diff --git a/crates/originweave-core/src/attached_tab_assurance.rs b/crates/originweave-core/src/attached_tab_assurance.rs
new file mode 100644
index 000000000..4d2cd5954
--- /dev/null
+++ b/crates/originweave-core/src/attached_tab_assurance.rs
@@ -0,0 +1,50 @@
+/// Browser control surface represented by assurance evidence.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum BrowserAttachmentKind {
+    /// OriginWeave is attached to an existing person-controlled browser tab.
+    AttachedHumanTab,
+    /// OriginWeave operates in a task-isolated browser profile.
+    IsolatedProfile,
+}
+
+/// Trusted adapter evidence about extension influence on page state.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum ExtensionInfluenceEvidence {
+    /// The trusted adapter established that an extension can influence page state.
+    CanInfluencePageState,
+    /// This bounded rule has no trusted evidence of extension influence.
+    ///
+    /// Absence of known influence is not proof that extensions are absent or unable
+    /// to interfere.
+    NoKnownExtensionInfluence,
+}
+
+/// A specific reason that one browser context has reduced assurance.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum ReducedAssuranceReason {
+    /// An attached human tab can be influenced by an existing browser extension.
+    AttachedTabExtensionInfluence,
+}
+
+/// Classify the narrow attached-tab extension-influence assurance reduction.
+///
+/// `None` means only that this specific rule found no such reduction; it is not
+/// proof of full trust, extension absence, inability to interfere, or high
+/// assurance.
+#[must_use]
+pub const fn classify_reduced_assurance(
+    attachment: BrowserAttachmentKind,
+    extension_influence: ExtensionInfluenceEvidence,
+) -> Option<ReducedAssuranceReason> {
+    if matches!(
+        (attachment, extension_influence),
+        (
+            BrowserAttachmentKind::AttachedHumanTab,
+            ExtensionInfluenceEvidence::CanInfluencePageState
+        )
+    ) {
+        Some(ReducedAssuranceReason::AttachedTabExtensionInfluence)
+    } else {
+        None
+    }
+}
diff --git a/crates/originweave-core/src/root.rs b/crates/originweave-core/src/root.rs
index c47a136d4..12f4c1828 100644
--- a/crates/originweave-core/src/root.rs
+++ b/crates/originweave-core/src/root.rs
@@ -11,6 +11,9 @@ mod contracts;
 
 pub use contracts::*;
 
+mod attached_tab_assurance;
+pub use attached_tab_assurance::*;
+
 /// Stateless MCP routing validation that maps only explicit tools to typed actions.
 pub mod mcp;
 /// Deterministic fail-closed release benchmark acceptance aggregation.
diff --git a/crates/originweave-core/tests/attached_tab_assurance.rs b/crates/originweave-core/tests/attached_tab_assurance.rs
new file mode 100644
index 000000000..a9bb52cc6
--- /dev/null
+++ b/crates/originweave-core/tests/attached_tab_assurance.rs
@@ -0,0 +1,37 @@
+use originweave_core::{
+    BrowserAttachmentKind, ExtensionInfluenceEvidence, ReducedAssuranceReason,
+    classify_reduced_assurance,
+};
+
+#[test]
+fn attached_human_tab_with_extension_influence_is_explicitly_reduced_assurance() {
+    assert_eq!(
+        classify_reduced_assurance(
+            BrowserAttachmentKind::AttachedHumanTab,
+            ExtensionInfluenceEvidence::CanInfluencePageState,
+        ),
+        Some(ReducedAssuranceReason::AttachedTabExtensionInfluence)
+    );
+}
+
+#[test]
+fn attached_human_tab_without_known_extension_influence_has_no_extension_reduction() {
+    assert_eq!(
+        classify_reduced_assurance(
+            BrowserAttachmentKind::AttachedHumanTab,
+            ExtensionInfluenceEvidence::NoKnownExtensionInfluence,
+        ),
+        None
+    );
+}
+
+#[test]
+fn isolated_profile_is_not_relabelled_by_attached_tab_rule() {
+    assert_eq!(
+        classify_reduced_assurance(
+            BrowserAttachmentKind::IsolatedProfile,
+            ExtensionInfluenceEvidence::CanInfluencePageState,
+        ),
+        None
+    );
+}