diff --git a/AGENTS.md b/AGENTS.md
index bcd2ca6..77b0170 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -177,6 +177,13 @@ The circuit list editor must remain a custom spreadsheet-like component with con
 
 Avoid permanent Bootstrap form controls inside table cells. Table cells should show static text by default and switch to inputs only while editing.
 
+## Browser Runtime
+
+The web UI is also opened over plain HTTP from other machines in the LAN, which
+is not a secure context. Do not call secure-context-only browser APIs such as
+`crypto.randomUUID()` directly; create client-side ids with `createClientId()`
+from `src/frontend/utils/client-id.ts`.
+
 ## Drag-and-Drop Rules
 
 Dragging from the circuit identifier / circuit handle moves the whole circuit.
diff --git a/src/frontend/components/circuit-tree-editor.tsx b/src/frontend/components/circuit-tree-editor.tsx
index 03aa38d..8dc993e 100644
--- a/src/frontend/components/circuit-tree-editor.tsx
+++ b/src/frontend/components/circuit-tree-editor.tsx
@@ -76,6 +76,7 @@ import {
   type MutableDistributionBoardComponentRole,
 } from "../utils/distribution-board-component-editing";
 import { loadCircuitEditorSnapshot } from "../utils/circuit-editor-history";
+import { createClientId } from "../utils/client-id";
 import type {
   CellKey,
   ColumnDef,
@@ -977,7 +978,7 @@ export function CircuitTreeEditor(props: { projectId: string; circuitListId: str
     defaultSortOrder: number
   ) {
     return buildCircuitDeviceRowInsertSnapshot({
-      id: crypto.randomUUID(),
+      id: createClientId(),
       circuitId,
       values,
       defaultSortOrder,
@@ -1001,7 +1002,7 @@ export function CircuitTreeEditor(props: { projectId: string; circuitListId: str
       ),
       data
     );
-    const circuitId = crypto.randomUUID();
+    const circuitId = createClientId();
     const deviceRows = deviceRowValues.map((row, index) =>
       createDeviceRowSnapshot(circuitId, row, (index + 1) * 10)
     );
@@ -1169,7 +1170,7 @@ export function CircuitTreeEditor(props: { projectId: string; circuitListId: str
                         (section) => section.id === intent.sectionId
                       )?.components ?? [];
                 const snapshot = buildDistributionBoardComponentSnapshot({
-                  id: crypto.randomUUID(),
+                  id: createClientId(),
                   circuitListId,
                   role: intent.role,
                   sectionId: intent.sectionId,
@@ -1234,7 +1235,7 @@ export function CircuitTreeEditor(props: { projectId: string; circuitListId: str
                 projectId,
                 getExpectedProjectRevision(),
                 buildNewCircuitGroupSnapshot({
-                  id: crypto.randomUUID(),
+                  id: createClientId(),
                   circuitListId,
                   category: values.category,
                   displayName: values.displayName,
diff --git a/src/frontend/utils/client-id.ts b/src/frontend/utils/client-id.ts
new file mode 100644
index 0000000..f3804d5
--- /dev/null
+++ b/src/frontend/utils/client-id.ts
@@ -0,0 +1,27 @@
+/**
+ * Creates a random RFC 4122 version 4 UUID for new entities built in the
+ * browser.
+ *
+ * crypto.randomUUID() exists only in secure contexts (HTTPS or localhost).
+ * The app is also opened over plain HTTP from other machines in the LAN, where
+ * it is undefined, so fall back to crypto.getRandomValues(), which is
+ * available in every context.
+ */
+export function createClientId(
+  source: Pick<Crypto, "getRandomValues"> & Partial<Pick<Crypto, "randomUUID">> = crypto
+): string {
+  if (typeof source.randomUUID === "function") {
+    return source.randomUUID();
+  }
+  const bytes = source.getRandomValues(new Uint8Array(16));
+  bytes[6] = (bytes[6] & 0x0f) | 0x40; // version 4
+  bytes[8] = (bytes[8] & 0x3f) | 0x80; // RFC 4122 variant
+  const hex = Array.from(bytes, (byte) => byte.toString(16).padStart(2, "0")).join("");
+  return [
+    hex.slice(0, 8),
+    hex.slice(8, 12),
+    hex.slice(12, 16),
+    hex.slice(16, 20),
+    hex.slice(20),
+  ].join("-");
+}
diff --git a/tests/client-id.test.ts b/tests/client-id.test.ts
new file mode 100644
index 0000000..95baa60
--- /dev/null
+++ b/tests/client-id.test.ts
@@ -0,0 +1,45 @@
+import assert from "node:assert/strict";
+import { webcrypto } from "node:crypto";
+import { describe, it } from "node:test";
+import { z } from "zod";
+import { createClientId } from "../src/frontend/utils/client-id.js";
+
+const uuidV4 = /^[0-9a-f]{8}-[0-9a-f]{4}-4[0-9a-f]{3}-[89ab][0-9a-f]{3}-[0-9a-f]{12}$/;
+
+describe("client id", () => {
+  it("uses crypto.randomUUID when the context provides it", () => {
+    const id = createClientId({
+      getRandomValues: () => {
+        throw new Error("fallback must not be used");
+      },
+      randomUUID: () => "11111111-2222-4333-8444-555555555555",
+    });
+    assert.equal(id, "11111111-2222-4333-8444-555555555555");
+  });
+
+  it("falls back to getRandomValues outside a secure context", () => {
+    const insecure = {
+      getRandomValues: webcrypto.getRandomValues.bind(webcrypto),
+    } as Pick<Crypto, "getRandomValues">;
+    const ids = new Set<string>();
+    for (let index = 0; index < 200; index += 1) {
+      const id = createClientId(insecure);
+      assert.match(id, uuidV4);
+      assert.equal(z.string().uuid().safeParse(id).success, true);
+      ids.add(id);
+    }
+    assert.equal(ids.size, 200);
+  });
+
+  it("sets version and variant bits regardless of the random bytes", () => {
+    const allOnes = {
+      getRandomValues: <T extends ArrayBufferView | null>(array: T) => {
+        if (array instanceof Uint8Array) {
+          array.fill(0xff);
+        }
+        return array;
+      },
+    } as Pick<Crypto, "getRandomValues">;
+    assert.equal(createClientId(allOnes), "ffffffff-ffff-4fff-bfff-ffffffffffff");
+  });
+});