diff --git a/crates/tui/src/core/engine.rs b/crates/tui/src/core/engine.rs
index 54ba0c243..68d34a6f5 100644
--- a/crates/tui/src/core/engine.rs
+++ b/crates/tui/src/core/engine.rs
@@ -1091,7 +1091,7 @@ impl Engine {
                     } else if messages.is_empty() && system_prompt.is_none() {
                         self.session.id = uuid::Uuid::new_v4().to_string();
                     }
-                    self.session.messages = messages;
+                    self.session.messages = messages.into();
                     self.session.compaction_summary_prompt =
                         extract_compaction_summary_prompt(system_prompt.clone());
                     self.session.system_prompt = system_prompt;
@@ -1182,7 +1182,7 @@ impl Engine {
             .tx_event
             .send(Event::SessionUpdated {
                 session_id: self.session.id.clone(),
-                messages: self.session.messages.clone(),
+                messages: self.session.messages.clone().into(),
                 system_prompt: self.session.system_prompt.clone(),
                 model: self.session.model.clone(),
                 workspace: self.session.workspace.clone(),
@@ -1673,7 +1673,7 @@ impl Engine {
             Ok(result) => {
                 if !result.messages.is_empty() || self.session.messages.is_empty() {
                     let messages_after = result.messages.len();
-                    self.session.messages = result.messages;
+                    self.session.messages = result.messages.into();
                     self.merge_compaction_summary(result.summary_prompt);
                     self.emit_session_updated().await;
                     let removed = messages_before.saturating_sub(messages_after);
@@ -1769,7 +1769,7 @@ impl Engine {
         {
             Ok(result) => {
                 let messages_after = result.messages.len();
-                self.session.messages = result.messages;
+                self.session.messages = result.messages.into();
                 self.emit_session_updated().await;
 
                 let summary = format!(
@@ -1864,7 +1864,7 @@ impl Engine {
         {
             Ok(result) => {
                 retries_used = result.retries_used;
-                compacted_messages = result.messages;
+                compacted_messages = result.messages.into();
                 summary_prompt = result.summary_prompt;
             }
             Err(err) => {
@@ -1952,7 +1952,7 @@ impl Engine {
             self.session.model.clone(),
             self.session.workspace.clone(),
             self.session.system_prompt.clone(),
-            self.session.messages.clone(),
+            self.session.messages.clone().into(),
         ))
         .with_cancel_token(self.cancel_token.clone())
         .with_trusted_external_paths(trusted_external_paths);
@@ -2310,7 +2310,7 @@ impl Engine {
         );
 
         // 5. Atomic swap.
-        self.session.messages = seed_messages;
+        self.session.messages = seed_messages.into();
         self.session.cycle_count = to;
         self.session.current_cycle_started = now;
         self.session.cycle_briefings.push(briefing.clone());
diff --git a/crates/tui/src/core/engine/capacity_flow.rs b/crates/tui/src/core/engine/capacity_flow.rs
index fe3577622..a17375f01 100644
--- a/crates/tui/src/core/engine/capacity_flow.rs
+++ b/crates/tui/src/core/engine/capacity_flow.rs
@@ -418,7 +418,7 @@ impl Engine {
             {
                 Ok(result) => {
                     if !result.messages.is_empty() || self.session.messages.is_empty() {
-                        self.session.messages = result.messages;
+                        self.session.messages = result.messages.into();
                         self.merge_compaction_summary(result.summary_prompt);
                         refreshed = true;
                     }
diff --git a/crates/tui/src/core/engine/tests.rs b/crates/tui/src/core/engine/tests.rs
index 48491277e..e631cb45e 100644
--- a/crates/tui/src/core/engine/tests.rs
+++ b/crates/tui/src/core/engine/tests.rs
@@ -2104,7 +2104,7 @@ fn messages_with_turn_metadata_preserves_stored_messages_for_prefix_cache() {
     let first_user = engine.user_text_message_with_turn_metadata("inspect src/lib.rs".to_string());
     engine.session.add_message(first_user.clone());
     let first_request = engine.messages_with_turn_metadata();
-    assert_eq!(first_request, engine.session.messages);
+    assert_eq!(&first_request, &*engine.session.messages);
 
     engine.session.add_message(Message {
         role: "assistant".to_string(),
@@ -2121,7 +2121,7 @@ fn messages_with_turn_metadata_preserves_stored_messages_for_prefix_cache() {
     engine.session.add_message(second_user);
 
     let second_request = engine.messages_with_turn_metadata();
-    assert_eq!(second_request, engine.session.messages);
+    assert_eq!(&second_request, &*engine.session.messages);
     assert_eq!(second_request.first(), Some(&first_user));
 }
 
diff --git a/crates/tui/src/core/engine/turn_loop.rs b/crates/tui/src/core/engine/turn_loop.rs
index 6b7ffd1ff..4b87b279f 100644
--- a/crates/tui/src/core/engine/turn_loop.rs
+++ b/crates/tui/src/core/engine/turn_loop.rs
@@ -148,7 +148,7 @@ impl Engine {
                         // Only update if we got valid messages (never corrupt state)
                         if !result.messages.is_empty() || self.session.messages.is_empty() {
                             let auto_messages_after = result.messages.len();
-                            self.session.messages = result.messages;
+                            self.session.messages = result.messages.into();
                             self.merge_compaction_summary(result.summary_prompt);
                             self.emit_session_updated().await;
                             let removed = auto_messages_before.saturating_sub(auto_messages_after);
@@ -2219,7 +2219,7 @@ impl Engine {
         // appended. Do not rewrite historical messages at request time: doing
         // so makes the API prefix differ from the bytes sent in earlier turns
         // and destroys DeepSeek's KV prefix cache reuse.
-        self.session.messages.clone()
+        self.session.messages.clone().into_inner()
     }
 }
 
diff --git a/crates/tui/src/core/session.rs b/crates/tui/src/core/session.rs
index df90caffe..9361045b0 100644
--- a/crates/tui/src/core/session.rs
+++ b/crates/tui/src/core/session.rs
@@ -6,7 +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::prompt_zones::{AppendLog, FrozenPrefix};
 use crate::tui::approval::ApprovalMode;
 use crate::working_set::WorkingSet;
 use chrono::{DateTime, Utc};
@@ -42,8 +42,8 @@ pub struct Session {
     /// Persisted summary blocks generated by context compaction.
     pub compaction_summary_prompt: Option<SystemPrompt>,
 
-    /// Conversation history (API format)
-    pub messages: Vec<Message>,
+    /// Conversation history (API format), backed by AppendLog (#2264).
+    pub messages: AppendLog,
 
     /// Total tokens used in this session
     pub total_usage: SessionUsage,
@@ -152,7 +152,7 @@ impl Session {
             system_prompt: None,
             system_prompt_override: false,
             compaction_summary_prompt: None,
-            messages: Vec::new(),
+            messages: AppendLog::new(),
             total_usage: SessionUsage::default(),
             allow_shell,
             trust_mode,
diff --git a/crates/tui/src/prompt_zones.rs b/crates/tui/src/prompt_zones.rs
index 581ac355d..c7162541d 100644
--- a/crates/tui/src/prompt_zones.rs
+++ b/crates/tui/src/prompt_zones.rs
@@ -195,16 +195,15 @@ impl std::fmt::Display for PrefixDrift {
 
 // ── AppendLog ──────────────────────────────────────────────────────────
 
-/// Append-only conversation history. Only exposes `push`-style mutations.
+/// Append-only conversation history. Derefs to [`Vec<Message>`] for
+/// transparent read access; mutations go through `push()` / `From`.
 ///
-/// **Phase 1 scaffolding** — not yet wired into the engine request path.
-#[allow(dead_code)]
+/// Phase 4: backing store for `Session.messages` (#2264).
 #[derive(Debug, Clone)]
 pub struct AppendLog {
     messages: Vec<Message>,
 }
 
-#[allow(dead_code)]
 impl AppendLog {
     pub fn new() -> Self {
         Self {
@@ -216,33 +215,47 @@ impl AppendLog {
         Self { messages }
     }
 
+    /// Append a message to the log.
     pub fn push(&mut self, message: Message) {
         self.messages.push(message);
     }
 
+    /// Consume and return the inner `Vec<Message>`.
     #[must_use]
-    pub fn len(&self) -> usize {
-        self.messages.len()
+    pub fn into_inner(self) -> Vec<Message> {
+        self.messages
     }
+}
 
-    #[must_use]
-    pub fn is_empty(&self) -> bool {
-        self.messages.is_empty()
+impl Default for AppendLog {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl From<Vec<Message>> for AppendLog {
+    fn from(messages: Vec<Message>) -> Self {
+        Self { messages }
     }
+}
 
-    pub fn iter(&self) -> impl Iterator<Item = &Message> {
-        self.messages.iter()
+impl From<AppendLog> for Vec<Message> {
+    fn from(log: AppendLog) -> Self {
+        log.messages
     }
+}
 
-    #[must_use]
-    pub fn as_slice(&self) -> &[Message] {
+impl std::ops::Deref for AppendLog {
+    type Target = Vec<Message>;
+
+    fn deref(&self) -> &Self::Target {
         &self.messages
     }
 }
 
-impl Default for AppendLog {
-    fn default() -> Self {
-        Self::new()
+impl std::ops::DerefMut for AppendLog {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        &mut self.messages
     }
 }
 
@@ -525,7 +538,6 @@ mod tests {
         let msgs = vec![make_message("user", "a"), make_message("assistant", "b")];
         let log = AppendLog::from_messages(msgs);
         assert_eq!(log.len(), 2);
-        assert_eq!(log.as_slice().len(), 2);
     }
 
     // ── TurnScratch ───────────────────────────────────────────────