diff --git a/bascule-gateway/src/breach.rs b/bascule-gateway/src/breach.rs
new file mode 100644
index 0000000..a7bf628
--- /dev/null
+++ b/bascule-gateway/src/breach.rs
@@ -0,0 +1,255 @@
+// Copyright 2026 Guildhouse Dev
+// SPDX-License-Identifier: Apache-2.0
+
+//! Breach evaluator — compares posture changes against active sessions
+//! and determines the appropriate response.
+
+use bascule_core::{derive_shell_class, ShellClass};
+use governance_types::{BreachResponse, PostureLevel};
+use uuid::Uuid;
+
+/// Action to take on an active session after a posture change.
+#[derive(Debug, Clone)]
+pub enum BreachAction {
+    /// Posture still satisfies session requirements.
+    NoAction,
+
+    /// Downgrade shell class (System → Application).
+    Downgrade {
+        session_id: Uuid,
+        from: ShellClass,
+        to: ShellClass,
+        reason: String,
+    },
+
+    /// Terminate the session immediately.
+    Terminate {
+        session_id: Uuid,
+        reason: String,
+    },
+}
+
+/// Evaluate a posture change for a specific session.
+///
+/// Compares new posture against the session's establishment posture
+/// and the Accord's `BreachResponse` to determine the action.
+pub fn evaluate_breach(
+    session_id: Uuid,
+    established_posture: u8,
+    established_shell_class: ShellClass,
+    new_posture_level: u8,
+    breach_response: &BreachResponse,
+    system_threshold: Option<PostureLevel>,
+) -> BreachAction {
+    let new_level = match PostureLevel::from_wire(new_posture_level) {
+        Some(l) => l,
+        None => return BreachAction::NoAction, // Invalid level — don't act on garbage
+    };
+
+    let new_shell_class = derive_shell_class(new_level, system_threshold);
+
+    // If the new shell class still satisfies the established one, no breach
+    if new_shell_class.satisfies(established_shell_class) {
+        return BreachAction::NoAction;
+    }
+
+    // Posture has degraded — apply BreachResponse policy
+    match breach_response {
+        BreachResponse::LogOnly => {
+            tracing::info!(
+                %session_id,
+                old = established_posture,
+                new = new_posture_level,
+                "Posture breach detected (LogOnly — no enforcement)"
+            );
+            BreachAction::NoAction
+        }
+        BreachResponse::AlertDelegates => {
+            // Delegate notification happens elsewhere (WitnessForwarder).
+            // For session purposes, same as LogOnly.
+            tracing::info!(
+                %session_id,
+                old = established_posture,
+                new = new_posture_level,
+                "Posture breach detected (AlertDelegates)"
+            );
+            BreachAction::NoAction
+        }
+        BreachResponse::ReducePosture { .. } => {
+            if established_shell_class == ShellClass::System {
+                BreachAction::Downgrade {
+                    session_id,
+                    from: ShellClass::System,
+                    to: ShellClass::Application,
+                    reason: format!(
+                        "posture degraded from {} to {} (below System threshold)",
+                        established_posture, new_posture_level
+                    ),
+                }
+            } else {
+                BreachAction::NoAction
+            }
+        }
+        BreachResponse::SuspendTrust => BreachAction::Terminate {
+            session_id,
+            reason: format!(
+                "trust suspended: posture degraded to {}",
+                new_posture_level
+            ),
+        },
+        BreachResponse::RevokeAccord => BreachAction::Terminate {
+            session_id,
+            reason: "accord revoked due to posture breach".into(),
+        },
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn uuid() -> Uuid {
+        Uuid::nil()
+    }
+
+    #[test]
+    fn no_breach_posture_unchanged() {
+        let action = evaluate_breach(
+            uuid(),
+            5, // Normal
+            ShellClass::System,
+            5, // Still Normal
+            &BreachResponse::ReducePosture {
+                target_level: PostureLevel::Lockdown,
+            },
+            None,
+        );
+        assert!(matches!(action, BreachAction::NoAction));
+    }
+
+    #[test]
+    fn no_breach_posture_improved() {
+        let action = evaluate_breach(
+            uuid(),
+            4, // Elevated
+            ShellClass::Application,
+            5, // Normal (improved)
+            &BreachResponse::ReducePosture {
+                target_level: PostureLevel::Lockdown,
+            },
+            None,
+        );
+        assert!(matches!(action, BreachAction::NoAction));
+    }
+
+    #[test]
+    fn breach_reduce_posture_downgrades_system() {
+        let action = evaluate_breach(
+            uuid(),
+            5,
+            ShellClass::System,
+            3, // Restricted — below Normal threshold
+            &BreachResponse::ReducePosture {
+                target_level: PostureLevel::Restricted,
+            },
+            None,
+        );
+        assert!(matches!(action, BreachAction::Downgrade { .. }));
+    }
+
+    #[test]
+    fn breach_suspend_trust_terminates() {
+        let action = evaluate_breach(
+            uuid(),
+            5,
+            ShellClass::System,
+            3,
+            &BreachResponse::SuspendTrust,
+            None,
+        );
+        assert!(matches!(action, BreachAction::Terminate { .. }));
+    }
+
+    #[test]
+    fn breach_revoke_accord_terminates() {
+        let action = evaluate_breach(
+            uuid(),
+            5,
+            ShellClass::System,
+            3,
+            &BreachResponse::RevokeAccord,
+            None,
+        );
+        assert!(matches!(action, BreachAction::Terminate { .. }));
+    }
+
+    #[test]
+    fn already_application_reduce_posture_no_action() {
+        let action = evaluate_breach(
+            uuid(),
+            4,
+            ShellClass::Application,
+            2, // Critical — but already Application
+            &BreachResponse::ReducePosture {
+                target_level: PostureLevel::Lockdown,
+            },
+            None,
+        );
+        assert!(matches!(action, BreachAction::NoAction));
+    }
+
+    #[test]
+    fn breach_with_custom_threshold() {
+        // Threshold at Elevated(4) — Normal(5) and Elevated(4) both qualify
+        let action = evaluate_breach(
+            uuid(),
+            5,
+            ShellClass::System,
+            4, // Elevated — still above threshold
+            &BreachResponse::ReducePosture {
+                target_level: PostureLevel::Lockdown,
+            },
+            Some(PostureLevel::Elevated),
+        );
+        assert!(matches!(action, BreachAction::NoAction));
+
+        // Drop below custom threshold
+        let action = evaluate_breach(
+            uuid(),
+            5,
+            ShellClass::System,
+            3, // Restricted — below Elevated threshold
+            &BreachResponse::ReducePosture {
+                target_level: PostureLevel::Lockdown,
+            },
+            Some(PostureLevel::Elevated),
+        );
+        assert!(matches!(action, BreachAction::Downgrade { .. }));
+    }
+
+    #[test]
+    fn log_only_no_enforcement() {
+        let action = evaluate_breach(
+            uuid(),
+            5,
+            ShellClass::System,
+            3,
+            &BreachResponse::LogOnly,
+            None,
+        );
+        assert!(matches!(action, BreachAction::NoAction));
+    }
+
+    #[test]
+    fn invalid_posture_level_no_action() {
+        let action = evaluate_breach(
+            uuid(),
+            5,
+            ShellClass::System,
+            0, // Invalid wire value
+            &BreachResponse::SuspendTrust,
+            None,
+        );
+        assert!(matches!(action, BreachAction::NoAction));
+    }
+}
diff --git a/bascule-gateway/src/main.rs b/bascule-gateway/src/main.rs
index bf63878..eb1b019 100644
--- a/bascule-gateway/src/main.rs
+++ b/bascule-gateway/src/main.rs
@@ -1,5 +1,6 @@
 mod audit_pipeline;
 mod auth;
+mod breach;
 mod ceremony;
 mod config;
 mod executor;
@@ -168,6 +169,23 @@ spec:
         reaper_manager.run_reaper().await;
     });
 
+    // 11b. Spawn posture polling task for breach detection
+    let breach_manager = session_manager.clone();
+    tokio::spawn(async move {
+        let mut interval = tokio::time::interval(std::time::Duration::from_secs(30));
+        let breach_response = governance_types::BreachResponse::ReducePosture {
+            target_level: governance_types::PostureLevel::Lockdown,
+        };
+        tracing::info!("Posture breach polling loop started (30s interval)");
+        loop {
+            interval.tick().await;
+            let level = server::read_posture_level().await;
+            breach_manager
+                .on_posture_change(level.to_wire(), &breach_response)
+                .await;
+        }
+    });
+
     if let Some(cm) = &ceremony_manager {
         let cm = cm.clone();
         tokio::spawn(async move {
diff --git a/bascule-gateway/src/server.rs b/bascule-gateway/src/server.rs
index 8856f3c..fa661c2 100644
--- a/bascule-gateway/src/server.rs
+++ b/bascule-gateway/src/server.rs
@@ -419,10 +419,11 @@ fn proto_scope_to_core(proto: &bascule_proto::bascule_v1::SessionScope) -> Sessi
         pathways: proto.pathways.iter().map(|p| parse_pathway(p)).collect(),
         mutation_budget: proto.mutation_budget,
         can_delegate: proto.can_delegate,
-        // ShellClass is server-derived from posture, not client-requested.
-        // Set to default here; stamped by the ceremony grant path.
+        // ShellClass and delegation are server-derived, not client-requested.
+        // Set to defaults here; stamped by the ceremony grant path.
         shell_class: bascule_core::ShellClass::default(),
         posture_level_at_establishment: None,
+        delegation: bascule_core::DelegationScope::default(),
     }
 }
 
@@ -523,7 +524,7 @@ fn core_global_to_proto(core: &GlobalScope) -> bascule_proto::bascule_v1::Global
 /// Read the cluster's current operational posture level from the
 /// `posture-current` ConfigMap.  Falls back to `PostureLevel::Lockdown`
 /// (fail-closed) if the ConfigMap is missing or unreadable.
-async fn read_posture_level() -> governance_types::PostureLevel {
+pub(crate) async fn read_posture_level() -> governance_types::PostureLevel {
     use governance_types::PostureLevel;
 
     let client = match kube::Client::try_default().await {
diff --git a/bascule-gateway/src/session_manager.rs b/bascule-gateway/src/session_manager.rs
index cf0a101..18dec6e 100644
--- a/bascule-gateway/src/session_manager.rs
+++ b/bascule-gateway/src/session_manager.rs
@@ -195,6 +195,77 @@ impl SessionManager {
         }
     }
 
+    /// Apply a breach action to an active session.
+    pub async fn apply_breach_action(&self, action: crate::breach::BreachAction) {
+        use crate::breach::BreachAction;
+        match action {
+            BreachAction::NoAction => {}
+            BreachAction::Downgrade {
+                session_id,
+                from,
+                to,
+                reason,
+            } => {
+                if let Some(mut session) = self.sessions.get_mut(&session_id) {
+                    session.scope.shell_class = to;
+                    tracing::warn!(
+                        session_id = %session_id,
+                        from = %from,
+                        to = %to,
+                        reason = %reason,
+                        "Session shell class downgraded due to posture breach"
+                    );
+                }
+            }
+            BreachAction::Terminate {
+                session_id,
+                reason,
+            } => {
+                tracing::error!(
+                    session_id = %session_id,
+                    reason = %reason,
+                    "Session terminated due to posture breach"
+                );
+                self.end_session(&session_id).await;
+            }
+        }
+    }
+
+    /// Handle a posture change notification.
+    ///
+    /// Iterates all active sessions and evaluates breach for each.
+    /// Called by the posture polling loop when the posture-current
+    /// ConfigMap reports a new level.
+    pub async fn on_posture_change(
+        &self,
+        new_level: u8,
+        breach_response: &governance_types::BreachResponse,
+    ) {
+        use crate::breach::evaluate_breach;
+        use bascule_core::session::SessionState;
+
+        let actions: Vec<crate::breach::BreachAction> = self
+            .sessions
+            .iter()
+            .filter(|entry| entry.value().state == SessionState::Active)
+            .map(|entry| {
+                let session = entry.value();
+                evaluate_breach(
+                    *entry.key(),
+                    session.scope.posture_level_at_establishment.unwrap_or(0),
+                    session.scope.shell_class,
+                    new_level,
+                    breach_response,
+                    None,
+                )
+            })
+            .collect();
+
+        for action in actions {
+            self.apply_breach_action(action).await;
+        }
+    }
+
     /// Create a default read-only scope for the given namespaces.
     pub fn default_read_scope(namespaces: &[String]) -> SessionScope {
         use bascule_core::scope::{
@@ -223,6 +294,7 @@ impl SessionManager {
             can_delegate: false,
             shell_class: bascule_core::ShellClass::default(),
             posture_level_at_establishment: None,
+            delegation: bascule_core::DelegationScope::default(),
         }
     }
 }