diff --git a/crates/tui/src/core/engine/tests.rs b/crates/tui/src/core/engine/tests.rs
index 10a27e28a..6e730f077 100644
--- a/crates/tui/src/core/engine/tests.rs
+++ b/crates/tui/src/core/engine/tests.rs
@@ -1586,6 +1586,41 @@ fn non_yolo_mode_retains_default_defer_policy() {
     assert!(should_default_defer_tool("git_blame", &always_load));
 }
 
+#[test]
+fn default_defer_lookup_matches_linear_scan_over_active_native_tools() {
+    // Parity guard for #4152: `should_default_defer_tool` now consults an O(1)
+    // side set built from DEFAULT_ACTIVE_NATIVE_TOOLS instead of a linear
+    // `.iter().any(...)` scan. Assert the set returns the SAME hit/miss as an
+    // explicit linear scan over the ordered array — every array member is a hit
+    // (not deferred); names outside the array miss (deferred by default).
+    let always_load = HashSet::new();
+    let active = default_active_native_tool_names();
+
+    for name in active {
+        // Reference linear scan == what the converted lookup must agree with.
+        let linear_hit = active.iter().any(|core| core == name);
+        assert!(linear_hit, "reference scan should find array member {name}");
+        assert!(
+            !should_default_defer_tool(name, &always_load),
+            "array member {name} must stay active (not deferred)"
+        );
+    }
+
+    for name in [
+        "git_blame",
+        "task_shell_start",
+        REQUEST_USER_INPUT_NAME,
+        "definitely_not_a_tool",
+    ] {
+        let linear_hit = active.iter().any(|core| *core == name);
+        assert!(!linear_hit, "non-member {name} should be absent from array");
+        assert!(
+            should_default_defer_tool(name, &always_load),
+            "non-member {name} must default to deferred"
+        );
+    }
+}
+
 #[test]
 fn model_tool_catalog_applies_native_and_mcp_deferral() {
     let always_load = HashSet::new();
diff --git a/crates/tui/src/core/engine/tool_catalog.rs b/crates/tui/src/core/engine/tool_catalog.rs
index 3d5e3c33e..72f591ebc 100644
--- a/crates/tui/src/core/engine/tool_catalog.rs
+++ b/crates/tui/src/core/engine/tool_catalog.rs
@@ -125,6 +125,21 @@ fn cached_fallbacks() -> &'static [CachedFallback] {
     })
 }
 
+/// Membership index over [`DEFAULT_ACTIVE_NATIVE_TOOLS`], built once for the
+/// process lifetime. The array stays the source of truth for *ordered*
+/// iteration (see [`tool_catalog_consistency_issues`] and
+/// `engine::default_active_native_tool_names`); this set only accelerates the
+/// hot membership check in [`should_default_defer_tool`], which runs once per
+/// catalog tool on every catalog rebuild (i.e. per turn) — an O(n·m) linear
+/// scan over the array collapses to O(1) hashed lookups.
+static DEFAULT_ACTIVE_NATIVE_TOOLS_SET: std::sync::OnceLock<HashSet<&'static str>> =
+    std::sync::OnceLock::new();
+
+fn default_active_native_tools_set() -> &'static HashSet<&'static str> {
+    DEFAULT_ACTIVE_NATIVE_TOOLS_SET
+        .get_or_init(|| DEFAULT_ACTIVE_NATIVE_TOOLS.iter().copied().collect())
+}
+
 pub(super) fn should_default_defer_tool(name: &str, always_load: &HashSet<String>) -> bool {
     if always_load.contains(name) {
         return false;
@@ -134,9 +149,10 @@ pub(super) fn should_default_defer_tool(name: &str, always_load: &HashSet<String
         return false;
     }
 
-    !DEFAULT_ACTIVE_NATIVE_TOOLS
-        .iter()
-        .any(|core_tool| core_tool == &name)
+    // Membership-only test (no ordering dependency): the side set built from
+    // DEFAULT_ACTIVE_NATIVE_TOOLS returns identical hit/miss results as the
+    // former `.iter().any(...)` linear scan.
+    !default_active_native_tools_set().contains(name)
 }
 
 pub(super) fn apply_native_tool_deferral(catalog: &mut [Tool], always_load: &HashSet<String>) {