diff --git a/frontend/src/__tests__/protocol-parts.test.ts b/frontend/src/__tests__/protocol-parts.test.ts
index 55de1a24..de94b89b 100644
--- a/frontend/src/__tests__/protocol-parts.test.ts
+++ b/frontend/src/__tests__/protocol-parts.test.ts
@@ -17,6 +17,7 @@
 import { describe, it, expect, beforeEach, afterEach, vi } from 'vitest';
 import { PartSimulationRegistry } from '../simulation/parts/PartSimulationRegistry';
 import { dispatchSensorUpdate } from '../simulation/SensorUpdateRegistry';
+import { useSimulatorStore } from '../store/useSimulatorStore';
 import '../simulation/parts/ProtocolParts';
 
 // ─── Globals ──────────────────────────────────────────────────────────────────
@@ -228,31 +229,35 @@ describe('ssd1306 — protocol auto-detect', () => {
     expect(sim.addI2CDevice).not.toHaveBeenCalled();
   });
 
-  it('runs SPI when DC is wired to a GPIO', () => {
-    const sim = makeSPISim();
+  it('runs I2C when only DC is wired (DC is the I2C address-select, not SPI)', () => {
+    const sim = makeI2CSim();
     PartSimulationRegistry.get('ssd1306')!.attachEvents!(
       makeElement(),
       sim as any,
       pinMap({ DC: 4 }),
     );
-    expect(typeof sim.spi.onByte).toBe('function');
-    expect(sim.addI2CDevice).not.toHaveBeenCalled();
-  });
-
-  it('legacy ssd1306-i2c alias forces I2C even with CS wired', () => {
-    const sim = makeI2CSim();
-    PartSimulationRegistry.get('ssd1306-i2c')!.attachEvents!(
-      makeElement(),
-      sim as any,
-      pinMap({ CS: 5 }),
-    );
     expect(sim.addI2CDevice).toHaveBeenCalledOnce();
   });
 
-  it('legacy ssd1306-spi alias forces SPI even with nothing wired', () => {
-    const sim = makeSPISim();
-    PartSimulationRegistry.get('ssd1306-spi')!.attachEvents!(makeElement(), sim as any, noPins);
-    expect(typeof sim.spi.onByte).toBe('function');
+  it('honors an explicit protocol property (migrated legacy projects)', () => {
+    // A project migrated from the old ssd1306-spi entry carries protocol:'spi';
+    // it must run SPI even though nothing SPI-specific is wired (CS absent).
+    useSimulatorStore.setState({
+      components: [{ id: 'oled-legacy', metadataId: 'ssd1306', properties: { protocol: 'spi' } }],
+    } as any);
+    try {
+      const sim = makeSPISim();
+      PartSimulationRegistry.get('ssd1306')!.attachEvents!(
+        makeElement(),
+        sim as any,
+        noPins,
+        'oled-legacy',
+      );
+      expect(typeof sim.spi.onByte).toBe('function');
+      expect(sim.addI2CDevice).not.toHaveBeenCalled();
+    } finally {
+      useSimulatorStore.setState({ components: [] } as any);
+    }
   });
 });
 
diff --git a/frontend/src/simulation/parts/ProtocolParts.ts b/frontend/src/simulation/parts/ProtocolParts.ts
index fd998356..17835fc6 100644
--- a/frontend/src/simulation/parts/ProtocolParts.ts
+++ b/frontend/src/simulation/parts/ProtocolParts.ts
@@ -329,8 +329,8 @@ function attachSSD1306SPI(
 
 /**
  * Internal: SSD1306 attach logic, parameterised over the wire protocol.
- * Used by the three picker entries (the generic `ssd1306` plus the two
- * dedicated `ssd1306-i2c` / `ssd1306-spi` shortcuts).
+ * Called by the single `ssd1306` entry once the protocol has been resolved
+ * (auto-detected from the wiring, or read from an explicit `protocol` property).
  */
 function attachSSD1306(
   element: HTMLElement,
@@ -376,52 +376,43 @@ function attachSSD1306(
 }
 
 /**
- * Which wire protocol did the user build?  A real SSD1306 breakout is ONE
- * board that talks either I2C or SPI depending on how it is wired: SPI drives
- * the chip-select (CS) and data/command (DC) lines from MCU GPIOs, while I2C
- * leaves them tied to power (address select) or unconnected.  So if CS or DC is
- * wired to a GPIO we decode SPI; otherwise I2C.  This mirrors the physical part
- * — one component, no protocol switch to set, just wire it up.
+ * Which wire protocol did the user build?  A real SSD1306 breakout is ONE board
+ * that talks either I2C or SPI depending on how it is wired.  The definitive
+ * SPI-only signal is chip-select (CS): I2C never uses it.  (DC deliberately does
+ * NOT count — on the 8-pin module DC doubles as the I2C address-select/SA0 line,
+ * so many I2C circuits wire it too.)  So CS wired to a GPIO => SPI, otherwise
+ * I2C.  This mirrors the physical part — one component, no protocol switch to
+ * set, just wire it up.
  *
- * Pure wiring check: it deliberately does NOT read `simulator.spi`, whose
- * getter on some boards (RP2040, and the ESP32/STM32 bridge shims) lazily
- * re-routes the board SPI bus as a side effect and must not fire in I2C mode.
+ * Pure wiring check: it deliberately does NOT read `simulator.spi`, whose getter
+ * on some boards (RP2040, and the ESP32/STM32 bridge shims) lazily re-routes the
+ * board SPI bus as a side effect and must not fire in I2C mode.
  */
 function detectSSD1306Protocol(getPin: (n: string) => number | null): 'i2c' | 'spi' {
-  return getPin('CS') !== null || getPin('DC') !== null ? 'spi' : 'i2c';
+  return getPin('CS') !== null ? 'spi' : 'i2c';
 }
 
 /**
  * SSD1306 OLED — a single component that works on every board with an I2C or
- * SPI bus (AVR, RP2040, ESP32, STM32), auto-detecting the protocol from the
- * wiring like the physical module.  Consolidates the old ssd1306 / ssd1306-i2c
- * / ssd1306-spi picker entries into one (issues #101 / #215).
+ * SPI bus (AVR, RP2040, ESP32, STM32).  New projects just wire it up and the
+ * protocol is auto-detected from the wiring like the physical module; a
+ * `protocol` property, when present, pins it explicitly (projects migrated from
+ * the old ssd1306-i2c / ssd1306-spi entries carry it so their behaviour is
+ * preserved exactly).  Consolidates the old three picker entries into one
+ * (issues #101 / #215).
  */
 PartSimulationRegistry.register('ssd1306', {
   attachEvents: (element, simulator, getPin, componentId) => {
     const { components } = useSimulatorStore.getState();
     const comp = components.find((c) => c.id === componentId);
     const i2cAddr = parseI2cAddress(comp?.properties?.i2cAddress, 0x3c);
-    const protocol = detectSSD1306Protocol(getPin);
+    const explicit = comp?.properties?.protocol;
+    const protocol: 'i2c' | 'spi' =
+      explicit === 'i2c' || explicit === 'spi' ? explicit : detectSSD1306Protocol(getPin);
     return attachSSD1306(element, simulator, getPin, protocol, i2cAddr);
   },
 });
 
-/**
- * Legacy aliases kept ONLY for projects saved before the picker entries merged
- * into the single auto-detecting `ssd1306` above.  New projects never carry
- * these ids.  They force a fixed protocol (no auto-detect) to reproduce the old
- * behaviour exactly.
- */
-PartSimulationRegistry.register('ssd1306-i2c', {
-  attachEvents: (element, simulator, getPin) =>
-    attachSSD1306(element, simulator, getPin, 'i2c'),
-});
-PartSimulationRegistry.register('ssd1306-spi', {
-  attachEvents: (element, simulator, getPin) =>
-    attachSSD1306(element, simulator, getPin, 'spi'),
-});
-
 // ─── DS1307 RTC ──────────────────────────────────────────────────────────────
 
 /**
diff --git a/frontend/src/store/useSimulatorStore.ts b/frontend/src/store/useSimulatorStore.ts
index a60b26a4..33dcef22 100644
--- a/frontend/src/store/useSimulatorStore.ts
+++ b/frontend/src/store/useSimulatorStore.ts
@@ -1374,8 +1374,23 @@ export const useSimulatorStore = create<SimulatorState>((set, get) => {
       // (createFileGroup is a no-op for existing ids) — overwrite their files.
       useEditorStore.getState().replaceFileGroups(payload.fileGroups);
 
-      // Components and wires
-      setComponents(payload.components);
+      // Components and wires. Normalize the retired ssd1306-i2c / ssd1306-spi
+      // ids (merged into the single auto-detecting `ssd1306`, issues #101/#215)
+      // so old .vlx files and pre-migration snapshots still render and simulate;
+      // the old id's protocol is pinned so behaviour is preserved exactly.
+      const normalizedComponents = payload.components.map((c) =>
+        c.metadataId === 'ssd1306-i2c' || c.metadataId === 'ssd1306-spi'
+          ? {
+              ...c,
+              metadataId: 'ssd1306',
+              properties: {
+                protocol: c.metadataId === 'ssd1306-spi' ? 'spi' : 'i2c',
+                ...(c.properties ?? {}),
+              },
+            }
+          : c,
+      );
+      setComponents(normalizedComponents);
       setWires(payload.wires);
 
       // Active board: prefer the saved one, fall back to the first.