diff --git a/nodes/machineGroupControl b/nodes/machineGroupControl
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+++ b/nodes/machineGroupControl
@@ -1 +1 @@
-Subproject commit 96b84d312491687bc9afde60de8a33ef7d65635f
+Subproject commit df74ea0facd750a37ae7027dac40412079015097
diff --git a/test/mgc-overactive-demand-serialization.integration.test.js b/test/mgc-overactive-demand-serialization.integration.test.js
new file mode 100644
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+// Regression: MGC must serialize concurrent handleInput calls.
+//
+// Live observation (2026-05-09): PS sends a fresh demand% every 1 s as
+// basin level drifts. Each MGC.handleInput unconditionally calls
+// abortActiveMovements — so an in-flight pump ramp gets killed, the
+// pump's setpoint is replaced, the new ramp gets killed by the next
+// tick, and the loop never settles. Real symptom: 120
+// "Aborting active movements ..." log lines per 2 min while a single
+// pump randomly leads (138 m³/h) and the others are clamped at minFlow
+// (60 m³/h, "near_curve_edge").
+//
+// Proper design (mirrors rotatingMachine state.delayedMove): when a
+// handleInput is already in-flight (pumps still moving), save the new
+// demand to a delayed slot and return. When the in-flight dispatch
+// finishes, pick up the latest delayed demand. Latest-wins —
+// intermediate values are stomped because they were obsolete by the
+// time the pumps were ready for them.
+//
+// Fail mode this catches: any future change that re-introduces
+// concurrent handleInput entries (or removes the gate) will explode the
+// abort count and leave pumps unbalanced.
+
+const test = require('node:test');
+const assert = require('node:assert/strict');
+const { buildPlant, injectPumpPressure } = require('./lib/wiring');
+
+test('MGC serializes overactive demand — one dispatch in flight at a time, latest queued for pickup', async () => {
+  const plant = buildPlant({ initialBasinLevel: 2.6 });
+  const { mgc, pumps, restore } = plant;
+  try {
+    // Realistic ramp time so the in-flight window is wide enough that
+    // multiple PS calls land during it.
+    for (const p of pumps) {
+      p.state.config.time = { starting: 1, warmingup: 1, stopping: 1, coolingdown: 1 };
+      injectPumpPressure(p, 19620, 117720);
+    }
+    // Bring pumps to operational once so the test focuses on
+    // STEADY-STATE thrash (not startup).
+    for (const p of pumps) await p.handleInput('parent', 'execsequence', 'startup');
+
+    // Count how many times MGC actually issues an abort. Wrap the
+    // existing method so the contract is enforced regardless of
+    // implementation details.
+    let abortCount = 0;
+    const originalAbort = mgc.abortActiveMovements.bind(mgc);
+    mgc.abortActiveMovements = async (reason) => {
+      abortCount += 1;
+      return originalAbort(reason);
+    };
+
+    // Simulate PS jitter: 30 demand calls fired back-to-back without
+    // awaiting (mirrors PS._applyMachineGroupLevelControl firing into
+    // an MGC whose previous handleInput is still settling pumps).
+    // Tiny percControl drift around 14 % matches what we observed live.
+    const calls = [];
+    const baseDemand = 14;
+    for (let i = 0; i < 30; i++) {
+      const d = baseDemand + (i % 5) * 0.05;
+      // Fire-and-forget — the gate must absorb the burst.
+      calls.push(mgc.handleInput('parent', d).catch(() => {}));
+    }
+    await Promise.all(calls);
+    // Let any deferred pickup settle.
+    await new Promise((r) => setImmediate(r));
+    await new Promise((r) => setImmediate(r));
+
+    // Contract: with a serialization gate, concurrent burst yields at
+    // most TWO real dispatches (the first that wins entry, plus one
+    // queued pickup carrying the latest value). Without the gate, every
+    // call aborts → abortCount equals call count.
+    //
+    // We assert ≤ 5 to allow for legitimate sequential dispatch that
+    // span a few ticks but block the runaway-thrash mode.
+    assert.ok(abortCount <= 5,
+      `MGC issued ${abortCount} aborts for 30 concurrent demand calls — gate not serializing dispatches (live system showed 1 abort/sec / 120 per 2 min with this exact bug).`);
+
+    // Whatever combination the optimizer picks, all selected pumps
+    // must reach a non-floor ctrl. Pump_b stuck at the curve floor was
+    // the live failure — the gate fixes it because the ramp completes
+    // before the next demand starts.
+    const finalCtrls = pumps.map((p) => Number(p.state.getCurrentPosition?.()) || 0);
+    const activePumps = finalCtrls.filter((c) => c > 0);
+    if (activePumps.length >= 2) {
+      const min = Math.min(...activePumps);
+      const max = Math.max(...activePumps);
+      assert.ok(max / Math.max(min, 1) < 3,
+        `active pump ctrls=${activePumps.map((c) => c.toFixed(1))} — disparity > 3× indicates one pump clamped at curve floor (the live "138 vs 60" symptom).`);
+    }
+  } finally {
+    restore();
+  }
+});
+
+test('MGC serialization preserves latest-wins semantic — intermediate values stomped, last value applied', async () => {
+  const plant = buildPlant({ initialBasinLevel: 2.6 });
+  const { mgc, pumps, restore } = plant;
+  try {
+    for (const p of pumps) {
+      p.state.config.time = { starting: 1, warmingup: 1, stopping: 1, coolingdown: 1 };
+      injectPumpPressure(p, 19620, 117720);
+    }
+    for (const p of pumps) await p.handleInput('parent', 'execsequence', 'startup');
+
+    // Fire 10 demands quickly: 25, 50, 25, 50, ..., 100. Final must be 100.
+    const sequence = [25, 50, 25, 50, 25, 50, 25, 50, 25, 100];
+    const calls = sequence.map((d) => mgc.handleInput('parent', d).catch(() => {}));
+    await Promise.all(calls);
+    // Allow one extra event-loop turn for the deferred pickup.
+    await new Promise((r) => setImmediate(r));
+    await new Promise((r) => setImmediate(r));
+
+    // After settling, the LATEST demand (100 %) wins — pumps should be
+    // at high ctrl, not stuck on the first burst-leader value.
+    const finalCtrls = pumps.map((p) => Number(p.state.getCurrentPosition?.()) || 0);
+    const maxCtrl = Math.max(...finalCtrls);
+    assert.ok(maxCtrl > 70,
+      `latest demand was 100 % but max pump ctrl=${maxCtrl.toFixed(1)} — gate is dropping the queued value instead of picking it up.`);
+  } finally {
+    restore();
+  }
+});