diff --git a/CHANGELOG.md b/CHANGELOG.md
index 6e4bd39c..569bb0cf 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,7 @@ All notable changes to OriginWeave are documented in this file. The format follo
 
 ### Added
 
+- Explicit reduced-assurance classification for attached human tabs when trusted adapter evidence says an existing extension can influence page state; the narrow rule does not detect extensions, prove extension absence, grant Agent authority, or turn an unclassified context into high-assurance evidence.
 - Rust workspace for independently reusable core, policy, destination, network, TLS, resource, and evidence modules.
 - Canonical HTTPS and loopback-origin boundary with case-normalized schemes and hosts, default-port normalization, IPv4/IPv6 handling, browser-special numeric-host rejection, and explicit malformed-input errors.
 - Typed browser actions, capabilities, risk classes, execution modes, robots decisions, secret-delivery contracts, immutable canonical action-intent digests, and intent-bound approval scopes.
diff --git a/crates/originweave-core/src/lib.rs b/crates/originweave-core/src/lib.rs
index 75303407..de817f13 100644
--- a/crates/originweave-core/src/lib.rs
+++ b/crates/originweave-core/src/lib.rs
@@ -1181,3 +1181,53 @@ pub fn evaluate_native_messaging_access(
     }
     NativeMessagingAccessDecision::Allow
 }
+
+/// 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 rule did not identify this specific reduction. It
+/// is not evidence of full trust, extension absence, or high assurance. A future
+/// trusted Chromium adapter must supply the attachment and influence evidence and
+/// evaluate any other applicable assurance rules separately.
+#[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/tests/attached_tab_assurance.rs b/crates/originweave-core/tests/attached_tab_assurance.rs
new file mode 100644
index 00000000..a9bb52cc
--- /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
+    );
+}