diff --git a/.changeset/prototype-shadow-dom-isolation.md b/.changeset/prototype-shadow-dom-isolation.md
new file mode 100644
index 00000000..1bd59695
--- /dev/null
+++ b/.changeset/prototype-shadow-dom-isolation.md
@@ -0,0 +1,7 @@
+---
+'@youversion/platform-core': major
+'@youversion/platform-react-hooks': major
+'@youversion/platform-react-ui': major
+---
+
+Prototype automatic Shadow DOM style isolation on `YouVersionAuthButton`, plus a shadow-local native top-layer strategy for isolated Popovers. The spike also constrains Popovers to Radix's available collision height.
diff --git a/.changeset/shadow-aware-verse-actions.md b/.changeset/shadow-aware-verse-actions.md
new file mode 100644
index 00000000..cd961cbd
--- /dev/null
+++ b/.changeset/shadow-aware-verse-actions.md
@@ -0,0 +1,5 @@
+---
+'@youversion/platform-react-ui': patch
+---
+
+Keep verse action popovers in their component's shadow-local portal and restore focus after dismissal.
diff --git a/.changeset/tidy-shadows-reset.md b/.changeset/tidy-shadows-reset.md
new file mode 100644
index 00000000..69a5c445
--- /dev/null
+++ b/.changeset/tidy-shadows-reset.md
@@ -0,0 +1,5 @@
+---
+'@youversion/platform-react-ui': patch
+---
+
+Harden Shadow DOM style isolation so only text direction crosses the boundary and host custom properties cannot alter known SDK spacing or radius values.
diff --git a/.changeset/validate-shadow-dialog.md b/.changeset/validate-shadow-dialog.md
new file mode 100644
index 00000000..28f500ca
--- /dev/null
+++ b/.changeset/validate-shadow-dialog.md
@@ -0,0 +1,5 @@
+---
+'@youversion/platform-react-ui': patch
+---
+
+Validate shadow-local top-layer Dialog behavior, including focus containment and modal background inertness.
diff --git a/docs/adr/0006-prototype-shadow-dom-style-isolation.md b/docs/adr/0006-prototype-shadow-dom-style-isolation.md
new file mode 100644
index 00000000..76835e91
--- /dev/null
+++ b/docs/adr/0006-prototype-shadow-dom-style-isolation.md
@@ -0,0 +1,93 @@
+# ADR 0006: Prototype automatic Shadow DOM style isolation
+
+Status: Proposed (validated experimentally; not approved for production rollout)
+
+Host applications can apply unlayered global CSS, including Tailwind preflight,
+that outranks the UI package's layered styles. Resets, stronger selectors,
+`!important`, cascade layers, and `@scope` remain part of the host document's
+cascade and cannot prevent outside selectors from matching SDK internals. The
+prototype therefore uses Shadow DOM as the browser-enforced style boundary.
+
+## Decision for the prototype
+
+`YouVersionAuthButton` automatically creates an open shadow root and renders its
+existing implementation into it through a React portal. Consumers continue to
+use the same component API; they do not need to discover or enable isolation.
+The SDK's compiled Tailwind CSS is installed inside the root, the light-DOM host
+receives a protected box reset, and an internal wrapper resets inherited visual
+properties.
+
+Writing direction is the only intentional inherited visual input: both reset
+boundaries explicitly preserve `direction`, while `all: initial` restores
+horizontal writing, mixed text orientation, SDK typography, and other visual
+properties. Vertical host writing modes and host typography are unsupported.
+Known ambient custom-property dependencies are closed by using SDK-owned
+`--yv-spacing` and `--yv-radius` values and by defining a local `--spacing`
+compatibility alias for `tw-animate-css`. YPE-5400 owns the full custom-property
+inventory and a compiled-CSS prevention guard.
+
+Constructable stylesheets are cached per owning `Document`, because a sheet from
+the top-level document cannot be adopted into a same-origin iframe's shadow
+root. Environments without constructable stylesheets receive a `
+ ))}
+
+
+
Automatic Shadow DOM isolation POC
+
+ This branch automatically isolates only YouVersionAuthButton. The plain host
+ controls are positive witnesses: they should look broken when an attack is active, while
+ the SDK button should remain stable. The font-face option demonstrates a known Shadow DOM
+ limitation.
+
+
+
+
+
+
+
+
LIGHT DOM — SHOULD BE AFFECTED
+
+
+
Plain host text for inherited-property attacks.
+
+ Host-box witness — this should disappear during the host attack.
+
+
+ Pseudo-element witness — generated content should appear above this text.
+
+
+ Host text requesting Inter for the font-face collision.
+
+
+
+
+
+
SDK POC — SHOULD RESIST
+
+ console.error('Auth error:', error)}
+ />
+
+
+ Other SDK components are intentionally absent: automatic isolation has not been rolled
+ out to them on this POC branch.
+
+
+
+
+ );
+}
diff --git a/packages/ui/AGENTS.md b/packages/ui/AGENTS.md
index ada2ee13..06c4549d 100644
--- a/packages/ui/AGENTS.md
+++ b/packages/ui/AGENTS.md
@@ -38,10 +38,20 @@ export — treat those two as public API and breaking-change territory.
## STYLING
**React 19 `
+ ) : null}
+ {/* Host selectors cannot reach this reset boundary. */}
+