diff --git a/crates/specialized/rtx-cfd/src/solvers/incompressible/overset/mod.rs b/crates/specialized/rtx-cfd/src/solvers/incompressible/overset/mod.rs
index ca44415..d37eb0e 100644
--- a/crates/specialized/rtx-cfd/src/solvers/incompressible/overset/mod.rs
+++ b/crates/specialized/rtx-cfd/src/solvers/incompressible/overset/mod.rs
@@ -329,6 +329,14 @@ impl OversetPisoSolver {
         })
     }
 
+    /// PERF-2 P2-c: the background pressure solve's inner stop as a
+    /// multiple of the Schwarz tolerance (the constructor's 0.1; a knob for
+    /// the band-gated ladder — an accuracy change, never bit-identical).
+    pub fn set_inner_stop_multiplier(&mut self, multiplier: f64) {
+        self.background
+            .set_inner_stop_factor(multiplier * self.params.schwarz_tolerance);
+    }
+
     /// PERF-2 P2: threads for the background multigrid's red-black maps.
     pub fn set_poisson_threads(&mut self, threads: usize) {
         self.background.set_poisson_threads(threads);
diff --git a/crates/specialized/rtx-fsi/tests/fsi2_harness/overset.rs b/crates/specialized/rtx-fsi/tests/fsi2_harness/overset.rs
index 4347470..7a40330 100644
--- a/crates/specialized/rtx-fsi/tests/fsi2_harness/overset.rs
+++ b/crates/specialized/rtx-fsi/tests/fsi2_harness/overset.rs
@@ -451,6 +451,18 @@ impl OversetFluid {
             ..OversetParameters::default()
         };
         let mut solver = OversetPisoSolver::new(background, patch, (nx, ny, h, h), params)?;
+        // PERF-2 P2-c: `RTX_FSI2O_INNER_STOP=m` — the background Poisson's inner
+        // stop at m × the Schwarz tolerance (default 0.1; band-gated ladder).
+        let mut solver = solver;
+        if let Some(m) = std::env::var("RTX_FSI2O_INNER_STOP")
+            .ok()
+            .and_then(|v| v.parse::<f64>().ok())
+        {
+            solver.set_inner_stop_multiplier(m);
+            println!(
+                "  background Poisson inner stop: {m} × the Schwarz tolerance (RTX_FSI2O_INNER_STOP; default 0.1)"
+            );
+        }
         let mut field = OversetField {
             background: FlowField::new(nx, ny, h, h)?,
             patch: patch_field,