diff --git a/frontend/src/App.jsx b/frontend/src/App.jsx
index cbfa37d..89a2bea 100644
--- a/frontend/src/App.jsx
+++ b/frontend/src/App.jsx
@@ -13,6 +13,7 @@ import FileBrowser from './FileBrowser';
 import * as api from './api';
 import { toBlob } from 'html-to-image';
 import { embedWorkflow, extractWorkflow } from './pngMetadata';
+import { captureViewportBlob as captureWorkflowViewportBlob } from './workflowCapture';
 import { hydrateWorkflowState } from './workflowHydration';
 import { serializeWorkflowState } from './workflowSerialization';
 
@@ -867,7 +868,7 @@ function Flow() {
     const imageHeight = Math.ceil(bounds.height * (1 + pad * 2));
     const vp = getViewportForBounds(bounds, imageWidth, imageHeight, 0.5, 1, pad);
 
-    const blob = await captureViewportBlob(viewportEl, {
+    const blob = await captureWorkflowViewportBlob(viewportEl, {
       backgroundColor: '#1a1a1a',
       width: imageWidth,
       height: imageHeight,
diff --git a/frontend/src/workflowCapture.js b/frontend/src/workflowCapture.js
new file mode 100644
index 0000000..e5bd537
--- /dev/null
+++ b/frontend/src/workflowCapture.js
@@ -0,0 +1,192 @@
+import { toBlob } from 'html-to-image';
+
+export const OVERLAY_CAPTURE_SELECTORS = [
+  '.lineplot-overlay',
+  '.cs-overlay',
+  '.crop-overlay',
+];
+
+function encodeBase64(bytes) {
+  if (typeof Buffer !== 'undefined') {
+    return Buffer.from(bytes).toString('base64');
+  }
+
+  let binary = '';
+  for (let i = 0; i < bytes.length; i += 1) {
+    binary += String.fromCharCode(bytes[i]);
+  }
+  return btoa(binary);
+}
+
+async function blobToDataUrl(blob) {
+  if (!blob) return null;
+  const bytes = new Uint8Array(await blob.arrayBuffer());
+  return `data:${blob.type || 'image/png'};base64,${encodeBase64(bytes)}`;
+}
+
+function getElementSize(el) {
+  const rect = el.getBoundingClientRect?.() ?? { width: 0, height: 0 };
+  return {
+    width: Math.max(1, Math.round(el.clientWidth || rect.width || 0)),
+    height: Math.max(1, Math.round(el.clientHeight || rect.height || 0)),
+  };
+}
+
+export async function waitForImageElement(img) {
+  if (img.complete && img.naturalWidth > 0) return;
+  if (typeof img.decode === 'function') {
+    try {
+      await img.decode();
+      return;
+    } catch {
+      // Fall back to load/error listeners below.
+    }
+  }
+  await new Promise((resolve) => {
+    const done = () => {
+      img.removeEventListener('load', done);
+      img.removeEventListener('error', done);
+      resolve();
+    };
+    img.addEventListener('load', done, { once: true });
+    img.addEventListener('error', done, { once: true });
+  });
+}
+
+export async function getCaptureImageDataUrl(img) {
+  const src = img.currentSrc || img.src;
+  if (!src) return null;
+  if (!src.startsWith('data:')) return src;
+
+  const { width, height } = getElementSize(img);
+  const scale = Math.min(2, globalThis.window?.devicePixelRatio || 1);
+
+  const canvas = globalThis.document.createElement('canvas');
+  canvas.width = Math.max(1, Math.round(width * scale));
+  canvas.height = Math.max(1, Math.round(height * scale));
+
+  const ctx = canvas.getContext('2d');
+  if (!ctx) return src;
+
+  try {
+    ctx.drawImage(img, 0, 0, canvas.width, canvas.height);
+    return canvas.toDataURL('image/png');
+  } catch {
+    return src;
+  }
+}
+
+export function createCapturePlaceholder(
+  el,
+  dataUrl,
+  { stretch = false, documentRef = globalThis.document, getComputedStyleFn = globalThis.window?.getComputedStyle?.bind(globalThis.window) } = {},
+) {
+  const rect = el.getBoundingClientRect();
+  const style = getComputedStyleFn(el);
+  const placeholder = documentRef.createElement('div');
+
+  placeholder.style.display = style.display === 'inline' ? 'inline-block' : style.display;
+  placeholder.style.width = `${el.clientWidth || rect.width}px`;
+  placeholder.style.height = `${el.clientHeight || rect.height}px`;
+  placeholder.style.maxWidth = style.maxWidth;
+  placeholder.style.maxHeight = style.maxHeight;
+  placeholder.style.minWidth = style.minWidth;
+  placeholder.style.minHeight = style.minHeight;
+  placeholder.style.borderRadius = style.borderRadius;
+  placeholder.style.backgroundImage = `url("${dataUrl}")`;
+  placeholder.style.backgroundRepeat = 'no-repeat';
+  placeholder.style.backgroundPosition = 'center';
+  placeholder.style.backgroundSize = stretch ? '100% 100%' : 'contain';
+  placeholder.style.flexShrink = style.flexShrink;
+
+  return placeholder;
+}
+
+async function renderCanvasToDataUrl(canvas) {
+  try {
+    return canvas.toDataURL('image/png');
+  } catch {
+    return null;
+  }
+}
+
+async function renderElementToDataUrl(el, toBlobImpl) {
+  const { width, height } = getElementSize(el);
+  const blob = await toBlobImpl(el, {
+    width,
+    height,
+    backgroundColor: '#0f172a',
+    style: {
+      width: `${width}px`,
+      height: `${height}px`,
+      transform: 'none',
+    },
+  });
+  return blobToDataUrl(blob);
+}
+
+async function replaceElementsWithPlaceholders(elements, renderDataUrl, createPlaceholderFn, stretch) {
+  const restorers = [];
+
+  for (const el of elements) {
+    if (!el?.parentNode) continue;
+    const dataUrl = await renderDataUrl(el);
+    if (!dataUrl) continue;
+
+    const placeholder = createPlaceholderFn(el, dataUrl, { stretch });
+    el.parentNode.replaceChild(placeholder, el);
+    restorers.push(() => {
+      if (placeholder.parentNode) {
+        placeholder.parentNode.replaceChild(el, placeholder);
+      }
+    });
+  }
+
+  return restorers;
+}
+
+function defaultQueryAll(root, selector) {
+  return Array.from(root.querySelectorAll(selector));
+}
+
+function defaultNextFrame() {
+  return new Promise((resolve) => requestAnimationFrame(() => resolve()));
+}
+
+export async function captureViewportBlob(viewportEl, options, deps = {}) {
+  const queryAll = deps.queryAll ?? defaultQueryAll;
+  const toBlobImpl = deps.toBlobImpl ?? toBlob;
+  const waitForImage = deps.waitForImageElement ?? waitForImageElement;
+  const renderImage = deps.renderImageToDataUrl ?? getCaptureImageDataUrl;
+  const renderCanvas = deps.renderCanvasToDataUrl ?? renderCanvasToDataUrl;
+  const renderOverlay = deps.renderOverlayToDataUrl ?? ((el) => renderElementToDataUrl(el, toBlobImpl));
+  const createPlaceholderFn = deps.createPlaceholder ?? createCapturePlaceholder;
+  const nextFrame = deps.nextFrame ?? defaultNextFrame;
+  const overlaySelectors = deps.overlaySelectors ?? OVERLAY_CAPTURE_SELECTORS;
+  const restorers = [];
+
+  const overlays = [];
+  const seen = new Set();
+  for (const selector of overlaySelectors) {
+    for (const el of queryAll(viewportEl, selector)) {
+      if (seen.has(el)) continue;
+      seen.add(el);
+      overlays.push(el);
+    }
+  }
+
+  const images = queryAll(viewportEl, 'img');
+  await Promise.all(images.map(waitForImage));
+
+  try {
+    restorers.push(...await replaceElementsWithPlaceholders(overlays, renderOverlay, createPlaceholderFn, true));
+    restorers.push(...await replaceElementsWithPlaceholders(queryAll(viewportEl, 'img'), renderImage, createPlaceholderFn, false));
+    restorers.push(...await replaceElementsWithPlaceholders(queryAll(viewportEl, 'canvas'), renderCanvas, createPlaceholderFn, true));
+
+    await nextFrame();
+    await nextFrame();
+    return await toBlobImpl(viewportEl, options);
+  } finally {
+    restorers.reverse().forEach((restore) => restore());
+  }
+}
diff --git a/frontend/tests/workflowCapture.test.mjs b/frontend/tests/workflowCapture.test.mjs
new file mode 100644
index 0000000..09f160a
--- /dev/null
+++ b/frontend/tests/workflowCapture.test.mjs
@@ -0,0 +1,136 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+
+import { OVERLAY_CAPTURE_SELECTORS, captureViewportBlob } from '../src/workflowCapture.js';
+
+function makeElement(name, { tagName = 'DIV', width = 160, height = 90, src = '' } = {}) {
+  return {
+    name,
+    tagName,
+    src,
+    currentSrc: src,
+    complete: true,
+    naturalWidth: 1,
+    clientWidth: width,
+    clientHeight: height,
+    parentNode: null,
+    getBoundingClientRect() {
+      return { width, height };
+    },
+  };
+}
+
+function makeParent(initialChild) {
+  return {
+    currentChild: initialChild,
+    replaceChild(nextChild, prevChild) {
+      assert.equal(this.currentChild, prevChild);
+      this.currentChild = nextChild;
+      nextChild.parentNode = this;
+    },
+  };
+}
+
+function makeViewport({ overlay, image, canvas }) {
+  return {
+    querySelectorAll(selector) {
+      if (selector === '.lineplot-overlay') {
+        return overlay.parentNode?.currentChild === overlay ? [overlay] : [];
+      }
+      if (selector === '.cs-overlay' || selector === '.crop-overlay') {
+        return [];
+      }
+      if (selector === 'img') {
+        return image.parentNode?.currentChild === image ? [image] : [];
+      }
+      if (selector === 'canvas') {
+        return canvas.parentNode?.currentChild === canvas ? [canvas] : [];
+      }
+      return [];
+    },
+  };
+}
+
+test('captureViewportBlob rasterizes custom overlays before the final workflow snapshot', async () => {
+  const overlay = makeElement('overlay', { width: 320, height: 220 });
+  const image = makeElement('image', { tagName: 'IMG', width: 180, height: 120, src: 'data:image/png;base64,abc' });
+  const canvas = makeElement('canvas', { tagName: 'CANVAS', width: 128, height: 128 });
+  canvas.toDataURL = () => 'data:image/png;base64,canvas';
+
+  const overlayParent = makeParent(overlay);
+  const imageParent = makeParent(image);
+  const canvasParent = makeParent(canvas);
+  overlay.parentNode = overlayParent;
+  image.parentNode = imageParent;
+  canvas.parentNode = canvasParent;
+
+  const selectorsSeen = [];
+  const viewport = makeViewport({ overlay, image, canvas });
+  const blob = await captureViewportBlob(viewport, { width: 800, height: 600 }, {
+    queryAll(root, selector) {
+      selectorsSeen.push(selector);
+      return root.querySelectorAll(selector);
+    },
+    waitForImageElement: async () => {},
+    renderOverlayToDataUrl: async (el) => `data:image/png;base64,overlay-${el.name}`,
+    renderImageToDataUrl: async (el) => `data:image/png;base64,image-${el.name}`,
+    renderCanvasToDataUrl: async (el) => `data:image/png;base64,canvas-${el.name}`,
+    createPlaceholder: (el, dataUrl) => ({ placeholderFor: el.name, dataUrl, parentNode: null }),
+    nextFrame: async () => {},
+    toBlobImpl: async (target) => {
+      if (target === viewport) {
+        assert.notEqual(overlayParent.currentChild, overlay);
+        assert.notEqual(imageParent.currentChild, image);
+        assert.notEqual(canvasParent.currentChild, canvas);
+        return new Blob(['viewport'], { type: 'image/png' });
+      }
+      throw new Error('Unexpected element capture path');
+    },
+  });
+
+  assert.equal(await blob.text(), 'viewport');
+  assert.equal(overlayParent.currentChild, overlay);
+  assert.equal(imageParent.currentChild, image);
+  assert.equal(canvasParent.currentChild, canvas);
+  assert.deepEqual(
+    selectorsSeen.filter((selector) => OVERLAY_CAPTURE_SELECTORS.includes(selector)),
+    OVERLAY_CAPTURE_SELECTORS,
+  );
+});
+
+test('captureViewportBlob restores live elements when final capture fails', async () => {
+  const overlay = makeElement('overlay', { width: 320, height: 220 });
+  const image = makeElement('image', { tagName: 'IMG', width: 180, height: 120, src: 'data:image/png;base64,abc' });
+  const canvas = makeElement('canvas', { tagName: 'CANVAS', width: 128, height: 128 });
+  canvas.toDataURL = () => 'data:image/png;base64,canvas';
+
+  const overlayParent = makeParent(overlay);
+  const imageParent = makeParent(image);
+  const canvasParent = makeParent(canvas);
+  overlay.parentNode = overlayParent;
+  image.parentNode = imageParent;
+  canvas.parentNode = canvasParent;
+
+  const viewport = makeViewport({ overlay, image, canvas });
+
+  await assert.rejects(
+    captureViewportBlob(viewport, { width: 800, height: 600 }, {
+      queryAll: (root, selector) => root.querySelectorAll(selector),
+      waitForImageElement: async () => {},
+      renderOverlayToDataUrl: async () => 'data:image/png;base64,overlay',
+      renderImageToDataUrl: async () => 'data:image/png;base64,image',
+      renderCanvasToDataUrl: async () => 'data:image/png;base64,canvas',
+      createPlaceholder: (el) => ({ placeholderFor: el.name, parentNode: null }),
+      nextFrame: async () => {},
+      toBlobImpl: async (target) => {
+        if (target === viewport) throw new Error('capture failed');
+        return new Blob(['unexpected'], { type: 'image/png' });
+      },
+    }),
+    /capture failed/,
+  );
+
+  assert.equal(overlayParent.currentChild, overlay);
+  assert.equal(imageParent.currentChild, image);
+  assert.equal(canvasParent.currentChild, canvas);
+});