diff --git a/crates/specialized/rtx-cfd/tests/embedded3_added_mass.rs b/crates/specialized/rtx-cfd/tests/embedded3_added_mass.rs
index 45189c4..431292c 100644
--- a/crates/specialized/rtx-cfd/tests/embedded3_added_mass.rs
+++ b/crates/specialized/rtx-cfd/tests/embedded3_added_mass.rs
@@ -167,7 +167,7 @@ fn cut_cell_added_mass() {
     let (cs, cc, c0) = fit(&op);
     let (cs_r, cc_r, _) = fit(&rec);
     println!(
-        "  ADDED MASS n = {n}: C_m operator {:.4} (reconstructed {:.4}) vs Stokes {:.3}; viscous {:.4} (reconstructed {:.4}) vs {:.3}; offset {:+.3e}; worst residual {worst:.1e}; {:.0} s",
+        "  ADDED MASS n = {n}: C_m operator {:.4} (reconstructed {:.4}) vs Stokes {:.3} unbounded / 1.135 confined (potential flow 1.071 in the unit box + 0.064); viscous {:.4} (reconstructed {:.4}) vs {:.3}; offset {:+.3e}; worst residual {worst:.1e}; {:.0} s",
         cs / scale,
         cs_r / scale,
         1.0 + stokes,
diff --git a/crates/specialized/rtx-cfd/tests/overset_added_mass.rs b/crates/specialized/rtx-cfd/tests/overset_added_mass.rs
index 4d78f4b..4253d6d 100644
--- a/crates/specialized/rtx-cfd/tests/overset_added_mass.rs
+++ b/crates/specialized/rtx-cfd/tests/overset_added_mass.rs
@@ -109,7 +109,7 @@ async fn oscillating_cylinder_added_mass_on_the_moving_patch() -> CfdResult<()>
     let dt = period / steps_per_period as f64;
     let beta = R * R * OMEGA / NU;
     println!(
-        "  n = {n}, h = {h:.4}, patch inner edge {hp:.4}, dt = {dt:.3e} ({steps_per_period} per period), A/r = {:.3}, KC = {:.3}, β = {beta:.0}: Stokes C_m ≈ {:.3}",
+        "  n = {n}, h = {h:.4}, patch inner edge {hp:.4}, dt = {dt:.3e} ({steps_per_period} per period), A/r = {:.3}, KC = {:.3}, β = {beta:.0}: Stokes C_m ≈ {:.3} (unbounded; the unit box confines the cylinder: potential-flow C_m 1.071 + Stokes 0.064 = 1.135, R2-b 2026-09-21)",
         AMP / R,
         2.0 * PI * AMP / R,
         1.0 + 4.0 / (PI * beta).sqrt()
diff --git a/crates/specialized/rtx-fsi/tests/fsi2_harness/overset_march.rs b/crates/specialized/rtx-fsi/tests/fsi2_harness/overset_march.rs
index e810a7f..d7b4bbf 100644
--- a/crates/specialized/rtx-fsi/tests/fsi2_harness/overset_march.rs
+++ b/crates/specialized/rtx-fsi/tests/fsi2_harness/overset_march.rs
@@ -376,6 +376,16 @@ pub fn run_march_overset(case: BenchmarkCase, config: &OversetMarchConfig) -> Ov
         writeln!(f, "t,ux,uy,drag,lift").unwrap();
         f
     });
+    // R4 probe (2026-09-21): every step's RAW force (the CSV above carries
+    // 10-step medians) with the accepted fluid step's overlap defects —
+    // `RTX_FSI2O_STEP_CSV=<path>`: t, ux, uy, drag, lift, Σ|div| on the
+    // patch acceptors, Σ|div| on the background fringe, the patch's max
+    // cell imbalance.
+    let mut step_csv = std::env::var("RTX_FSI2O_STEP_CSV").ok().map(|p| {
+        let mut f = std::fs::File::create(p).expect("step csv");
+        writeln!(f, "t,ux,uy,drag,lift,defect_patch,defect_bg,patch_div").unwrap();
+        f
+    });
     let (t_fluid, t_structure) = (std::cell::Cell::new(0.0_f64), std::cell::Cell::new(0.0_f64));
     // PERF-2 P0: the FEA predictor step (outside both buckets before).
     let t_predictor = std::cell::Cell::new(0.0_f64);
@@ -560,6 +570,14 @@ pub fn run_march_overset(case: BenchmarkCase, config: &OversetMarchConfig) -> Ov
         ux_series.push(ux);
         uy_series.push(uy);
         let (drag_now, lift_now) = fluid.borrow().measure_force();
+        if let Some(f) = step_csv.as_mut() {
+            let (dp, db, pd) = fluid.borrow().last_defects.get();
+            writeln!(
+                f,
+                "{t_now:.6},{ux:.6e},{uy:.6e},{drag_now:.6e},{lift_now:.6e},{dp:.3e},{db:.3e},{pd:.3e}"
+            )
+            .unwrap();
+        }
         interval_drag.push(drag_now);
         interval_lift.push(lift_now);
         if (step + 1) % 10 == 0 {