diff --git a/agent_docs/simulation-engine.md b/agent_docs/simulation-engine.md
index 0f373c6..d7b0ad6 100644
--- a/agent_docs/simulation-engine.md
+++ b/agent_docs/simulation-engine.md
@@ -13,7 +13,7 @@ On every canvas state change, `simulate(nodes, edges, timeSeconds)`:
 4. **Mixed Island Detection**: Flags unsupported direct analog-to-gate connections with a warning on the analog node.
 
 ## Digital Evaluation & Relaxation (`digital.ts`)
-- Iterative relaxation loop (up to 50 iterations) to settle combinational feedback, latches, and subcircuits.
+- Iterative relaxation loop (up to 50 iterations) to settle combinational feedback, latches, and subcircuits. Pre-indexes incoming edges by target node and sorts by target handle once per run to avoid repeated linear scans.
 - Supports hierarchical subcircuit evaluation by looking up definitions in `localStorage` (`customComponents`).
 - Dispatches legacy primitive gates via `evaluateGate.ts` for backwards compatibility.
 
diff --git a/frontend/src/logic/simulation/digital.test.ts b/frontend/src/logic/simulation/digital.test.ts
index 18a785c..2712e9c 100644
--- a/frontend/src/logic/simulation/digital.test.ts
+++ b/frontend/src/logic/simulation/digital.test.ts
@@ -1,3 +1,9 @@
+/**
+ * @file digital.test.ts
+ * @description Unit tests for digital circuit evaluation and relaxation simulation,
+ * including hierarchical components, input handle ordering, and adder settling.
+ */
+
 import { runSimulation } from './digital';
 import type { DigiEdge, DigiNode } from '../../types';
 
@@ -85,4 +91,87 @@ describe('runSimulation', () => {
     const updated = runSimulation(nodes, edges);
     expect(updated.length).toBe(2);
   });
+
+  test('orders gate inputs by target handle regardless of edge list order', () => {
+    // mux: a = sel, b = data0, c = data1. Edges are deliberately listed c, b, a.
+    const nodes: DigiNode[] = [
+      { id: 'sel', type: 'input', data: { label: 'sel', value: 1 }, position: { x: 0, y: 0 } },
+      { id: 'd0', type: 'input', data: { label: 'd0', value: 0 }, position: { x: 0, y: 0 } },
+      { id: 'd1', type: 'input', data: { label: 'd1', value: 1 }, position: { x: 0, y: 0 } },
+      { id: 'mux', type: 'mux', data: { label: 'mux' }, position: { x: 0, y: 0 } },
+      { id: 'out', type: 'output', data: { label: 'out' }, position: { x: 0, y: 0 } },
+    ];
+    const edges: DigiEdge[] = [
+      { id: 'e3', source: 'd1', target: 'mux', targetHandle: 'c' },
+      { id: 'e2', source: 'd0', target: 'mux', targetHandle: 'b' },
+      { id: 'e1', source: 'sel', target: 'mux', targetHandle: 'a' },
+      { id: 'e4', source: 'mux', target: 'out' },
+    ];
+
+    expect(runSimulation(nodes, edges).find((n) => n.id === 'out')?.data.value).toBe(1);
+    nodes[0] = { ...nodes[0], data: { ...nodes[0].data, value: 0 } };
+    expect(runSimulation(nodes, edges).find((n) => n.id === 'out')?.data.value).toBe(0);
+  });
+
+  test('settles a ripple-carry adder with nodes listed in reverse dependency order', () => {
+    const bits = 8;
+    const a = 0b10110101;
+    const b = 0b01101110;
+    const nodes: DigiNode[] = [];
+    const edges: DigiEdge[] = [];
+    /**
+     * Append a directed edge targeting a handle.
+     * @param source - Source node ID
+     * @param target - Target node ID
+     * @param handle - Target handle name
+     */
+    const wire = (source: string, target: string, handle: string): void => {
+      edges.push({ id: `e${edges.length}`, source, target, targetHandle: handle } as DigiEdge);
+    };
+
+    /**
+     * Append a circuit node.
+     * @param id - Unique node ID
+     * @param type - Component type
+     * @param value - Optional initial value
+     */
+    const node = (id: string, type: string, value?: number): void => {
+      nodes.push({ id, type, data: { label: id, ...(value === undefined ? {} : { value }) }, position: { x: 0, y: 0 } } as DigiNode);
+    };
+    node('cin', 'input', 0);
+    let carry = 'cin';
+    for (let i = 0; i < bits; i++) {
+      node(`a${i}`, 'input', (a >> i) & 1);
+      node(`b${i}`, 'input', (b >> i) & 1);
+      node(`x${i}`, 'xorGate');
+      node(`s${i}`, 'xorGate');
+      node(`n1${i}`, 'andGate');
+      node(`n2${i}`, 'andGate');
+      node(`c${i}`, 'orGate');
+      node(`o${i}`, 'output');
+      wire(`a${i}`, `x${i}`, 'a');
+      wire(`b${i}`, `x${i}`, 'b');
+      wire(`x${i}`, `s${i}`, 'a');
+      wire(carry, `s${i}`, 'b');
+      wire(`a${i}`, `n1${i}`, 'a');
+      wire(`b${i}`, `n1${i}`, 'b');
+      wire(`x${i}`, `n2${i}`, 'a');
+      wire(carry, `n2${i}`, 'b');
+      wire(`n1${i}`, `c${i}`, 'a');
+      wire(`n2${i}`, `c${i}`, 'b');
+      wire(`s${i}`, `o${i}`, 'a');
+      carry = `c${i}`;
+    }
+    node('cout', 'output');
+    wire(carry, 'cout', 'a');
+
+    // Sum bits and carry-out must match integer addition in either node order.
+    for (const ordered of [nodes, [...nodes].reverse()]) {
+      const byId = new Map(runSimulation(ordered, edges).map((n) => [n.id, n.data.value]));
+      let sum = 0;
+      for (let i = 0; i < bits; i++) sum |= (byId.get(`o${i}`) as number) << i;
+      sum |= (byId.get('cout') as number) << bits;
+      expect(sum).toBe(a + b);
+    }
+  });
 });
diff --git a/frontend/src/logic/simulation/digital.ts b/frontend/src/logic/simulation/digital.ts
index 2d1d2e8..3cc951e 100644
--- a/frontend/src/logic/simulation/digital.ts
+++ b/frontend/src/logic/simulation/digital.ts
@@ -1,6 +1,6 @@
 /**
  * @file digital.ts
- * @description Digital circuit evaluation — Relaxation algorithm propagates 0/1/Z/X
+ * @description Digital circuit evaluation - Relaxation algorithm propagates 0/1/Z/X
  * logic values gate-by-gate until stable, resolving feedback loops and latches.
  * Supports hierarchical subcircuit evaluation via customComponents and
  * switch-level CMOS transistor networks (NMOS/PMOS + supplies) so hand-wired
@@ -17,10 +17,27 @@ export type SimulateCircuit = (
   timeSeconds?: number
 ) => DigiNode[];
 
+const NO_EDGES: readonly DigiEdge[] = [];
+
+/**
+ * Simulate circuit logic values across nodes and edges using relaxation evaluation.
+ * @param currentNodes - Array of circuit nodes to evaluate
+ * @param currentEdges - Array of circuit edges defining connections
+ * @param timeSeconds - Simulation elapsed time in seconds
+ * @returns Array of evaluated nodes with updated states
+ */
 export const runSimulation: SimulateCircuit = (currentNodes, currentEdges, timeSeconds = 0) => {
   return runSimulationWithCycleGuard(currentNodes, currentEdges, timeSeconds, new Set());
 };
 
+/**
+ * Relaxation evaluation loop with cycle detection for hierarchical subcircuits.
+ * @param currentNodes - Array of circuit nodes to evaluate
+ * @param currentEdges - Array of circuit edges defining connections
+ * @param timeSeconds - Simulation elapsed time in seconds
+ * @param visitedSubcircuits - Set of visited subcircuit IDs to guard against recursion cycles
+ * @returns Array of evaluated nodes with updated states
+ */
 const runSimulationWithCycleGuard = (
   currentNodes: DigiNode[],
   currentEdges: DigiEdge[],
@@ -33,6 +50,19 @@ const runSimulationWithCycleGuard = (
   }));
   const nodeMap = new Map<string, DigiNode>(newNodes.map((node) => [node.id, node]));
 
+  // Index incoming edges once per run (O(E)) instead of rescanning every edge
+  // for every node on every relaxation pass. Each list keeps edge order and is
+  // sorted by handle name ('a', 'b', 'c', ...) or index, exactly as before.
+  const incomingByTarget = new Map<string, DigiEdge[]>();
+  for (const edge of currentEdges) {
+    const list = incomingByTarget.get(edge.target);
+    if (list) list.push(edge);
+    else incomingByTarget.set(edge.target, [edge]);
+  }
+  incomingByTarget.forEach((list) => {
+    list.sort((e1, e2) => (e1.targetHandle || '').localeCompare(e2.targetHandle || ''));
+  });
+
   let changed = true;
   let iterations = 0;
   const MAX_ITERATIONS = 50; // Enough to settle combinational loops and latches
@@ -64,14 +94,7 @@ const runSimulationWithCycleGuard = (
         continue;
       }
 
-      const incomingEdges = currentEdges.filter((edge) => edge.target === node.id);
-      
-      // Sort edges by handle name ('a', 'b', 'c', ...) or index
-      incomingEdges.sort((e1, e2) => {
-        const h1 = e1.targetHandle || '';
-        const h2 = e2.targetHandle || '';
-        return h1.localeCompare(h2);
-      });
+      const incomingEdges = incomingByTarget.get(node.id) ?? NO_EDGES;
 
       const inputs = incomingEdges.map((edge) => {
         const src = nodeMap.get(edge.source);