diff --git a/frontend/src/__tests__/chipbus-netkey.test.ts b/frontend/src/__tests__/chipbus-netkey.test.ts
new file mode 100644
index 00000000..2f7af5fc
--- /dev/null
+++ b/frontend/src/__tests__/chipbus-netkey.test.ts
@@ -0,0 +1,153 @@
+/**
+ * Multi-chip digital bus — Phase 0 go/no-go proof (project/multichip-bus/).
+ *
+ * D-008: the cheapest falsification of the core assumption. If a shared net
+ * key does NOT make a byte written by one chip visible to another, the keying
+ * model is wrong and we stop before building the kernel. These tests prove:
+ *
+ *   1. Root cause A is fixed — two chips on one wire resolve to the SAME key.
+ *   2. The bug is real — per-endpoint syntheticChipPin keys differ.
+ *   3. Byte exchange works — a write on the driver's keys is visible
+ *      synchronously to watchers the reader registered on its own keys.
+ *   4. The flag gates it — off by default (legacy path untouched).
+ *   5. No regression — a single-chip chip-to-component net is NOT collapsed,
+ *      so rules 2/3 still own it.
+ *
+ * This is WASM-free on purpose: it exercises the resolver keying + the real
+ * PinManager fan-out directly. The full two-real-chips-light-8-LEDs milestone
+ * is verified live in the app once the flag is flipped (see 03-phases.md).
+ */
+import { describe, it, expect, beforeEach, afterEach } from 'vitest';
+import { PinManager } from '../simulation/PinManager';
+import {
+  resolveChipNetKey,
+  setChipBusEnabledForTest,
+  resetChipNetIndexForTest,
+  type ChipNetState,
+} from '../simulation/customChips/chipNets';
+import { syntheticChipPin } from '../simulation/customChips/syntheticPins';
+
+// ── Builders ─────────────────────────────────────────────────────────────────
+
+const chip = (id: string) => ({ id, metadataId: 'custom-chip' });
+const part = (id: string, metadataId: string) => ({ id, metadataId });
+const wire = (aId: string, aPin: string, bId: string, bPin: string) => ({
+  start: { componentId: aId, pinName: aPin },
+  end: { componentId: bId, pinName: bPin },
+});
+const range = (n: number) => Array.from({ length: n }, (_, i) => i);
+
+// A CPU chip and a ROM chip with D0..D7 wired straight across, no board.
+function busState(): ChipNetState {
+  return {
+    wires: range(8).map((i) => wire('cpu', `D${i}`, 'rom', `D${i}`)),
+    components: [chip('cpu'), chip('rom')],
+    boards: [],
+  };
+}
+
+describe('chipbus Phase 0 — net-identity shared key', () => {
+  beforeEach(() => {
+    setChipBusEnabledForTest(true);
+    resetChipNetIndexForTest();
+  });
+  afterEach(() => {
+    setChipBusEnabledForTest(null);
+    resetChipNetIndexForTest();
+  });
+
+  it('two chips on one wire resolve to the SAME key (root cause A fixed)', () => {
+    const state = busState();
+    const kCpu = resolveChipNetKey(state, 'cpu', 'D0');
+    const kRom = resolveChipNetKey(state, 'rom', 'D0');
+    expect(kCpu).not.toBeNull();
+    expect(kCpu).toBe(kRom);
+  });
+
+  it('distinct data lines get distinct keys (no cross-talk between D0 and D1)', () => {
+    const state = busState();
+    expect(resolveChipNetKey(state, 'cpu', 'D0')).not.toBe(
+      resolveChipNetKey(state, 'cpu', 'D1'),
+    );
+  });
+
+  it('documents the bug: per-endpoint synthetic keys differ for one net', () => {
+    expect(syntheticChipPin('cpu', 'D0')).not.toBe(syntheticChipPin('rom', 'D0'));
+  });
+
+  it('byte exchange — a write on the driver is visible synchronously to the reader', () => {
+    const state = busState();
+    const pm = new PinManager();
+
+    // Reader (ROM) registers a watcher on EACH of its resolved data-bus keys,
+    // exactly as vx_pin_watch would after the net key fix.
+    let received = 0;
+    for (const i of range(8)) {
+      const key = resolveChipNetKey(state, 'rom', `D${i}`)!;
+      pm.onPinChange(key, (_p, s) => {
+        if (s) received |= 1 << i;
+        else received &= ~(1 << i);
+      });
+    }
+
+    // Driver (CPU) writes 0xA5 onto ITS resolved keys (vx_pin_write).
+    const byte = 0xa5;
+    for (const i of range(8)) {
+      const key = resolveChipNetKey(state, 'cpu', `D${i}`)!;
+      pm.triggerPinChange(key, ((byte >> i) & 1) === 1);
+    }
+
+    // The reader latched exactly the driver's byte, within the same call stack.
+    expect(received).toBe(0xa5);
+  });
+
+  it('the same key reads back the driven level via getPinState', () => {
+    const state = busState();
+    const pm = new PinManager();
+    const driveKey = resolveChipNetKey(state, 'cpu', 'D3')!;
+    const readKey = resolveChipNetKey(state, 'rom', 'D3')!;
+    pm.triggerPinChange(driveKey, true);
+    expect(pm.getPinState(readKey)).toBe(true);
+  });
+
+  it('flag OFF (default): chip-to-chip net is NOT collapsed (legacy path)', () => {
+    setChipBusEnabledForTest(false);
+    resetChipNetIndexForTest();
+    expect(resolveChipNetKey(busState(), 'cpu', 'D0')).toBeNull();
+  });
+
+  it('chip-to-component (single chip on net) returns null — rules 2/3 preserved', () => {
+    const state: ChipNetState = {
+      wires: [wire('chip', 'LED0', 'led1', 'A')],
+      components: [chip('chip'), part('led1', 'led')],
+      boards: [],
+    };
+    expect(resolveChipNetKey(state, 'chip', 'LED0')).toBeNull();
+  });
+
+  it('a board on the net defers to board priority (returns null)', () => {
+    const state: ChipNetState = {
+      wires: [
+        wire('cpu', 'D0', 'rom', 'D0'),
+        wire('cpu', 'D0', 'uno', '7'),
+      ],
+      components: [chip('cpu'), chip('rom')],
+      boards: [{ id: 'uno', boardKind: 'arduino-uno' }],
+    };
+    expect(resolveChipNetKey(state, 'cpu', 'D0')).toBeNull();
+  });
+
+  it('three chips on one bus line all share one key', () => {
+    const state: ChipNetState = {
+      wires: [wire('cpu', 'D0', 'rom', 'D0'), wire('rom', 'D0', 'ram', 'D0')],
+      components: [chip('cpu'), chip('rom'), chip('ram')],
+      boards: [],
+    };
+    const a = resolveChipNetKey(state, 'cpu', 'D0');
+    const b = resolveChipNetKey(state, 'rom', 'D0');
+    const c = resolveChipNetKey(state, 'ram', 'D0');
+    expect(a).not.toBeNull();
+    expect(a).toBe(b);
+    expect(b).toBe(c);
+  });
+});
diff --git a/frontend/src/components/DynamicComponent.tsx b/frontend/src/components/DynamicComponent.tsx
index 9d482aeb..7d831106 100644
--- a/frontend/src/components/DynamicComponent.tsx
+++ b/frontend/src/components/DynamicComponent.tsx
@@ -26,6 +26,7 @@ import {
 } from '../simulation/PinResolver';
 import { BOARD_PIN_GROUPS } from '../simulation/spice/boardPinGroups';
 import { syntheticChipPin } from '../simulation/customChips/syntheticPins';
+import { resolveChipNetKey } from '../simulation/customChips/chipNets';
 import { getMixedModeScheduler } from '../simulation/spice/MixedModeScheduler';
 import { getBoardLogicFamily } from '../simulation/LogicFamilies';
 
@@ -169,6 +170,23 @@ function traceDetailed(
     }
   }
 
+  // No board pin reachable. Multi-chip digital bus (chipbus flag, Phase 0 of
+  // project/multichip-bus/): when this net has two or more chip endpoints and
+  // no board pin, collapse every endpoint onto ONE net-canonical synthetic key
+  // so a write on one chip is visible to another through the synchronous
+  // PinManager fan-out (fixes root cause A: per-endpoint keys never matching).
+  // resolveChipNetKey returns null when the flag is off, when a board owns the
+  // net, or when there is a single chip endpoint — so the chip-to-component
+  // rules below (2 and 3) are left exactly as-is. Scoped to depth 0 (the
+  // starting chip pin); the key is net-bound, so a pin flipping INPUT<->OUTPUT
+  // keeps the same key with no re-trace.
+  if (depth === 0) {
+    const netKey = resolveChipNetKey(state, fromId, fromPin);
+    if (netKey !== null) {
+      return { arduinoPin: netKey, crossedActiveDevice: activeSeen };
+    }
+  }
+
   // No board pin reachable. Fall back to a custom-chip pin on this net so the
   // chip can still drive / read it through the synthetic-pin PinManager key.
   if (chipNeighbour) {
diff --git a/frontend/src/simulation/customChips/chipNets.ts b/frontend/src/simulation/customChips/chipNets.ts
new file mode 100644
index 00000000..f719cb62
--- /dev/null
+++ b/frontend/src/simulation/customChips/chipNets.ts
@@ -0,0 +1,204 @@
+/**
+ * Chip-to-chip net identity — Phase 0 of the multi-chip digital bus track
+ * (see project/multichip-bus/ in the velxio-prod repo).
+ *
+ * THE PROBLEM (root cause A, 00-problem-analysis.md section 2): a digital net
+ * is keyed by ONE integer pin number in the per-board PinManager. A board pin
+ * (Uno D7 = 7) is net-symmetric — everyone on the net shares the number. But a
+ * chip-to-chip net is keyed per-endpoint by `syntheticChipPin(chipId, pinName)`,
+ * so the two chips on one wire resolve to two DIFFERENT keys and never share a
+ * net. Each chip writes into a key the other never reads.
+ *
+ * THE FIX: assign every electrically-connected net a single canonical id via
+ * union-find over the wire graph, and mint ONE shared `syntheticNetPin(netId)`
+ * for any net that has two or more chip endpoints and no board pin. Every
+ * endpoint on that net resolves to the same key, so a write on one chip is
+ * visible to another through the existing synchronous PinManager fan-out.
+ *
+ * SCOPE (D-006, never-clone boundary): this module ONLY decides the shared key
+ * for pure chip-to-chip nets. It returns null for:
+ *   - nets with a board pin  -> traceDetailed's rule 1 (board priority) handles it
+ *   - nets with <2 chip endpoints -> traceDetailed's rules 2/3 (single-chip own
+ *     synthetic) handle the chip-to-component case unchanged
+ * Board emulation never enters this path; the regression surface is the
+ * existing chip-to-component examples, gated behind the `chipbus` flag (D-007).
+ */
+import { UnionFind } from '../spice/unionFind';
+import { isBoardComponent, boardPinToNumber } from '../../utils/boardPinMapping';
+import { syntheticNetPin } from './syntheticPins';
+
+// Structural view of the slice of simulator state this module needs. The real
+// useSimulatorStore state is a superset, so it satisfies this shape directly —
+// declaring it structurally keeps the module pure and unit-testable without
+// pulling in React / the Zustand store.
+interface NetEndpointRef {
+  componentId: string;
+  pinName: string;
+}
+interface WireLike {
+  start: NetEndpointRef;
+  end: NetEndpointRef;
+}
+interface ComponentLike {
+  id: string;
+  metadataId: string;
+}
+interface BoardLike {
+  id: string;
+  boardKind: string;
+}
+export interface ChipNetState {
+  wires: readonly WireLike[];
+  components: readonly ComponentLike[];
+  boards: readonly BoardLike[];
+}
+
+// Endpoint key = `${componentId}::${pinName}`. velxio chip ids
+// (`custom_chip_<ts>_<rand>`) and chip.json pin names are identifier-like and
+// never contain `::`, so the split back to (componentId, pinName) is exact.
+const SEP = '::';
+function epKey(componentId: string, pinName: string): string {
+  return `${componentId}${SEP}${pinName}`;
+}
+function parseEpKey(key: string): { componentId: string; pinName: string } {
+  const i = key.indexOf(SEP);
+  return { componentId: key.slice(0, i), pinName: key.slice(i + SEP.length) };
+}
+
+interface NetInfo {
+  /** Lexicographically-smallest endpoint key in the net — stable canonical id
+   *  independent of union order, so the minted net pin number does not churn
+   *  between resolve passes. */
+  canonical: string;
+  /** True if any endpoint on the net is a board pin that resolves to a real
+   *  GPIO number (board priority defers to traceDetailed's rule 1). */
+  hasBoardPin: boolean;
+  /** Distinct custom-chip endpoint keys on the net. */
+  chipEndpoints: Set<string>;
+}
+
+interface ChipNetIndex {
+  /** Net representative for an endpoint key, or undefined if not on any wire. */
+  rootOf(key: string): string | undefined;
+  nets: Map<string, NetInfo>;
+}
+
+// ── Feature flag (D-007) ─────────────────────────────────────────────────────
+//
+// Off by default. Enable with `?chipbus=on` or
+// `localStorage.velxio.chipbus = 'on'`, mirroring sim-mixedmode's `?mixedmode`.
+// Guards every browser global so the module is safe under vitest/node.
+
+let testOverride: boolean | null = null;
+/** Test seam: force the flag on/off, or pass null to restore real detection. */
+export function setChipBusEnabledForTest(v: boolean | null): void {
+  testOverride = v;
+}
+
+export function chipBusEnabled(): boolean {
+  if (testOverride !== null) return testOverride;
+  try {
+    if (typeof window !== 'undefined' && window.location) {
+      const q = new URLSearchParams(window.location.search).get('chipbus');
+      if (q === 'on' || q === '1' || q === 'true') return true;
+      if (q === 'off' || q === '0' || q === 'false') return false;
+    }
+    if (typeof localStorage !== 'undefined') {
+      const v = localStorage.getItem('velxio.chipbus');
+      if (v === 'on' || v === '1' || v === 'true') return true;
+    }
+  } catch {
+    /* SecurityError on localStorage, missing globals in tests — treat as off */
+  }
+  return false;
+}
+
+// ── Net index (memoized by wire/component fingerprint) ───────────────────────
+
+let cache: { sig: string; index: ChipNetIndex } | null = null;
+
+function fingerprint(state: ChipNetState): string {
+  const w = state.wires
+    .map(
+      (x) =>
+        `${x.start.componentId}${SEP}${x.start.pinName}|${x.end.componentId}${SEP}${x.end.pinName}`,
+    )
+    .join(',');
+  const c = state.components.map((x) => `${x.id}:${x.metadataId}`).join(',');
+  const b = state.boards.map((x) => `${x.id}:${x.boardKind}`).join(',');
+  return `${w}#${c}#${b}`;
+}
+
+function buildIndex(state: ChipNetState): ChipNetIndex {
+  const uf = new UnionFind();
+  for (const wire of state.wires) {
+    const a = epKey(wire.start.componentId, wire.start.pinName);
+    const b = epKey(wire.end.componentId, wire.end.pinName);
+    uf.union(a, b);
+  }
+
+  const compById = new Map(state.components.map((c) => [c.id, c]));
+  const boardById = new Map(state.boards.map((b) => [b.id, b]));
+  const nets = new Map<string, NetInfo>();
+
+  for (const [key, root] of uf.entries()) {
+    let info = nets.get(root);
+    if (!info) {
+      info = { canonical: key, hasBoardPin: false, chipEndpoints: new Set() };
+      nets.set(root, info);
+    }
+    if (key < info.canonical) info.canonical = key;
+
+    const { componentId, pinName } = parseEpKey(key);
+    const board = boardById.get(componentId);
+    if (board || isBoardComponent(componentId)) {
+      const kind = board?.boardKind ?? componentId;
+      // A real numbered board pin (including -1 power/GND) means a board owns
+      // this net; defer to traceDetailed's board-priority rule.
+      if (boardPinToNumber(kind, pinName) !== null) info.hasBoardPin = true;
+    } else if (compById.get(componentId)?.metadataId === 'custom-chip') {
+      info.chipEndpoints.add(key);
+    }
+  }
+
+  return {
+    rootOf: (k) => (uf.has(k) ? uf.find(k) : undefined),
+    nets,
+  };
+}
+
+function getChipNetIndex(state: ChipNetState): ChipNetIndex {
+  const sig = fingerprint(state);
+  if (cache && cache.sig === sig) return cache.index;
+  const index = buildIndex(state);
+  cache = { sig, index };
+  return index;
+}
+
+/** Test seam: drop the memoized index (the fingerprint already invalidates it
+ *  on real input changes; this is only for deterministic unit tests). */
+export function resetChipNetIndexForTest(): void {
+  cache = null;
+}
+
+// ── Public resolver ──────────────────────────────────────────────────────────
+
+/**
+ * Shared net-canonical key for a chip pin on a pure chip-to-chip net, or null
+ * when the legacy resolver rules should handle it (flag off; board on the net;
+ * fewer than two chip endpoints). When non-null, EVERY endpoint of the same net
+ * gets the identical key, so writes and reads land on one PinManager slot.
+ */
+export function resolveChipNetKey(
+  state: ChipNetState,
+  componentId: string,
+  pinName: string,
+): number | null {
+  if (!chipBusEnabled()) return null;
+  const idx = getChipNetIndex(state);
+  const root = idx.rootOf(epKey(componentId, pinName));
+  if (root === undefined) return null;
+  const info = idx.nets.get(root);
+  if (!info || info.hasBoardPin || info.chipEndpoints.size < 2) return null;
+  return syntheticNetPin(info.canonical);
+}
diff --git a/frontend/src/simulation/customChips/syntheticPins.ts b/frontend/src/simulation/customChips/syntheticPins.ts
index 147551ce..2de0b0a4 100644
Binary files a/frontend/src/simulation/customChips/syntheticPins.ts and b/frontend/src/simulation/customChips/syntheticPins.ts differ