diff --git a/frontend/src/__tests__/simulation-parts.test.ts b/frontend/src/__tests__/simulation-parts.test.ts
index 15d9edc8..45aa3341 100644
--- a/frontend/src/__tests__/simulation-parts.test.ts
+++ b/frontend/src/__tests__/simulation-parts.test.ts
@@ -557,6 +557,50 @@ describe('7segment — attachEvents', () => {
     expect(unsubMock).toHaveBeenCalledTimes(8);
   });
 
+  it('re-attach after digits changes 1→4 subscribes DIG pins (agent build order)', () => {
+    // The agent adds the display with default digits=1, THEN sets digits=4,
+    // then wires, compiles (hexEpoch bump → re-attach) and runs. The cached
+    // per-element state must rebuild for the new digit count, or the
+    // re-attach keeps subscribing COM.1/COM.2 and the display stays blank
+    // until a page reload.
+    const logic = PartSimulationRegistry.get('7segment')!;
+    const el = makeElement({ values: new Array(8).fill(0), digits: 1 });
+    const sim = makeSimulator();
+    const subscribed: number[] = [];
+    sim.pinManager.onPinChange.mockImplementation((pin: number) => {
+      subscribed.push(pin);
+      return () => {};
+    });
+
+    // First attach while digits=1 (mid-build): subscribes COM pins only.
+    const pins1 = pinMap({ 'COM.1': 30, 'COM.2': 31, DIG1: 10, DIG2: 11, DIG3: 12, DIG4: 13, A: 2 });
+    const cleanup1 = logic.attachEvents!(el, sim as any, pins1);
+    expect(subscribed).toContain(30); // COM.1 (single-digit mode)
+    expect(subscribed).not.toContain(10); // DIG1 not subscribed yet
+    cleanup1();
+
+    // Property set later: digits=4. Re-attach (compile/run does this).
+    (el as any).digits = 4;
+    subscribed.length = 0;
+    const callbacks = new Map<number, (pin: number, state: boolean) => void>();
+    sim.pinManager.onPinChange.mockImplementation(
+      (pin: number, cb: (pin: number, state: boolean) => void) => {
+        subscribed.push(pin);
+        callbacks.set(pin, cb);
+        return () => {};
+      },
+    );
+    logic.attachEvents!(el, sim as any, pins1);
+    expect(subscribed).toEqual(expect.arrayContaining([10, 11, 12, 13])); // DIG1..4
+
+    // And the display actually lights: segment A high + DIG1 enabled.
+    callbacks.get(2)!(2, true);   // segment A high
+    callbacks.get(10)!(10, true); // DIG1 on → latches segments
+    const values = (el as any).values as number[];
+    expect(values.length).toBe(32); // 4 digits × 8 segments
+    expect(values[0]).toBe(1); // digit 0, segment A lit
+  });
+
   it('CLN pin drives colon + colonValue (attachEvents and onPinStateChange)', () => {
     const logic = PartSimulationRegistry.get('7segment')!;
 
diff --git a/frontend/src/simulation/parts/ChipParts.ts b/frontend/src/simulation/parts/ChipParts.ts
index 42ec58a4..639000b9 100644
--- a/frontend/src/simulation/parts/ChipParts.ts
+++ b/frontend/src/simulation/parts/ChipParts.ts
@@ -94,8 +94,20 @@ function getDigitsCount(element: HTMLElement): number {
 
 function get7SegState(element: HTMLElement): SevenSegState {
   let s = sevenSegState.get(element);
+  const digits = getDigitsCount(element);
+  // Rebuild when the digit count CHANGED since the state was created. The
+  // agent adds the display with the default digits=1 and only then sets
+  // digits=4 — the stale 1-digit state made every later attachEvents
+  // subscribe COM.1/COM.2 (which don't exist on a 4-digit part) instead of
+  // DIG1..DIG4, and since those resolvers "attached", the all-digits-on
+  // fallback never kicked in either: the display stayed permanently blank
+  // in-session and only a page reload (fresh element, digits already 4)
+  // fixed it.
+  if (s && s.digits !== digits) {
+    if (s.flushTimer !== null) clearTimeout(s.flushTimer);
+    s = undefined;
+  }
   if (!s) {
-    const digits = getDigitsCount(element);
     s = {
       digits,
       segments: [0, 0, 0, 0, 0, 0, 0, 0],