diff --git a/crates/tui/src/core/engine.rs b/crates/tui/src/core/engine.rs
index 291e2a26..be683200 100644
--- a/crates/tui/src/core/engine.rs
+++ b/crates/tui/src/core/engine.rs
@@ -536,6 +536,18 @@ impl Engine {
         session.last_system_prompt_hash = Some(system_prompt_hash(stable_prompt.as_ref()));
         session.system_prompt = stable_prompt;
 
+        // Initialize prefix-cache stability monitor (lazy-pin).
+        // The system prompt is available now but the tool catalog isn't
+        // fully built until the first turn, so we start unpinned. The
+        // first `check_and_update` call in the turn loop will pin the
+        // fingerprint automatically.
+        let _ = session.prefix_stability.get_or_insert_with(|| {
+            // Use the tool registry's spec names for fingerprinting.
+            // At this point tool spec builders may not be registered yet,
+            // so we start with None — fingerprint will pin on first request.
+            crate::prefix_cache::PrefixStabilityManager::new_unpinned()
+        });
+
         let subagent_manager =
             new_shared_subagent_manager(config.workspace.clone(), config.max_subagents);
         let shell_manager = config
diff --git a/crates/tui/src/core/engine/turn_loop.rs b/crates/tui/src/core/engine/turn_loop.rs
index 7927246b..bfcbf3e2 100644
--- a/crates/tui/src/core/engine/turn_loop.rs
+++ b/crates/tui/src/core/engine/turn_loop.rs
@@ -235,6 +235,49 @@ impl Engine {
                 &self.session.messages,
             );
 
+            // Check prefix-cache stability before building the request.
+            // This detects system-prompt or tool-set drift that would
+            // invalidate DeepSeek's KV prefix cache for this turn.
+            // Sends an event on EVERY check so the TUI can maintain
+            // its own counter for the stable-checks tally.
+            if let Some(pm) = self.session.prefix_stability.as_mut() {
+                let system_text =
+                    crate::prefix_cache::system_prompt_text(self.session.system_prompt.as_ref());
+                let tools_ref: Option<&[crate::models::Tool]> = active_tools.as_deref();
+                match pm.check_and_update(&system_text, tools_ref) {
+                    Err(change) => {
+                        tracing::debug!(
+                            target: "prefix_cache",
+                            "{}",
+                            change.description()
+                        );
+                        let _ = self
+                            .tx_event
+                            .send(Event::PrefixCacheChange {
+                                description: change.description(),
+                                system_prompt_changed: change.system_changed,
+                                tools_changed: change.tools_changed,
+                                stability_pct: (pm.stability_ratio() * 100.0).round() as u32,
+                                changed: true,
+                            })
+                            .await;
+                    }
+                    Ok(_) => {
+                        // Stable check — keep the TUI counter in sync.
+                        let _ = self
+                            .tx_event
+                            .send(Event::PrefixCacheChange {
+                                description: String::new(),
+                                system_prompt_changed: false,
+                                tools_changed: false,
+                                stability_pct: (pm.stability_ratio() * 100.0).round() as u32,
+                                changed: false,
+                            })
+                            .await;
+                    }
+                }
+            }
+
             let request = MessageRequest {
                 model: self.session.model.clone(),
                 messages: self.messages_with_turn_metadata(),
diff --git a/crates/tui/src/core/events.rs b/crates/tui/src/core/events.rs
index 41ca417f..ea338409 100644
--- a/crates/tui/src/core/events.rs
+++ b/crates/tui/src/core/events.rs
@@ -261,6 +261,24 @@ pub enum Event {
         blocked_network: bool,
         blocked_write: bool,
     },
+
+    // === Prefix-Cache Stability Events ===
+    /// The prefix (system prompt + tool specs) changed between turns,
+    /// which invalidates DeepSeek's KV prefix cache. Carries diagnostics
+    /// for the TUI to surface.
+    PrefixCacheChange {
+        /// Human-readable description of what changed.
+        description: String,
+        /// Whether the system prompt component changed.
+        system_prompt_changed: bool,
+        /// Whether the tool set component changed.
+        tools_changed: bool,
+        /// Overall prefix stability percentage (100 = fully stable).
+        stability_pct: u32,
+        /// True when the prefix actually changed (cache invalidated).
+        /// False for routine stable-check heartbeats.
+        changed: bool,
+    },
 }
 
 impl Event {
diff --git a/crates/tui/src/core/session.rs b/crates/tui/src/core/session.rs
index 65ea5f16..60aa9187 100644
--- a/crates/tui/src/core/session.rs
+++ b/crates/tui/src/core/session.rs
@@ -4,6 +4,7 @@
 
 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::tui::approval::ApprovalMode;
 use crate::working_set::WorkingSet;
@@ -82,6 +83,11 @@ pub struct Session {
     /// Briefings produced at past cycle boundaries, in chronological order.
     /// Bounded growth: one entry per cycle, briefing capped at ~3,000 tokens.
     pub cycle_briefings: Vec<CycleBriefing>,
+
+    /// Prefix-cache stability monitor (inspired by Reasonix's Pillar 1).
+    /// 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>,
 }
 
 /// Cumulative usage statistics for a session.
@@ -155,6 +161,7 @@ impl Session {
             cycle_count: 0,
             current_cycle_started: Utc::now(),
             cycle_briefings: Vec::new(),
+            prefix_stability: None,
         }
     }
 
diff --git a/crates/tui/src/main.rs b/crates/tui/src/main.rs
index 5a84a625..9a90bee9 100644
--- a/crates/tui/src/main.rs
+++ b/crates/tui/src/main.rs
@@ -47,6 +47,7 @@ mod memory;
 mod models;
 mod network_policy;
 mod palette;
+mod prefix_cache;
 mod pricing;
 mod project_context;
 mod project_doc;
diff --git a/crates/tui/src/prefix_cache.rs b/crates/tui/src/prefix_cache.rs
new file mode 100644
index 00000000..8d631ffa
--- /dev/null
+++ b/crates/tui/src/prefix_cache.rs
@@ -0,0 +1,496 @@
+//! Prefix-cache stability manager (inspired by Reasonix's Pillar 1).
+//!
+//! DeepSeek's automatic prefix caching activates only when the *exact*
+//! byte prefix of a request matches the prior request. Any system-prompt
+//! drift, tool-list reordering, or message-rewriting busts the cache
+//! for every token after the changed byte.
+//!
+//! This module provides a `PrefixStabilityManager` that:
+//!
+//! 1. **Fingerprints** the immutable prefix (system prompt + tool specs)
+//!    at session start, using SHA-256 for strong collision resistance.
+//! 2. **Detects drift** by comparing the current prefix against the
+//!    pinned fingerprint before every request.
+//! 3. **Diagnoses** the cause of drift — did the system prompt change?
+//!    Did the tool set change? Both?
+//! 4. **Emits events** so the TUI can surface stability to the user.
+//!
+//! ## Three-region model (from Reasonix)
+//!
+//! ```text
+//! ┌─────────────────────────────────────────┐
+//! │ IMMUTABLE PREFIX                        │ ← fixed for session
+//! │   system + tool_specs                    │   cache hit candidate
+//! ├─────────────────────────────────────────┤
+//! │ APPEND-ONLY HISTORY                     │ ← grows monotonically
+//! │   [assistant₁][tool₁][assistant₂]...    │   preserves prefix of prior turns
+//! ├─────────────────────────────────────────┤
+//! │ LATEST USER TURN                        │ ← the only new content per request
+//! └─────────────────────────────────────────┘
+//! ```
+
+use serde::{Deserialize, Serialize};
+use sha2::{Digest, Sha256};
+
+use crate::models::{SystemPrompt, Tool};
+
+/// A snapshot of the immutable prefix's fingerprint.
+///
+/// Two snapshots with the same `combined` hash are guaranteed to
+/// produce the same byte prefix when serialized for the API.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct PrefixFingerprint {
+    /// SHA-256 of the system prompt text.
+    pub system_sha256: String,
+    /// SHA-256 of the concatenated, sorted tool names.
+    pub tools_sha256: String,
+    /// SHA-256 of system_sha256 ++ tools_sha256 (combined).
+    pub combined_sha256: String,
+}
+
+impl PrefixFingerprint {
+    /// Compute a fingerprint from system prompt text and tool list.
+    pub fn compute(system_text: &str, tools: Option<&[Tool]>) -> Self {
+        let system_sha256 = sha256_hex(system_text.as_bytes());
+
+        let tools_sha256 = match tools {
+            Some(tools) if !tools.is_empty() => {
+                // Sort tool names deterministically so the hash is
+                // stable regardless of registration order.
+                let mut tool_names: Vec<&str> = tools.iter().map(|t| t.name.as_str()).collect();
+                tool_names.sort();
+                let joined = tool_names.join(",");
+                sha256_hex(joined.as_bytes())
+            }
+            _ => sha256_hex(b""),
+        };
+
+        let combined = format!("{}:{}", system_sha256, tools_sha256);
+        let combined_sha256 = sha256_hex(combined.as_bytes());
+
+        Self {
+            system_sha256,
+            tools_sha256,
+            combined_sha256,
+        }
+    }
+}
+
+/// A change record describing what drifted in the prefix.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct PrefixChange {
+    /// The old fingerprint (before the change).
+    pub old: PrefixFingerprint,
+    /// The new fingerprint (after the change).
+    pub new: PrefixFingerprint,
+    /// Whether the system prompt component changed.
+    pub system_changed: bool,
+    /// Whether the tool set component changed.
+    pub tools_changed: bool,
+}
+
+#[allow(dead_code)]
+impl PrefixChange {
+    /// Returns a human-readable description of what changed.
+    pub fn description(&self) -> String {
+        let mut parts = Vec::new();
+        if self.system_changed {
+            parts.push("system prompt");
+        }
+        if self.tools_changed {
+            parts.push("tool set");
+        }
+        if parts.is_empty() {
+            return "unknown (fingerprint mismatch but no component detected)".to_string();
+        }
+        format!("prefix cache invalidated: {} changed", parts.join(" and "))
+    }
+
+    /// Returns a short label for TUI chip display.
+    pub fn label(&self) -> &'static str {
+        if self.system_changed && self.tools_changed {
+            "sys+tools"
+        } else if self.system_changed {
+            "sys"
+        } else if self.tools_changed {
+            "tools"
+        } else {
+            "prefix"
+        }
+    }
+}
+
+/// Monitors and manages prefix-cache stability across turns.
+///
+/// This is the core abstraction, mirroring Reasonix's `ImmutablePrefix`
+/// concept but adapted to DeepSeek-TUI's existing architecture where the
+/// system prompt is rebuilt each turn and tools are registered at startup.
+///
+/// Usage:
+/// ```ignore
+/// let mgr = PrefixStabilityManager::new(system_text, tools);
+/// if mgr.check_and_update(system_text, tools) {
+///     println!("Prefix is stable (cache-friendly)");
+/// } else {
+///     let change = mgr.last_change().unwrap();
+///     println!("Prefix drifted: {}", change.description());
+/// }
+/// ```
+#[derive(Debug, Clone)]
+pub struct PrefixStabilityManager {
+    /// The pinned fingerprint from session start or last stabilization.
+    pinned: Option<PrefixFingerprint>,
+    /// The most recent fingerprint (computed during last check).
+    current: Option<PrefixFingerprint>,
+    /// The last detected change, if any.
+    last_change: Option<PrefixChange>,
+    /// Total number of prefix changes detected this session.
+    change_count: u64,
+    /// Total number of stability checks performed.
+    check_count: u64,
+}
+
+#[allow(dead_code)]
+impl PrefixStabilityManager {
+    /// Create a new manager and immediately pin the first fingerprint.
+    pub fn new(system_text: &str, tools: Option<&[Tool]>) -> Self {
+        let fp = PrefixFingerprint::compute(system_text, tools);
+        Self {
+            pinned: Some(fp.clone()),
+            current: Some(fp),
+            last_change: None,
+            change_count: 0,
+            check_count: 0,
+        }
+    }
+
+    /// Create a manager in "unpinned" state — no initial fingerprint.
+    /// Call `pin()` or `check_and_update()` to establish the baseline.
+    pub fn new_unpinned() -> Self {
+        Self {
+            pinned: None,
+            current: None,
+            last_change: None,
+            change_count: 0,
+            check_count: 0,
+        }
+    }
+
+    /// Explicitly pin a fingerprint, replacing any prior pinned state.
+    /// Returns `true` if this is the first pin, or `false` if replacing.
+    /// Note: does NOT increment `check_count` — that counter is reserved
+    /// for `check_and_update` calls so `stability_ratio()` stays accurate.
+    pub fn pin(&mut self, system_text: &str, tools: Option<&[Tool]>) -> bool {
+        let fp = PrefixFingerprint::compute(system_text, tools);
+        let was_unpinned = self.pinned.is_none();
+        self.pinned = Some(fp.clone());
+        self.current = Some(fp);
+        was_unpinned
+    }
+
+    /// Check whether the current prefix matches the pinned fingerprint.
+    /// Updates internal state and returns:
+    /// - `Ok(true)` if the prefix is stable (fingerprint matches pinned).
+    /// - `Ok(false)` if the prefix changed but was automatically re-pinned.
+    /// - `Err(change)` if the prefix changed; caller should surface this.
+    ///
+    /// After calling this, `last_change()` returns the detected change.
+    pub fn check_and_update(
+        &mut self,
+        system_text: &str,
+        tools: Option<&[Tool]>,
+    ) -> Result<bool, PrefixChange> {
+        let fp = PrefixFingerprint::compute(system_text, tools);
+        let old_fp = std::mem::replace(&mut self.current, Some(fp.clone()));
+        self.check_count += 1;
+
+        let pinned = match &self.pinned {
+            Some(p) => p,
+            None => {
+                // First check: pin now.
+                self.pinned = Some(fp);
+                self.last_change = None;
+                return Ok(true);
+            }
+        };
+
+        if fp.combined_sha256 == pinned.combined_sha256 {
+            // Stable — no change.
+            Ok(true)
+        } else {
+            // Change detected.
+            let old = old_fp.unwrap_or_else(|| pinned.clone());
+            let system_changed = fp.system_sha256 != pinned.system_sha256;
+            let tools_changed = fp.tools_sha256 != pinned.tools_sha256;
+
+            let change = PrefixChange {
+                old,
+                new: fp.clone(),
+                system_changed,
+                tools_changed,
+            };
+
+            self.last_change = Some(change.clone());
+            self.change_count += 1;
+
+            // Re-pin to the new prefix so subsequent checks are
+            // against the latest baseline. Use the original fp
+            // (avoid recomputing the hash — clone was for the change record).
+            self.pinned = Some(fp);
+
+            Err(change)
+        }
+    }
+
+    /// Returns the most recent prefix change, if any.
+    pub fn last_change(&self) -> Option<&PrefixChange> {
+        self.last_change.as_ref()
+    }
+
+    /// Returns the pinned fingerprint.
+    pub fn pinned_fingerprint(&self) -> Option<&PrefixFingerprint> {
+        self.pinned.as_ref()
+    }
+
+    /// Returns the current (most recently computed) fingerprint.
+    pub fn current_fingerprint(&self) -> Option<&PrefixFingerprint> {
+        self.current.as_ref()
+    }
+
+    /// Returns the total number of prefix changes detected.
+    pub fn change_count(&self) -> u64 {
+        self.change_count
+    }
+
+    /// Returns the total number of stability checks performed.
+    pub fn check_count(&self) -> u64 {
+        self.check_count
+    }
+
+    /// Returns the prefix stability rate as a fraction (0.0 – 1.0).
+    /// 1.0 means the prefix has never changed. Returns 1.0 when no
+    /// checks have been performed (to avoid division by zero).
+    pub fn stability_ratio(&self) -> f64 {
+        if self.check_count == 0 {
+            1.0
+        } else {
+            let stable_checks = self.check_count - self.change_count;
+            stable_checks as f64 / self.check_count as f64
+        }
+    }
+
+    /// Returns a human-readable stability summary.
+    pub fn summary(&self) -> String {
+        let pct = self.stability_ratio() * 100.0;
+        let pinned_short = self
+            .pinned
+            .as_ref()
+            .map(|fp| {
+                if fp.combined_sha256.len() >= 12 {
+                    &fp.combined_sha256[..12]
+                } else {
+                    &fp.combined_sha256
+                }
+            })
+            .unwrap_or("none");
+
+        format!(
+            "Prefix stability: {pct:.1}% ({stable}/{total} checks stable) | fingerprint: {pinned_short} | changes: {changes}",
+            pct = pct,
+            stable = self.check_count.saturating_sub(self.change_count),
+            total = self.check_count,
+            pinned_short = pinned_short,
+            changes = self.change_count,
+        )
+    }
+}
+
+/// Compute the SHA-256 hex digest of a byte slice.
+fn sha256_hex(bytes: &[u8]) -> String {
+    let mut hasher = Sha256::new();
+    hasher.update(bytes);
+    format!("{:x}", hasher.finalize())
+}
+
+/// Extract the system prompt text from an optional SystemPrompt,
+/// returning an owned String. This is used for prefix fingerprinting
+/// and avoids lifetime/leak issues with the rare SystemPrompt::Blocks case.
+pub fn system_prompt_text(system: Option<&SystemPrompt>) -> String {
+    match system {
+        Some(SystemPrompt::Text(text)) => text.clone(),
+        Some(SystemPrompt::Blocks(blocks)) => {
+            let mut text = String::new();
+            for block in blocks {
+                text.push_str(&block.text);
+                text.push('\n');
+            }
+            text
+        }
+        None => String::new(),
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn make_tool(name: &str) -> Tool {
+        Tool {
+            name: name.to_string(),
+            description: String::new(),
+            input_schema: serde_json::Value::Null,
+            tool_type: None,
+            allowed_callers: None,
+            defer_loading: None,
+            input_examples: None,
+            strict: None,
+            cache_control: None,
+        }
+    }
+
+    #[test]
+    fn same_prefix_produces_same_fingerprint() {
+        let a = PrefixFingerprint::compute("hello world", None);
+        let b = PrefixFingerprint::compute("hello world", None);
+        assert_eq!(a.combined_sha256, b.combined_sha256);
+    }
+
+    #[test]
+    fn different_system_produces_different_fingerprint() {
+        let a = PrefixFingerprint::compute("hello", None);
+        let b = PrefixFingerprint::compute("world", None);
+        assert_ne!(a.combined_sha256, b.combined_sha256);
+    }
+
+    #[test]
+    fn tool_order_does_not_affect_fingerprint() {
+        let tools_a = vec![make_tool("read_file"), make_tool("write_file")];
+        let tools_b = vec![make_tool("write_file"), make_tool("read_file")];
+        let a = PrefixFingerprint::compute("system", Some(&tools_a));
+        let b = PrefixFingerprint::compute("system", Some(&tools_b));
+        assert_eq!(a.combined_sha256, b.combined_sha256);
+    }
+
+    #[test]
+    fn different_tools_produce_different_fingerprint() {
+        let tools_a = vec![make_tool("read_file")];
+        let tools_b = vec![make_tool("write_file")];
+        let a = PrefixFingerprint::compute("system", Some(&tools_a));
+        let b = PrefixFingerprint::compute("system", Some(&tools_b));
+        assert_ne!(a.combined_sha256, b.combined_sha256);
+    }
+
+    #[test]
+    fn manager_starts_stable() {
+        let mut mgr = PrefixStabilityManager::new("system prompt", None);
+        assert!(mgr.check_and_update("system prompt", None).unwrap());
+        assert_eq!(mgr.change_count(), 0);
+        assert_eq!(mgr.check_count(), 1);
+    }
+
+    #[test]
+    fn manager_detects_change() {
+        let mut mgr = PrefixStabilityManager::new("system prompt", None);
+        let result = mgr.check_and_update("different prompt", None);
+        assert!(result.is_err());
+        assert_eq!(mgr.change_count(), 1);
+        let change = mgr.last_change().unwrap();
+        assert!(change.system_changed);
+        assert!(!change.tools_changed);
+    }
+
+    #[test]
+    fn manager_detects_tool_change() {
+        let tools_a = vec![make_tool("read_file")];
+        let tools_b = vec![make_tool("write_file")];
+        let mut mgr = PrefixStabilityManager::new("system", Some(&tools_a));
+        let result = mgr.check_and_update("system", Some(&tools_b));
+        assert!(result.is_err());
+        let change = mgr.last_change().unwrap();
+        assert!(!change.system_changed);
+        assert!(change.tools_changed);
+    }
+
+    #[test]
+    fn manager_re_pins_after_change() {
+        let mut mgr = PrefixStabilityManager::new("old", None);
+        let _ = mgr.check_and_update("new", None);
+        // After re-pin, the new "new" should be stable.
+        assert!(mgr.check_and_update("new", None).unwrap());
+        assert_eq!(mgr.change_count(), 1);
+    }
+
+    #[test]
+    fn stability_ratio_is_one_for_no_changes() {
+        let mut mgr = PrefixStabilityManager::new("hello", None);
+        mgr.check_and_update("hello", None).unwrap();
+        mgr.check_and_update("hello", None).unwrap();
+        assert!((mgr.stability_ratio() - 1.0).abs() < f64::EPSILON);
+        assert_eq!(mgr.check_count(), 2);
+        assert_eq!(mgr.change_count(), 0);
+    }
+
+    #[test]
+    fn stability_ratio_reflects_change_rate() {
+        let mut mgr = PrefixStabilityManager::new("hello", None);
+        mgr.check_and_update("hello", None).unwrap(); // check 1: stable
+        let _ = mgr.check_and_update("world", None); // check 2: changed
+        mgr.check_and_update("world", None).unwrap(); // check 3: stable
+        // 2 stable out of 3 checks = 0.666...
+        // (check_count=0 at start, so 3 checks: 3 checks - 1 change = 2 stable)
+        assert!((mgr.stability_ratio() - 2.0 / 3.0).abs() < 0.01);
+        assert_eq!(mgr.check_count(), 3);
+        assert_eq!(mgr.change_count(), 1);
+    }
+
+    #[test]
+    fn empty_tools_and_none_tools_produce_same_hash() {
+        let empty = PrefixFingerprint::compute("system", Some(&[]));
+        let none = PrefixFingerprint::compute("system", None);
+        // Both should produce sha256(b"") for the tool component
+        assert_eq!(empty.tools_sha256, none.tools_sha256);
+    }
+
+    #[test]
+    fn empty_system_produces_sha256_of_empty_string() {
+        let fp = PrefixFingerprint::compute("", None);
+        let expected = sha256_hex(b"");
+        assert_eq!(fp.system_sha256, expected);
+    }
+
+    #[test]
+    fn prefix_change_description_is_informative() {
+        let old = PrefixFingerprint::compute("old", None);
+        let new = PrefixFingerprint::compute("new", None);
+        let change = PrefixChange {
+            old,
+            new,
+            system_changed: true,
+            tools_changed: false,
+        };
+        assert_eq!(
+            change.description(),
+            "prefix cache invalidated: system prompt changed"
+        );
+        assert_eq!(change.label(), "sys");
+    }
+
+    #[test]
+    fn new_unpinned_has_no_change_history() {
+        let mut mgr = PrefixStabilityManager::new_unpinned();
+        assert!(mgr.pinned_fingerprint().is_none());
+        assert!(mgr.current_fingerprint().is_none());
+        assert!(mgr.last_change().is_none());
+        assert_eq!(mgr.change_count(), 0);
+        assert_eq!(mgr.check_count(), 0);
+        // First check should pin automatically and count as a check.
+        assert!(mgr.check_and_update("hello", None).unwrap());
+        assert!(mgr.pinned_fingerprint().is_some());
+        assert_eq!(mgr.check_count(), 1);
+    }
+
+    #[test]
+    fn system_prompt_text_returns_empty_for_none() {
+        assert_eq!(system_prompt_text(None), "");
+    }
+}
diff --git a/crates/tui/src/tui/app.rs b/crates/tui/src/tui/app.rs
index 0ff37b51..73fdb97d 100644
--- a/crates/tui/src/tui/app.rs
+++ b/crates/tui/src/tui/app.rs
@@ -1084,6 +1084,16 @@ pub struct App {
     /// Used by `/cycles` and `/cycle <n>` slash commands.
     pub cycle_briefings: Vec<CycleBriefing>,
 
+    // === Prefix-Cache Stability Tracking ===
+    /// Number of times the prefix (system prompt + tool specs) has changed.
+    pub prefix_change_count: u64,
+    /// Total number of prefix stability checks performed.
+    pub prefix_checks_total: u64,
+    /// Current prefix stability percentage, if known.
+    pub prefix_stability_pct: Option<u32>,
+    /// Description of the last prefix change, if any.
+    pub last_prefix_change_desc: Option<String>,
+
     /// Active cycle configuration (token threshold, briefing cap, per-model
     /// overrides). Loaded from config and forwarded to the engine.
     pub cycle: CycleConfig,
@@ -1582,6 +1592,10 @@ impl App {
             quit_armed_until: None,
             cycle_count: 0,
             cycle_briefings: Vec::new(),
+            prefix_change_count: 0,
+            prefix_checks_total: 0,
+            prefix_stability_pct: None,
+            last_prefix_change_desc: None,
             cycle: CycleConfig::default(),
             collapsed_cells: HashSet::new(),
             collapsed_cell_map: Vec::new(),
diff --git a/crates/tui/src/tui/ui.rs b/crates/tui/src/tui/ui.rs
index 42ec28d0..4d24426f 100644
--- a/crates/tui/src/tui/ui.rs
+++ b/crates/tui/src/tui/ui.rs
@@ -1449,6 +1449,21 @@ async fn run_event_loop(
                     EngineEvent::CoherenceState { state, .. } => {
                         app.coherence_state = state;
                     }
+                    EngineEvent::PrefixCacheChange {
+                        description,
+                        stability_pct,
+                        changed,
+                        ..
+                    } => {
+                        app.prefix_checks_total = app.prefix_checks_total.saturating_add(1);
+                        app.prefix_stability_pct = Some(stability_pct);
+                        if changed {
+                            app.prefix_change_count = app.prefix_change_count.saturating_add(1);
+                            if !description.is_empty() {
+                                app.last_prefix_change_desc = Some(description);
+                            }
+                        }
+                    }
                     EngineEvent::CapacityDecision { .. } => {
                         // Telemetry-only event. Surface actual interventions and failures
                         // instead of replacing the footer with no-op guardrail chatter.
@@ -3472,6 +3487,37 @@ fn format_cache_warmup_result(usage: &Usage) -> String {
     )
 }
 
+/// Format prefix stability info for the TUI footer chip.
+fn format_prefix_stability_chip(app: &App) -> Option<(String, ratatui::style::Color)> {
+    let pct = app.prefix_stability_pct?;
+    let changes = app.prefix_change_count;
+
+    let color = if changes == 0 {
+        // Perfect stability: green
+        ratatui::style::Color::Green
+    } else if pct >= 95 {
+        // Excellent: green
+        ratatui::style::Color::Green
+    } else if pct >= 80 {
+        // Good: yellow
+        ratatui::style::Color::Yellow
+    } else {
+        // Poor: red — cache is churning
+        ratatui::style::Color::Red
+    };
+
+    let label = if changes == 0 {
+        format!("P {pct}%")
+    } else {
+        format!(
+            "P {pct}% ({changes} change{})",
+            if changes == 1 { "" } else { "s" }
+        )
+    };
+
+    Some((label, color))
+}
+
 fn format_available_models_message(current_model: &str, models: &[String]) -> String {
     let mut lines = vec![format!("Available models ({})", models.len())];
     for model in models {
@@ -5241,10 +5287,25 @@ async fn apply_command_result(
                 app.status_message = Some("Warming DeepSeek cache...".to_string());
                 match run_cache_warmup(app, config).await {
                     Ok(usage) => {
-                        let message = format_cache_warmup_result(&usage);
-                        app.add_message(HistoryCell::System {
-                            content: message.clone(),
-                        });
+                        let mut message = format_cache_warmup_result(&usage);
+                        // Append prefix-cache stability info.
+                        if app.prefix_checks_total > 0 {
+                            let changes = app.prefix_change_count;
+                            let total = app.prefix_checks_total;
+                            let stable = total.saturating_sub(changes);
+                            let pct = app
+                                .prefix_stability_pct
+                                .map(|p| format!("{p}%"))
+                                .unwrap_or_else(|| "--".to_string());
+                            message.push_str(&format!(
+                                "\n\nPrefix stability: {pct} ({stable}/{total} checks stable, {changes} change{})",
+                                if changes == 1 { "" } else { "s" }
+                            ));
+                            if let Some(ref desc) = app.last_prefix_change_desc {
+                                message.push_str(&format!("\nLast prefix change: {desc}"));
+                            }
+                        }
+                        app.add_message(HistoryCell::System { content: message });
                         app.status_message = Some("Cache warmup complete".to_string());
                     }
                     Err(error) => {
@@ -8038,20 +8099,29 @@ fn should_show_footer_cost(displayed_cost: f64) -> bool {
 fn footer_auxiliary_spans(app: &App, max_width: usize) -> Vec<Span<'static>> {
     // Context % is already shown in the header signal bar — don't
     // duplicate it in the footer. The footer carries unique info only:
-    // coherence, in-flight sub-agents, reasoning replay tokens, cache hit
-    // rate, and session cost.
+    // prefix stability, coherence, in-flight sub-agents, reasoning
+    // replay tokens, cache hit rate, and session cost.
     let coherence_spans = footer_coherence_spans(app);
     let agents_spans =
         crate::tui::widgets::footer_agents_chip(running_agent_count(app), app.ui_locale);
     let replay_spans = footer_reasoning_replay_spans(app);
     let cache_spans = footer_cache_spans(app);
     let cost_spans = footer_cost_spans(app);
+    let prefix_spans = app
+        .prefix_stability_pct
+        .map(|_| {
+            let (label, color) = format_prefix_stability_chip(app)
+                .unwrap_or(("P --".to_string(), ratatui::style::Color::DarkGray));
+            vec![Span::styled(label, Style::default().fg(color))]
+        })
+        .unwrap_or_default();
 
     let parts: Vec<&Vec<Span<'static>>> = [
         &coherence_spans,
         &agents_spans,
         &replay_spans,
         &cache_spans,
+        &prefix_spans,
         &cost_spans,
     ]
     .iter()