diff --git a/crates/perry-codegen/src/collectors/segview.rs b/crates/perry-codegen/src/collectors/segview.rs
index 90da00a26f..990f31b646 100644
--- a/crates/perry-codegen/src/collectors/segview.rs
+++ b/crates/perry-codegen/src/collectors/segview.rs
@@ -1221,7 +1221,26 @@ fn rewrite_site(list: &mut Vec<Stmt>, i: usize, site: &SegmentForOfSite, fresh:
             pick(cur, Expr::Undefined, decline_iter),
         ),
     );
-    3
+    // Clear the cursor at loop exit. The cursor local is declared in the
+    // ENCLOSING statement list, not inside the loop, so without this its slot
+    // stays a live GC root until the function returns. A cursor that spans a
+    // minor while the loop runs is promoted, and because it holds the input
+    // string in a traced slot it drags that string into the old generation
+    // with it — one per `open`, and `string-width` is entered thousands of
+    // times per reply. That is a candidate mechanism for I4 settling 45-65 MB
+    // ABOVE I3 after idle despite winning 20-50 MB of peak.
+    //
+    // One unconditional clear covers both paths: on the declined path the
+    // local holds `0.0`, a number, so clearing it is a no-op. `break` reaches
+    // this statement; `return` inside the body pops the frame, which is
+    // equally fine. It does not prevent promotion DURING the loop — nothing
+    // in the compiler can, since the cursor is genuinely live there — it stops
+    // the slot from keeping a dead cursor rooted for the rest of the function.
+    list.insert(
+        i + 5,
+        Stmt::Expr(Expr::LocalSet(cur, Box::new(Expr::Undefined))),
+    );
+    4
 }
 
 fn unwrap_for_mut(s: &mut Stmt) -> &mut Stmt {
diff --git a/crates/perry-codegen/src/collectors/segview_tests.rs b/crates/perry-codegen/src/collectors/segview_tests.rs
index 51b7ae71d5..9783faf640 100644
--- a/crates/perry-codegen/src/collectors/segview_tests.rs
+++ b/crates/perry-codegen/src/collectors/segview_tests.rs
@@ -509,3 +509,46 @@ fn a_site_with_an_unanswerable_use_stays_on_v1() {
         "v1 does not rewrite uses: {out}"
     );
 }
+
+/// The cursor local is declared in the enclosing statement list, so its slot is
+/// a live GC root until the function returns unless the lowering clears it. A
+/// cursor promoted during the loop holds the input string in a traced slot and
+/// drags it into the old generation; leaving the slot rooted afterwards keeps a
+/// DEAD cursor doing that for the rest of the function.
+#[test]
+fn the_cursor_is_cleared_at_loop_exit() {
+    let mut m = module_with(region(cc_body()));
+    assert_eq!(segview_rewrite_module(&mut m), 1);
+
+    // Structural, not string-matched: the statement AFTER the `For` must be a
+    // `LocalSet(<cursor>, Undefined)`, and the cursor is the local the `For`'s
+    // condition tests against zero.
+    let for_idx = m
+        .init
+        .iter()
+        .position(|s| matches!(s, Stmt::For { .. }))
+        .expect("the rewritten loop");
+    let cursor_id = match &m.init[for_idx] {
+        Stmt::For {
+            condition: Some(Expr::Conditional { condition, .. }),
+            ..
+        } => match condition.as_ref() {
+            Expr::Compare { left, .. } => match left.as_ref() {
+                Expr::LocalGet(id) => *id,
+                other => panic!("expected the cursor guard, got {other:?}"),
+            },
+            other => panic!("expected a compare, got {other:?}"),
+        },
+        _ => unreachable!(),
+    };
+    match m.init.get(for_idx + 1) {
+        Some(Stmt::Expr(Expr::LocalSet(id, v))) => {
+            assert_eq!(*id, cursor_id, "the cleared local must be the cursor");
+            assert!(
+                matches!(v.as_ref(), Expr::Undefined),
+                "the cursor slot must be cleared to undefined, got {v:?}"
+            );
+        }
+        other => panic!("no cursor clear after the loop: {other:?}"),
+    }
+}
diff --git a/crates/perry/src/commands/compile/build_cache.rs b/crates/perry/src/commands/compile/build_cache.rs
index 2e50446cf8..43d9730a4f 100644
--- a/crates/perry/src/commands/compile/build_cache.rs
+++ b/crates/perry/src/commands/compile/build_cache.rs
@@ -866,6 +866,19 @@ fn eligibility(args: &CompileArgs, project_root: &Path) -> Result<(), String> {
     if std::env::var("PERRY_SEGVIEW_DIAG").is_ok() {
         return Err("segview-diag".to_string());
     }
+    // #9843: `PERRY_SEGVIEW` is NOT a diagnostic — it changes the emitted
+    // code. It is not part of the build-cache fingerprint or any object-cache
+    // key, so without this a cached build can hand back a binary compiled with
+    // the OTHER setting: compile a file with the tier on, compile it again
+    // with the tier off, and the second can be served from the first. The
+    // A/B rig's whole shape is "one compiler binary, two compiles of one
+    // source differing only in this variable", which is exactly the collision.
+    // Excluded rather than keyed because the tier is experimental and default
+    // OFF; a cache key is the right fix when it ships on, and then a stale
+    // entry cannot silently become the measurement.
+    if std::env::var("PERRY_SEGVIEW").is_ok() {
+        return Err("segview-lowering".to_string());
+    }
     if args.verify_native_regions || args.emit_attest || args.emit_sandbox {
         return Err("sidecar-or-verify".to_string());
     }