diff --git a/crates/tui/src/models.rs b/crates/tui/src/models.rs
index c2cdc842..df096e0f 100644
--- a/crates/tui/src/models.rs
+++ b/crates/tui/src/models.rs
@@ -268,6 +268,48 @@ pub fn compaction_threshold_for_model(model: &str) -> usize {
     usize::try_from(threshold).unwrap_or(DEFAULT_COMPACTION_TOKEN_THRESHOLD)
 }
 
+/// Mode-aware soft context caps for V4 models.
+///
+/// DeepSeek V4 paper Figure 9 shows retrieval quality (MRCR MMR) collapses as
+/// context grows: 0.90 at 8K, 0.94 at 32K, 0.92 at 128K, 0.66 at 512K, 0.49
+/// at 1M. The paper's own eval harness uses budget tiers per §5.3.1:
+///
+/// | Mode / Reasoning tier | Soft cap | Paper eval window |
+/// |-----------------------|----------|-------------------|
+/// | Plan / Non-Think (off) |   64,000 |       8K-128K   |
+/// | Agent / High          |  192,000 |         128K     |
+/// | YOLO / Max            |  384,000 |      384K-512K   |
+///
+/// These caps keep the agent inside the regime DeepSeek tuned for, triggering
+/// compaction *before* recall quality degrades. The 1M hard ceiling remains —
+/// users can override via config or by declining the /compact suggestion.
+pub const V4_PLAN_SOFT_CAP: usize = 64_000;
+pub const V4_AGENT_SOFT_CAP: usize = 192_000;
+pub const V4_YOLO_SOFT_CAP: usize = 384_000;
+
+/// Compaction threshold keyed by model and caller-supplied effort tier.
+///
+/// For V4-family models the threshold is a mode-aware soft cap (see constants
+/// above). For all other models the legacy 80%-of-window rule applies.
+#[must_use]
+pub fn compaction_threshold_for_model_and_effort(
+    model: &str,
+    reasoning_effort: Option<&str>,
+) -> usize {
+    let lower = model.to_lowercase();
+    if !lower.contains("deepseek") || !(lower.contains("v4") || is_current_deepseek_v4_alias(&lower))
+    {
+        return compaction_threshold_for_model(model);
+    }
+
+    match reasoning_effort.map(str::trim).filter(|s| !s.is_empty()) {
+        Some("off" | "disabled" | "none" | "false") => V4_PLAN_SOFT_CAP,
+        Some("low" | "medium" | "high") => V4_AGENT_SOFT_CAP,
+        Some("max" | "maximum" | "xhigh") => V4_YOLO_SOFT_CAP,
+        _ => V4_AGENT_SOFT_CAP,
+    }
+}
+
 /// Derive a compaction message-count threshold from model context window.
 #[must_use]
 pub fn compaction_message_threshold_for_model(model: &str) -> usize {
@@ -446,12 +488,50 @@ mod tests {
 
     #[test]
     fn compaction_scales_for_deepseek_v4_1m_context() {
-        // 80% of 1M = 800k tokens before token-based compaction.
         assert_eq!(compaction_threshold_for_model("deepseek-v4-pro"), 800_000);
-        // 1M / 500 = 2k messages before message-count compaction.
         assert_eq!(
             compaction_message_threshold_for_model("deepseek-v4-pro"),
             2_000
         );
     }
+
+    #[test]
+    fn v4_mode_aware_soft_caps() {
+        assert_eq!(
+            compaction_threshold_for_model_and_effort("deepseek-v4-pro", Some("off")),
+            V4_PLAN_SOFT_CAP
+        );
+        assert_eq!(
+            compaction_threshold_for_model_and_effort("deepseek-v4-pro", Some("high")),
+            V4_AGENT_SOFT_CAP
+        );
+        assert_eq!(
+            compaction_threshold_for_model_and_effort("deepseek-v4-pro", Some("max")),
+            V4_YOLO_SOFT_CAP
+        );
+    }
+
+    #[test]
+    fn v4_soft_caps_only_apply_to_v4_models() {
+        assert_eq!(
+            compaction_threshold_for_model_and_effort("deepseek-v3.2-128k", Some("max")),
+            102_400
+        );
+        assert_eq!(
+            compaction_threshold_for_model_and_effort("unknown-model", Some("max")),
+            50_000
+        );
+    }
+
+    #[test]
+    fn v4_soft_cap_defaults_to_agent_when_effort_unknown() {
+        assert_eq!(
+            compaction_threshold_for_model_and_effort("deepseek-v4-pro", None),
+            V4_AGENT_SOFT_CAP
+        );
+        assert_eq!(
+            compaction_threshold_for_model_and_effort("deepseek-v4-pro", Some("unknown")),
+            V4_AGENT_SOFT_CAP
+        );
+    }
 }
diff --git a/crates/tui/src/tui/app.rs b/crates/tui/src/tui/app.rs
index 3488a3de..c8a0b38d 100644
--- a/crates/tui/src/tui/app.rs
+++ b/crates/tui/src/tui/app.rs
@@ -14,6 +14,7 @@ use crate::core::coherence::CoherenceState;
 use crate::hooks::{HookContext, HookEvent, HookExecutor, HookResult};
 use crate::models::{
     Message, SystemPrompt, compaction_message_threshold_for_model, compaction_threshold_for_model,
+    compaction_threshold_for_model_and_effort,
 };
 use crate::palette::{self, UiTheme};
 use crate::settings::Settings;
@@ -622,7 +623,10 @@ impl App {
         let max_input_history = settings.max_input_history;
         let ui_theme = palette::ui_theme(&settings.theme);
         let model = settings.default_model.clone().unwrap_or(model);
-        let compact_threshold = compaction_threshold_for_model(&model);
+        let compact_threshold = compaction_threshold_for_model_and_effort(
+            &model,
+            config.reasoning_effort(),
+        );
 
         // Start in YOLO mode if --yolo flag was passed
         let preferred_mode = AppMode::from_setting(&settings.default_mode);
@@ -1390,7 +1394,10 @@ impl App {
     }
 
     pub fn update_model_compaction_budget(&mut self) {
-        self.compact_threshold = compaction_threshold_for_model(&self.model);
+        self.compact_threshold = compaction_threshold_for_model_and_effort(
+            &self.model,
+            self.reasoning_effort.api_value(),
+        );
     }
 
     pub fn compaction_config(&self) -> CompactionConfig {