diff --git a/crates/tui/src/core/engine/turn_loop.rs b/crates/tui/src/core/engine/turn_loop.rs
index 9a3245e16..ec3939e9a 100644
--- a/crates/tui/src/core/engine/turn_loop.rs
+++ b/crates/tui/src/core/engine/turn_loop.rs
@@ -6,6 +6,7 @@
 //! checkpoints, and loop termination.
 
 use super::*;
+use crate::prompt_zones::PinnedPrefix;
 
 fn loop_guard_block_tool_result(message: String) -> ToolResult {
     ToolResult::error(message).with_metadata(json!({"loop_guard": "identical_tool_call"}))
@@ -310,6 +311,37 @@ impl Engine {
                 }
             }
 
+            // Three-zone prefix contract (#2264): freeze baseline on first
+            // turn, verify against it on subsequent turns. Operates alongside
+            // PrefixStabilityManager as an independent diagnostic layer.
+            // Phase 2: warn-only, auto-re-freeze on drift.
+            let system_text =
+                crate::prefix_cache::system_prompt_text(self.session.system_prompt.as_ref());
+            let current_tools: &[crate::models::Tool] = active_tools.as_deref().unwrap_or_default();
+
+            match &self.session.frozen_prefix {
+                Some(frozen) => {
+                    if let Err(drift) = frozen.verify(&system_text, current_tools) {
+                        tracing::debug!(
+                            target: "prefix_cache",
+                            "three-zone drift: {drift}"
+                        );
+                        let pinned = PinnedPrefix::new(
+                            self.session.system_prompt.as_ref(),
+                            current_tools.to_vec(),
+                        );
+                        self.session.frozen_prefix = Some(pinned.freeze());
+                    }
+                }
+                None => {
+                    let pinned = PinnedPrefix::new(
+                        self.session.system_prompt.as_ref(),
+                        current_tools.to_vec(),
+                    );
+                    self.session.frozen_prefix = Some(pinned.freeze());
+                }
+            }
+
             let request = MessageRequest {
                 model: self.session.model.clone(),
                 messages: self.messages_with_turn_metadata(),
diff --git a/crates/tui/src/core/session.rs b/crates/tui/src/core/session.rs
index cde29b737..df90caffe 100644
--- a/crates/tui/src/core/session.rs
+++ b/crates/tui/src/core/session.rs
@@ -6,6 +6,7 @@ use crate::cycle_manager::CycleBriefing;
 use crate::models::{Message, SystemPrompt, Usage};
 use crate::prefix_cache::PrefixStabilityManager;
 use crate::project_context::{ProjectContext, load_project_context_with_parents};
+use crate::prompt_zones::FrozenPrefix;
 use crate::tui::approval::ApprovalMode;
 use crate::working_set::WorkingSet;
 use chrono::{DateTime, Utc};
@@ -91,6 +92,11 @@ pub struct Session {
     /// Tracks the immutable prefix fingerprint and detects drift across turns.
     /// Set during engine construction; None until the first system prompt assembly.
     pub prefix_stability: Option<PrefixStabilityManager>,
+
+    /// Three-zone immutable prefix baseline (#2264). Frozen on the first
+    /// request of the session; verified against the current system+tool
+    /// state before every subsequent request. None until the first turn.
+    pub frozen_prefix: Option<FrozenPrefix>,
 }
 
 /// Cumulative usage statistics for a session.
@@ -166,6 +172,7 @@ impl Session {
             current_cycle_started: Utc::now(),
             cycle_briefings: Vec::new(),
             prefix_stability: None,
+            frozen_prefix: None,
         }
     }