)
+documents the underlying WB bug.
+
+---
+
 ## Indoor walking issue cluster (2026-05-19)
 
 The Phase 2 indoor cell rendering fix (floor now renders inside buildings)
diff --git a/docs/research/2026-05-19-indoor-followup-handoff.md b/docs/research/2026-05-19-indoor-followup-handoff.md
index 2027c06..096ab00 100644
--- a/docs/research/2026-05-19-indoor-followup-handoff.md
+++ b/docs/research/2026-05-19-indoor-followup-handoff.md
@@ -28,7 +28,7 @@ The `WbMeshAdapter` also now injects a `ConsoleErrorLogger<ObjectMeshManager>` (
 
 - Indoor ambient color is now retail-faithful `(0.20, 0.20, 0.20)` — was guessed `(0.10, 0.09, 0.08)`.
 - Indoor lighting triggers off **player** cell, not camera cell — fixes "darker when camera enters" with third-person chase.
-- WB submodule patched (`ObjectMeshManager.cs:1230` Setup-prefix guard at `TryGet<Setup>`). Lives on `eriknihlen/WorldBuilder@acdream` at SHA `34460c4`. Submodule pointer in acdream's `main` advanced.
+- WB submodule has a **one-line band-aid patch** (`ObjectMeshManager.cs:1230` Setup-prefix guard at `TryGet<Setup>`) on `eriknihlen/WorldBuilder@acdream` at SHA `34460c4`. Submodule pointer in acdream's `main` is advanced. **This is a band-aid** — see `docs/ISSUES.md` #87 for the proper fix (switch to WB's narrower `PrepareEnvCellGeomMeshDataAsync` API). Retire the patch when that issue lands.
 
 ---