diff --git a/viewer/README.md b/viewer/README.md
index 09b59ea..e7e068b 100644
--- a/viewer/README.md
+++ b/viewer/README.md
@@ -98,6 +98,12 @@ The ground reference grid is separate from the scientific signal lattice. Its sq
 
 `browser/reference-grid.mjs` verifies the reference grid and presentation lifecycle in Chromium against a running Vite server. It uses Playwright (`@playwright/test`) and its installed Chromium; a shared installation can be supplied through `MICROSIMULATOR_PLAYWRIGHT_MODULE` as an absolute module filename. Set `VIEWER_URL` if the server is not on `http://127.0.0.1:4320`, and `EVIDENCE_DIR` to choose the screenshot directory. The test observes renderer transforms through test-only request instrumentation and introduces no production debug interface.
 
+### Device geometry visibility
+
+Use **Show device geometry** in the Scene panel to hide all mechanical constraint meshes and their outlines. The control is disabled when the current frame contains no geometry, while its preference is retained for later frames. Visibility persists through live updates, reset, and replay seeks, and defaults to enabled on opening another dataset. Cells, selection, the reference grid, signal slices, camera pose, and the existing Fit bounds policy are independent of this display setting. No simulation constraint or transport obstacle is changed.
+
+`browser/device-visibility.mjs` verifies all four constraint types, outlines, keyboard toggling, cell picking, sibling visibility, frame/reset retention, missing geometry, camera and Fit invariance, and new-dataset defaults against Vite on port 4323. It uses the same Playwright module and evidence-directory options as the reference-grid browser test.
+
 ## Channel labels
 
 Model-defined species and signal names appear in channel selectors, the species legend, and cell inspection. Duplicate names include their channel indices; unnamed channels retain `Channel N`. Names are presentation text; indices continue to identify selected channels. Current readers accept scene v2 and v3, while writers emit v3. See the [authoring guide](../docs/models/channel-labels.md) and [scene v3 schema](../docs/formats/scene-v3.md).
diff --git a/viewer/browser/device-visibility.mjs b/viewer/browser/device-visibility.mjs
new file mode 100644
index 0000000..4b24c05
--- /dev/null
+++ b/viewer/browser/device-visibility.mjs
@@ -0,0 +1,273 @@
+// Run against the Vite development server. No production debug hooks are added.
+import assert from "node:assert/strict";
+import { createHash } from "node:crypto";
+import { mkdir } from "node:fs/promises";
+import canonicalize from "canonicalize";
+
+const { chromium, expect } = await import(
+  process.env.MICROSIMULATOR_PLAYWRIGHT_MODULE ?? "@playwright/test"
+);
+const url = process.env.VIEWER_URL ?? "http://127.0.0.1:4323";
+const evidence =
+  process.env.EVIDENCE_DIR ?? "/tmp/microsimulator-device-visibility";
+await mkdir(evidence, { recursive: true });
+const browser = await chromium.launch({ headless: true });
+const page = await browser.newPage({ viewport: { width: 1440, height: 960 } });
+const errors = [];
+page.on("pageerror", (error) => errors.push(error.message));
+
+// Observe the real application instance rather than replacing its renderer.
+await page.route("**/src/colony-viewer.ts*", async (route) => {
+  const response = await route.fetch();
+  const source = await response.text();
+  const marker = "this.onSelection = onSelection;";
+  assert.equal(source.split(marker).length, 2);
+  await route.fulfill({
+    response,
+    body: source.replace(marker, `${marker}\nglobalThis.__testViewer = this;`),
+  });
+});
+let socket;
+await page.routeWebSocket("**/api/v1/session?*", (connection) => {
+  socket = connection;
+});
+const boundary = { kind: "no_flux", values: [] };
+function signalGrid(shape = [5, 7, 9], signalCount = 3) {
+  return {
+    signal_count: signalCount,
+    shape,
+    origin: [-2, -3, 0],
+    spacing: [1, 1, 1],
+    boundaries: {
+      x_lower: boundary,
+      x_upper: boundary,
+      y_lower: boundary,
+      y_upper: boundary,
+      z_lower: boundary,
+      z_upper: boundary,
+    },
+    levels: Array(shape.reduce((a, b) => a * b, signalCount)).fill(1),
+  };
+}
+const cell = {
+  id: "1",
+  parent_id: null,
+  slot: 0,
+  position: [0, 0, 0.6],
+  direction: [1, 0, 0],
+  length: 2,
+  radius: 0.5,
+  growth_rate: 0.1,
+  cell_type: 0,
+  fixed: false,
+  species: [1, 2, 3],
+};
+const base = {
+  backend: {
+    kind: "cpu",
+    name: "CPU fixture",
+    device: "host",
+    device_index: 0,
+    native: true,
+  },
+  time: 0,
+  species_count: 3,
+  cells: [cell],
+  constraints: { boxes: [], cylinders: [], planes: [], spheres: [] },
+  signal_grid: signalGrid(),
+};
+function scene(frame) {
+  return {
+    format: "microsimulator-scene",
+    version: 2,
+    producer: { name: "microsimulator", version: "test" },
+    integrity: {
+      algorithm: "sha256",
+      frame: createHash("sha256").update(canonicalize(frame)).digest("hex"),
+    },
+    frame,
+  };
+}
+let revision = 0;
+async function send(frame) {
+  socket.send(
+    JSON.stringify({
+      type: "frame",
+      revision: revision++,
+      completed_steps: revision,
+      playing: false,
+      checkpoint_enabled: false,
+      scene: scene(frame),
+    }),
+  );
+  await expect(page.locator("#time-chip")).toHaveText(`t = ${frame.time}`);
+  await page.evaluate(
+    () =>
+      new Promise((resolve) =>
+        requestAnimationFrame(() => requestAnimationFrame(resolve)),
+      ),
+  );
+}
+const constraints = {
+  boxes: [
+    {
+      id: "1",
+      center: [4, 0, 1],
+      half_extents: [1, 2, 1],
+      allowed_region: "outside",
+      coefficient: 1,
+    },
+  ],
+  spheres: [
+    {
+      id: "2",
+      center: [-4, 0, 1],
+      radius: 1.5,
+      allowed_region: "outside",
+      coefficient: 1,
+    },
+  ],
+  cylinders: [
+    {
+      id: "3",
+      center: [0, 4, 1],
+      radius: 1,
+      half_height: 2,
+      allowed_region: "outside",
+      coefficient: 1,
+    },
+  ],
+  planes: [
+    { id: "4", point: [0, 0, -0.5], inward_normal: [0, 0, 1], coefficient: 1 },
+  ],
+};
+const all = { ...base, constraints };
+async function snapshot() {
+  return page.evaluate(() => {
+    const v = globalThis.__testViewer;
+    v.grid.updateMatrixWorld(true);
+    return {
+      camera: [
+        ...v.camera.position.toArray(),
+        ...v.camera.quaternion.toArray(),
+      ],
+      bounds: [v.sceneBounds.min.toArray(), v.sceneBounds.max.toArray()],
+      grid: v.grid.matrixWorld.elements.slice(),
+      visible: v.device.visible,
+      deviceChildren: v.device.children.length,
+      cells: v.colony.visible && v.cellMeshes.length > 0,
+      highlight: v.highlight.visible,
+      signal: v.signal.visible && v.signal.children.length > 0,
+    };
+  });
+}
+function assertSnapshot(actual, expected, message) {
+  actual.camera.forEach((value, index) =>
+    assert.ok(Math.abs(value - expected.camera[index]) < 1e-9, message),
+  );
+  assert.deepEqual(
+    { ...actual, camera: [] },
+    { ...expected, camera: [] },
+    message,
+  );
+}
+async function pickCell() {
+  const point = await page.evaluate(() => {
+    const v = globalThis.__testViewer;
+    const rect = v.renderer.domElement.getBoundingClientRect();
+    const p = v.camera.position.clone().set(0, 0, 0.6).project(v.camera);
+    return [
+      rect.left + ((p.x + 1) * rect.width) / 2,
+      rect.top + ((1 - p.y) * rect.height) / 2,
+    ];
+  });
+  await page.mouse.click(...point);
+  await expect(page.locator("#selection-title")).toHaveText("Cell 1");
+}
+try {
+  await page.goto(`${url}/?token=device-test`);
+  await expect.poll(() => socket !== undefined).toBe(true);
+  await send(all);
+  const toggle = page.getByLabel("Show device geometry", { exact: true });
+  await expect(toggle).toBeEnabled();
+  await expect(toggle).toBeChecked();
+  await pickCell();
+  const initial = await snapshot();
+  assert.equal(
+    initial.deviceChildren,
+    6,
+    "all four meshes plus box/cylinder outlines",
+  );
+  assert.ok(initial.cells && initial.signal && initial.highlight);
+  await page.screenshot({ path: `${evidence}/visible.png` });
+  await toggle.focus();
+  await page.keyboard.press("Space");
+  await expect(toggle).not.toBeChecked();
+  const hidden = await snapshot();
+  assertSnapshot(hidden, { ...initial, visible: false });
+  await page.screenshot({ path: `${evidence}/hidden.png` });
+  await page.locator("#clear-selection").click();
+  await pickCell();
+  for (const time of [1, 20, 3, 0]) {
+    await send({ ...all, time });
+    await expect(toggle).not.toBeChecked();
+    const current = await snapshot();
+    assertSnapshot(
+      current,
+      hidden,
+      "updates/reset/reverse-time retain display state",
+    );
+  }
+  await send({ ...base, time: 21 });
+  await expect(toggle).toBeDisabled();
+  await expect(toggle).not.toBeChecked();
+  await send({ ...all, time: 22 });
+  await expect(toggle).toBeEnabled();
+  await expect(toggle).not.toBeChecked();
+  await page.locator("#fit-button").click();
+  await expect
+    .poll(async () =>
+      page.evaluate(() => globalThis.__testViewer.cameraTransition === null),
+    )
+    .toBe(true);
+  const fittedHidden = await snapshot();
+  await toggle.focus();
+  await page.keyboard.press("Space");
+  const shown = await snapshot();
+  assertSnapshot(shown, { ...fittedHidden, visible: true });
+  await page.locator("#fit-button").click();
+  await expect
+    .poll(async () =>
+      page.evaluate(() => globalThis.__testViewer.cameraTransition === null),
+    )
+    .toBe(true);
+  assertSnapshot(
+    await snapshot(),
+    shown,
+    "Fit bounds and pose are visibility independent",
+  );
+  // A separate standalone file opening starts a new dataset and restores enabled.
+  await page.goto(url);
+  const file = {
+    name: "device.json",
+    mimeType: "application/json",
+    buffer: Buffer.from(JSON.stringify(scene(all))),
+  };
+  await page.locator("#scene-file").setInputFiles(file);
+  await expect(toggle).toBeChecked();
+  await toggle.uncheck();
+  await page.locator("#scene-file").setInputFiles(file);
+  await expect(toggle).toBeChecked();
+  await pickCell();
+  assert.deepEqual(errors, []);
+  console.log(
+    JSON.stringify({
+      result: "passed",
+      browser: browser.version(),
+      assertions:
+        "device meshes/outlines, sibling visibility, keyboard, picking, camera, Fit, live updates/reset, missing geometry, new dataset",
+    }),
+  );
+} finally {
+  await browser.close();
+}
diff --git a/viewer/index.html b/viewer/index.html
index 7a2945d..b66a1b4 100644
--- a/viewer/index.html
+++ b/viewer/index.html
@@ -112,6 +112,10 @@ <h2>Signal grid</h2>
 
           <section class="scene-notes">
             <h2>Scene</h2>
+            <label class="device-visibility">
+              <span>Show device geometry</span>
+              <input id="device-visible" type="checkbox" checked disabled />
+            </label>
             <dl>
               <div>
                 <dt>Backend</dt>
diff --git a/viewer/src/colony-viewer.ts b/viewer/src/colony-viewer.ts
index 7be01dc..85f0855 100644
--- a/viewer/src/colony-viewer.ts
+++ b/viewer/src/colony-viewer.ts
@@ -207,10 +207,16 @@ export class ColonyViewer {
   /** Call once when opening a file, live session, or recording. */
   public beginDataset(): void {
     this.referenceGrid.beginDataset();
+    this.setDeviceVisible(true);
     this.cancelCameraTransition();
     this.selectCell(null);
   }
 
+  /** Presentation only: the group retains visibility when its children rebuild. */
+  public setDeviceVisible(visible: boolean): void {
+    this.device.visible = visible;
+  }
+
   /** Frame updates, including reset/seek, retain camera and dataset state. */
   public setFrame(frame: SceneFrame, fit = false): void {
     this.configureReferenceGrid(frame);
diff --git a/viewer/src/main.ts b/viewer/src/main.ts
index 27942fb..a65fa47 100644
--- a/viewer/src/main.ts
+++ b/viewer/src/main.ts
@@ -46,6 +46,7 @@ const legendMaximum = required<HTMLElement>("legend-max");
 const legendTitle = required<HTMLElement>("legend-title");
 const signalSection = required<HTMLElement>("signal-section");
 const signalVisible = required<HTMLInputElement>("signal-visible");
+const deviceVisible = required<HTMLInputElement>("device-visible");
 const signalChannel = required<HTMLSelectElement>("signal-channel");
 const signalAxis = required<HTMLSelectElement>("signal-axis");
 const signalRange = required<HTMLInputElement>("signal-slice");
@@ -237,6 +238,11 @@ function presentScene(
   const display = presentation.forFrame(next);
   frame = next;
   viewer.setFrame(next, newDataset);
+  deviceVisible.checked = display.deviceVisible;
+  deviceVisible.disabled = !Object.values(next.constraints).some(
+    (constraints) => constraints.length > 0,
+  );
+  viewer.setDeviceVisible(display.deviceVisible);
   fitButton.disabled = false;
   colorMode.disabled = false;
   emptyState.hidden = true;
@@ -322,6 +328,10 @@ speciesChannel.addEventListener("change", () => {
   presentation.preferences.speciesChannel = selectedInteger(speciesChannel);
   updateColors();
 });
+deviceVisible.addEventListener("change", () => {
+  presentation.preferences.deviceVisible = deviceVisible.checked;
+  viewer.setDeviceVisible(deviceVisible.checked);
+});
 signalVisible.addEventListener("change", () => {
   presentation.preferences.signalVisible = signalVisible.checked;
   updateSignal();
diff --git a/viewer/src/presentation-state.ts b/viewer/src/presentation-state.ts
index 4dd9e61..7f6c2ed 100644
--- a/viewer/src/presentation-state.ts
+++ b/viewer/src/presentation-state.ts
@@ -6,6 +6,7 @@ export interface PresentationPreferences {
   colorMode: ColorMode;
   speciesChannel: number;
   signalVisible: boolean;
+  deviceVisible: boolean;
   signalChannel: number;
   signalAxis: SliceAxis;
   signalSlice: number | null;
@@ -16,6 +17,7 @@ function defaults(): PresentationPreferences {
     colorMode: "cell-type",
     speciesChannel: 0,
     signalVisible: true,
+    deviceVisible: true,
     signalChannel: 0,
     signalAxis: "z",
     signalSlice: null,
diff --git a/viewer/src/style.css b/viewer/src/style.css
index 4656f5f..2b36f98 100644
--- a/viewer/src/style.css
+++ b/viewer/src/style.css
@@ -721,3 +721,20 @@ dd {
     transition-duration: 0ms;
   }
 }
+
+.device-visibility {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  gap: 0.75rem;
+  margin: 0.75rem 0;
+  color: var(--muted);
+  font-size: 0.75rem;
+}
+
+.device-visibility input {
+  width: 1rem;
+  height: 1rem;
+  margin: 0;
+  accent-color: var(--accent);
+}
diff --git a/viewer/tests/presentation-state.test.ts b/viewer/tests/presentation-state.test.ts
index 299050e..609c28d 100644
--- a/viewer/tests/presentation-state.test.ts
+++ b/viewer/tests/presentation-state.test.ts
@@ -38,6 +38,17 @@ const frame: SceneFrame = {
 };
 
 describe("dataset presentation lifecycle", () => {
+  it("retains device preference through absent geometry and resets for a new dataset", () => {
+    const state = new DatasetPresentationState();
+    state.beginDataset();
+    state.preferences.deviceVisible = false;
+    for (const time of [0, 2, 1, 0]) {
+      expect(state.forFrame({ ...frame, time }).deviceVisible).toBe(false);
+    }
+    state.beginDataset();
+    expect(state.forFrame(frame).deviceVisible).toBe(true);
+  });
+
   it("restores the desired slice after an axis round trip", () => {
     const state = new DatasetPresentationState();
     state.beginDataset();
@@ -75,6 +86,7 @@ describe("dataset presentation lifecycle", () => {
       colorMode: "cell-type",
       speciesChannel: 0,
       signalVisible: true,
+      deviceVisible: true,
       signalChannel: 0,
       signalAxis: "z",
       signalSlice: 4,