diff --git a/CHANGELOG.md b/CHANGELOG.md
index 1093841d4..8e84819b5 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
 
+- Time-bounded managed extension admission for isolated Agent Task profiles with an explicit half-open validity window and caller-supplied trusted evaluation time; invalid, not-yet-valid, and expired policy states fail closed before exact extension allow-list membership is considered.
 - Fail-closed managed extension admission for isolated Agent Task profiles: an empty policy admits no extension, only exact canonical `ExtensionId` allow-list membership is accepted, duplicate entries cannot widen authority, and successful profile admission remains separate from `ExtensionAgentGrant` capability.
 - 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.
 - Bound explicit extension-to-Agent grants to exclusive trusted-time expiry in addition to extension identity, session, browsing context, and canonical origin, so a same-origin grant cannot be reused at or after the deadline.
diff --git a/crates/originweave-policy/src/lib.rs b/crates/originweave-policy/src/lib.rs
index 3dda94b02..2d2d8f687 100644
--- a/crates/originweave-policy/src/lib.rs
+++ b/crates/originweave-policy/src/lib.rs
@@ -26,6 +26,9 @@ use originweave_core::{
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct AgentTaskExtensionPolicy {
     managed_extensions: BTreeSet<ExtensionId>,
+    valid_from: u64,
+    valid_until: u64,
+    maximum_window: u64,
 }
 
 impl AgentTaskExtensionPolicy {
@@ -33,13 +36,28 @@ impl AgentTaskExtensionPolicy {
     ///
     /// Duplicate identifiers collapse to one exact managed identity. An empty
     /// iterator therefore represents the default policy that admits no extension.
+    /// `valid_from` is inclusive and `valid_until` is exclusive. `maximum_window`
+    /// is the reviewed local ceiling for that interval. All three values are opaque
+    /// timestamps or durations in the same caller-defined trusted time domain and
+    /// units supplied to [`evaluate_agent_task_extension`]. This constructor does
+    /// not authenticate policy provenance or attest a clock; an invalid, empty, or
+    /// overlong validity window is retained so evaluation can fail closed
+    /// deterministically.
     #[must_use]
-    pub fn new<I>(managed_extensions: I) -> Self
+    pub fn new<I>(
+        managed_extensions: I,
+        valid_from: u64,
+        valid_until: u64,
+        maximum_window: u64,
+    ) -> Self
     where
         I: IntoIterator<Item = ExtensionId>,
     {
         Self {
             managed_extensions: managed_extensions.into_iter().collect(),
+            valid_from,
+            valid_until,
+            maximum_window,
         }
     }
 }
@@ -51,19 +69,45 @@ pub enum AgentTaskExtensionDecision {
     AllowManagedExtension,
     /// The extension identity is absent from the managed allow-list.
     DenyNotManaged,
+    /// The configured validity window is empty/reversed or its local ceiling is zero.
+    DenyInvalidPolicyWindow,
+    /// The configured policy validity interval exceeds the reviewed local maximum.
+    DenyPolicyWindowExceedsMaximum,
+    /// The trusted evaluation time precedes the policy validity window.
+    DenyPolicyNotYetValid,
+    /// The trusted evaluation time is at or beyond the policy expiry boundary.
+    DenyPolicyExpired,
 }
 
 /// Evaluate extension admission without minting OriginWeave Agent capability.
 ///
 /// This pure boundary answers only whether the exact canonical extension may be
-/// present in the caller's managed Agent Task profile. Chromium permissions,
-/// installation state, native messaging, and [`originweave_core::ExtensionAgentGrant`]
-/// remain separate authorities.
+/// present in the caller's managed Agent Task profile at `trusted_time`.
+/// `trusted_time`, [`AgentTaskExtensionPolicy::new`] `valid_from`, `valid_until`,
+/// and `maximum_window` must use one caller-attested time domain and compatible
+/// units; this function does not read or attest a clock. The validity window is
+/// half-open (`valid_from <= trusted_time < valid_until`) and must not exceed the
+/// reviewed local maximum. Chromium permissions, installation state, native
+/// messaging, and [`originweave_core::ExtensionAgentGrant`] remain separate
+/// authorities.
 #[must_use]
 pub fn evaluate_agent_task_extension(
     extension_id: &ExtensionId,
     policy: &AgentTaskExtensionPolicy,
+    trusted_time: u64,
 ) -> AgentTaskExtensionDecision {
+    if policy.valid_from >= policy.valid_until || policy.maximum_window == 0 {
+        return AgentTaskExtensionDecision::DenyInvalidPolicyWindow;
+    }
+    if policy.valid_until - policy.valid_from > policy.maximum_window {
+        return AgentTaskExtensionDecision::DenyPolicyWindowExceedsMaximum;
+    }
+    if trusted_time < policy.valid_from {
+        return AgentTaskExtensionDecision::DenyPolicyNotYetValid;
+    }
+    if trusted_time >= policy.valid_until {
+        return AgentTaskExtensionDecision::DenyPolicyExpired;
+    }
     if policy.managed_extensions.contains(extension_id) {
         AgentTaskExtensionDecision::AllowManagedExtension
     } else {
diff --git a/crates/originweave-policy/tests/agent_task_extension_policy.rs b/crates/originweave-policy/tests/agent_task_extension_policy.rs
index 3549560ec..fae16ade5 100644
--- a/crates/originweave-policy/tests/agent_task_extension_policy.rs
+++ b/crates/originweave-policy/tests/agent_task_extension_policy.rs
@@ -26,11 +26,11 @@ fn origin() -> Origin {
 
 #[test]
 fn empty_agent_task_extension_policy_denies_every_extension() {
-    let policy = AgentTaskExtensionPolicy::new([]);
+    let policy = AgentTaskExtensionPolicy::new([], 10, 20, 10);
     let extension = extension_id("abcdefghijklmnopabcdefghijklmnop");
 
     assert_eq!(
-        evaluate_agent_task_extension(&extension, &policy),
+        evaluate_agent_task_extension(&extension, &policy, 15),
         AgentTaskExtensionDecision::DenyNotManaged
     );
 }
@@ -39,25 +39,96 @@ fn empty_agent_task_extension_policy_denies_every_extension() {
 fn managed_agent_task_extension_policy_allows_only_exact_identifiers() {
     let allowed = extension_id("abcdefghijklmnopabcdefghijklmnop");
     let other = extension_id("bcdefghijklmnopabcdefghijklmnopa");
-    let policy = AgentTaskExtensionPolicy::new([allowed.clone(), allowed.clone()]);
+    let policy = AgentTaskExtensionPolicy::new([allowed.clone(), allowed.clone()], 10, 20, 10);
 
     assert_eq!(
-        evaluate_agent_task_extension(&allowed, &policy),
+        evaluate_agent_task_extension(&allowed, &policy, 10),
         AgentTaskExtensionDecision::AllowManagedExtension
     );
     assert_eq!(
-        evaluate_agent_task_extension(&other, &policy),
+        evaluate_agent_task_extension(&other, &policy, 19),
         AgentTaskExtensionDecision::DenyNotManaged
     );
 }
 
+#[test]
+fn managed_agent_task_extension_policy_fails_closed_outside_its_validity_window() {
+    let extension = extension_id("abcdefghijklmnopabcdefghijklmnop");
+    let policy = AgentTaskExtensionPolicy::new([extension.clone()], 10, 20, 10);
+
+    assert_eq!(
+        evaluate_agent_task_extension(&extension, &policy, 9),
+        AgentTaskExtensionDecision::DenyPolicyNotYetValid
+    );
+    assert_eq!(
+        evaluate_agent_task_extension(&extension, &policy, 20),
+        AgentTaskExtensionDecision::DenyPolicyExpired
+    );
+    assert_eq!(
+        evaluate_agent_task_extension(&extension, &policy, u64::MAX),
+        AgentTaskExtensionDecision::DenyPolicyExpired
+    );
+}
+
+#[test]
+fn invalid_managed_extension_policy_window_fails_closed_before_membership() {
+    let extension = extension_id("abcdefghijklmnopabcdefghijklmnop");
+    let reversed = AgentTaskExtensionPolicy::new([extension.clone()], 20, 10, 10);
+    let empty = AgentTaskExtensionPolicy::new([extension.clone()], 20, 20, 10);
+    let zero_maximum = AgentTaskExtensionPolicy::new([extension.clone()], 20, 21, 0);
+
+    assert_eq!(
+        evaluate_agent_task_extension(&extension, &reversed, 15),
+        AgentTaskExtensionDecision::DenyInvalidPolicyWindow
+    );
+    assert_eq!(
+        evaluate_agent_task_extension(&extension, &empty, 20),
+        AgentTaskExtensionDecision::DenyInvalidPolicyWindow
+    );
+    assert_eq!(
+        evaluate_agent_task_extension(&extension, &zero_maximum, 20),
+        AgentTaskExtensionDecision::DenyInvalidPolicyWindow
+    );
+}
+
+#[test]
+fn managed_extension_policy_window_cannot_exceed_local_maximum() {
+    let extension = extension_id("abcdefghijklmnopabcdefghijklmnop");
+    let exact = AgentTaskExtensionPolicy::new([extension.clone()], 10, 20, 10);
+    let overlong = AgentTaskExtensionPolicy::new([extension.clone()], 10, 21, 10);
+
+    assert_eq!(
+        evaluate_agent_task_extension(&extension, &exact, 19),
+        AgentTaskExtensionDecision::AllowManagedExtension
+    );
+    assert_eq!(
+        evaluate_agent_task_extension(&extension, &overlong, 19),
+        AgentTaskExtensionDecision::DenyPolicyWindowExceedsMaximum
+    );
+}
+
+#[test]
+fn maximum_timestamp_window_remains_half_open_without_overflow() {
+    let extension = extension_id("abcdefghijklmnopabcdefghijklmnop");
+    let policy = AgentTaskExtensionPolicy::new([extension.clone()], u64::MAX - 1, u64::MAX, 1);
+
+    assert_eq!(
+        evaluate_agent_task_extension(&extension, &policy, u64::MAX - 1),
+        AgentTaskExtensionDecision::AllowManagedExtension
+    );
+    assert_eq!(
+        evaluate_agent_task_extension(&extension, &policy, u64::MAX),
+        AgentTaskExtensionDecision::DenyPolicyExpired
+    );
+}
+
 #[test]
 fn managed_agent_task_extension_admission_does_not_mint_agent_capability() {
     let extension = extension_id("abcdefghijklmnopabcdefghijklmnop");
-    let policy = AgentTaskExtensionPolicy::new([extension.clone()]);
+    let policy = AgentTaskExtensionPolicy::new([extension.clone()], 10, 20, 10);
 
     assert_eq!(
-        evaluate_agent_task_extension(&extension, &policy),
+        evaluate_agent_task_extension(&extension, &policy, 15),
         AgentTaskExtensionDecision::AllowManagedExtension
     );