diff --git a/docs/superpowers/specs/2026-07-05-modern-pipeline-design.md b/docs/superpowers/specs/2026-07-05-modern-pipeline-design.md
index fb0d3814..c8978b43 100644
--- a/docs/superpowers/specs/2026-07-05-modern-pipeline-design.md
+++ b/docs/superpowers/specs/2026-07-05-modern-pipeline-design.md
@@ -214,6 +214,20 @@ listed follow-up, not v1.
   class gone; user visual gate (no missing/wrong geometry); legacy decode path deleted
   from the streaming hot path in the gate commit.
 
+### 6.6 MP1c design note — EnvCell keying (recorded 2026-07-05, MP1b review finding)
+
+The runtime deduplicates interior-cell geometry under a **content hash**
+(`EnvCellRenderer.GetEnvCellGeomId(environmentId, cellStructure, surfaces)`,
+bit 33 set — verbatim WB), NOT under the cell's dat fileId; many cells share
+one geometry. The pak keys EnvCell blobs by cell fileId. MP1c's cutover
+therefore maps at the existing seam: `RegisterCell` has the concrete cell in
+hand when it wants geometry for a geomId — read `pak[cellFileId]`, register
+under the geomId, and post-load dedup continues exactly as today. Cost:
+cells sharing geometry duplicate blob content on disk (v1-acceptable;
+a one-blob-per-unique-content TOC-alias refinement is a future format
+option). Baking under geomId keys directly is NOT viable in format v1 —
+the hash is a full 64-bit value and does not fit PakKey's 56 usable bits.
+
 ## 7. MP2 — Distance-degrade port (hide-only first cut)
 
 The already-scoped retail port from the 2026-06-23 handoff §3 (read it + the named