diff --git a/TESTING.md b/TESTING.md
index d249e9d..ce81b09 100644
--- a/TESTING.md
+++ b/TESTING.md
@@ -305,6 +305,82 @@ Use the disposable fixture and managed-host prerequisites above. A maintainer
 live check needs a ComfyUI restart after adding the Python route, then opening
 `/prompt_manager/workspace`; the mockup preview server is a separate design tool.
 
+### Concept shell and comparison method (#249)
+
+**CSS base and drift check.** `web/js/workspace/concept.css` is a byte-identical copy
+of the approved concept's `docs/concepts/workspace/style.css`. Never edit the copy;
+it is only rewritten from the concept with `npm run --prefix tools/vendor sync:concept`.
+`npm run --prefix tools/vendor check` runs `sync-concept-css.mjs --check`, which fails
+on a missing copy or names the first differing byte. `workspace.html` loads, in order,
+the toolkit controls, Viewer.js, `concept.css`, the page stylesheets that their own
+PRs retire, and the production overlay `workspace.css` last.
+`tests/vendor/concept-css.test.mjs` asserts that order, the drift check, and that no
+other page or canvas script loads workspace CSS.
+
+**Overlay decision-marker lint.** `lintOverlay` in `tools/vendor/sync-concept-css.mjs`
+fails any `workspace.css` rule whose selector is also a concept selector unless the
+rule carries a `/* D<n> */` comment, before its selector or inside its block. The lint
+test fails if `workspace.css` is missing. The match is by exact selector, so a narrower
+selector over a concept rule (for example `.ca-tools dialog.prompt-sender`) passes the
+lint. Such rules, and the page-stylesheet leftovers such as `library.css`
+`.row-actions`, name the PR that retires them, by convention.
+
+**Comparison method.** Production must equal the concept except for the recorded
+deviations in the #249 design (spec §8, D-numbers). It is never compared against
+blessed production screenshots.
+- The fixture is `tests/browser/workspace_concept_fixture.py`, reset through
+  `POST /__test__/concept/reset`. It holds the concept's six prompts and pairs, and
+  24 PNGs with real links, runs and the concept's inspector facts. The sidebar counts
+  are therefore the concept's own: 24, 6 and 15.
+- The shared inventory is `tests/browser/concept-inventory.mjs`. One self-contained
+  in-page measurer runs the same concept selectors in the immutable concept
+  (`/__test__/canonical/workspace`) and in production. An absent element becomes
+  `null`, so the diff names its path (for example `chrome.breadcrumbs`) instead of
+  failing the measurement.
+- Each page's `map<Page>Concept(reference)` applies only the recorded deviations, each
+  commented with its D-number. `diffInventory` must then report nothing.
+- Sizes, offsets and heights that follow from a removed element are derived from the
+  concept's own measurements, never copied from production.
+- `mutate(page, {css, remove})` appends a stylesheet and detaches elements, and returns
+  an undo. Each comparison has a mutation check proving it fails. For the shell, a
+  `.sidebar nav a { min-height: 30px }` rule must fail at `chrome.navs.0.rows.0.minHeight`
+  and `.height`, and removing `.breadcrumbs` must fail at `chrome.breadcrumbs`.
+- Paired concept/production captures are attached as test artifacts. There are no PNG
+  baselines.
+
+`tests/browser/shell-concept.spec.mjs` compares the shell: the sidebar, topbar, page
+heading and footer, at 1600x1100 and 390x844, in dark and light, under normal and
+reduced motion, on Images, Prompt library and Tags. The heading is compared in full on
+Tags; on Images and Library only its frame and h1 are compared, because their copy and
+actions belong to their own PRs. The spec also compares the hover state of the first
+non-current nav row, the Theme select and the ComfyUI link, and the keyboard rings of
+the first nav link and heading button.
+
+Recorded shell deviations:
+- **D4**: a "Theme" select with Dark, Light and System. System follows the ComfyUI
+  palette, then the OS.
+- **D5**: no Activity button or Activity row until an activity center exists.
+- **D28**: no SAVED VIEWS or COLLECTIONS, and no Design review. The bottom nav is
+  System plus the production Overview hub (the concept's `activity` glyph, no count).
+  The connection reads "Your library, on your server" with a neutral muted dot.
+- **D29**: no prototype copy. There is no "Interactive concept" badge and no
+  "What changes & why" button. The footer reads "ComfyAssets · Local tools. Your
+  library.", and the title has no " concept" suffix.
+- **D31**: at `max-width: 950px` the overlay caps the one-column shell at
+  `minmax(0, 1fr)`, so extreme zoom cannot widen it to the sidebar's min-content.
+- **#245 D2**: no "Auto-tag images" on the Tags heading until the image-annotation
+  slice.
+- **Brand name**: not a deviation. The brand link's accessible name matches the
+  concept's, mark included, because production's `.brand-mark` is not `aria-hidden`
+  either. The inventory compares `brand.name` and `brand.mark.hidden`.
+
+```sh
+npm run --prefix tools/vendor check
+npm test --prefix tools/vendor
+npm run --prefix tools/vendor test:workflows -- shell-concept.spec.mjs \
+  --project chromium --project touch --project firefox --project webkit
+```
+
 ## Prompt sending and cross-pack compatibility
 
 `tests/js/prompt-bridge-contracts.test.js` runs the Toolkit's shared protocol
diff --git a/tests/browser/concept-inventory.mjs b/tests/browser/concept-inventory.mjs
index 7e67119..cc20579 100644
--- a/tests/browser/concept-inventory.mjs
+++ b/tests/browser/concept-inventory.mjs
@@ -56,6 +56,37 @@ export async function hoverStateOf(page,locator) {
     return state;
 }
 
+/**
+ * Mutate the open page so a comparison can be shown to notice a change (the A7 mutation check): `css` is appended as
+ * the last stylesheet, so it wins over equally specific concept rules, and every element matching a `remove` selector
+ * is detached. Returns an async undo that restores the stylesheet and the removed elements in place.
+ */
+export async function mutate(page,{css=null,remove=[]}={}) {
+    const token=await page.evaluate(({css,remove})=>{
+        const token=`mutation-${Math.random().toString(36).slice(2)}`;
+        const undo=[];
+        if(css) {
+            const style=document.createElement('style');style.dataset.mutation=token;style.textContent=css;
+            document.head.append(style);undo.push(()=>style.remove());
+        }
+        for(const selector of remove) {
+            const nodes=[...document.querySelectorAll(selector)];
+            if(!nodes.length)throw new Error(`mutate: nothing matches ${selector}`);
+            for(const node of nodes) {
+                const parent=node.parentNode,next=node.nextSibling;
+                node.remove();undo.push(()=>parent.insertBefore(node,next));
+            }
+        }
+        (window.__conceptMutations??=new Map()).set(token,undo);
+        return token;
+    },{css,remove});
+    return ()=>page.evaluate(token=>{
+        const undo=window.__conceptMutations?.get(token);
+        if(!undo)throw new Error('mutate: this mutation was already undone or the page reloaded');
+        undo.reverse().forEach(step=>step());window.__conceptMutations.delete(token);
+    },token);
+}
+
 /**
  * Inventory sections: {chrome, heading, toolbar, filterRow, caption, grid, cards, panel, library, history,
  * noPageScroll}. A section is null when its element is absent. Sections whose page PR has not landed yet
diff --git a/tests/browser/shell-concept.spec.mjs b/tests/browser/shell-concept.spec.mjs
index 9711365..6cb5197 100644
--- a/tests/browser/shell-concept.spec.mjs
+++ b/tests/browser/shell-concept.spec.mjs
@@ -1,7 +1,7 @@
 import {writeFile} from 'node:fs/promises';
 import {test,expect} from '../../tools/vendor/node_modules/@playwright/test/index.mjs';
 import {captureCanonicalState} from './history-canonical-comparison.mjs';
-import {SIZES,THEMES,MOTIONS,seedConcept,openConcept,openProduction,inventoryOf,diffInventory,focusRingOf,hoverStateOf} from './concept-inventory.mjs';
+import {SIZES,THEMES,MOTIONS,seedConcept,openConcept,openProduction,inventoryOf,diffInventory,focusRingOf,hoverStateOf,mutate} from './concept-inventory.mjs';
 
 // Task A6 of #249: the production shell (sidebar, topbar, heading, footer) against the immutable concept, with the
 // concept-shaped real fixture so the sidebar counts are the concept's own (24 images, 6 prompts, 15 tags). Both themes,
@@ -114,6 +114,12 @@ function compare(route,reference,actual) {
     ];
 }
 
+/** The comparison as a gate: throws, naming every differing path, unless production equals the mapped concept. */
+function assertShellMatches(route,reference,actual) {
+    const diff=compare(route,reference,actual);
+    if(diff.length)throw new Error(`the shell differs from the concept at ${diff.map(item=>item.path).join(', ')}`);
+}
+
 /** Keyboard focus rings on the first WORKSPACE nav link and the first heading button. */
 async function ringsOf(page,mainSelector) {
     // Any key press makes the next programmatic focus keyboard-modal (`:focus-visible`); WebKit does not Tab to links.
@@ -218,3 +224,29 @@ test('the concept fixture renders the concept inspector facts',async({page})=>{
     // D21 maps execution evidence to "Execution recorded" in PR D; today's panel names the same source this way.
     await expect(page.locator('#workspace-main').getByText('Execution prompt evidence',{exact:true})).toBeVisible();
 });
+
+// Task A7: the comparison is only worth something if it notices a change. One CSS mutation (a nav-row height the
+// concept never uses) and one DOM mutation (the breadcrumbs removed) are applied to production after an exact match,
+// and each must make the comparison fail with the path of what changed.
+for(const [width,height] of SIZES)test(`shell comparison detects mutations ${width}x${height}`,async({page})=>{
+    await page.setViewportSize({width,height});
+    await openConcept(page,'tags','dark');await page.mouse.move(0,0);
+    const reference=await inventoryOf(page,'#main');
+    await openProduction(page,'tags','dark');await page.mouse.move(0,0);
+    const measure=()=>inventoryOf(page,'#workspace-main');
+    const clean=await measure();
+    assertShellMatches('tags',reference,clean);
+
+    const restoreCss=await mutate(page,{css:'.sidebar nav a { min-height: 30px }'});
+    const cssMutated=await measure();
+    expect(()=>assertShellMatches('tags',reference,cssMutated)).toThrow(/navs\.0\.rows\.0\.minHeight/);
+    expect(()=>assertShellMatches('tags',reference,cssMutated)).toThrow(/navs\.0\.rows\.0\.height/);
+    await restoreCss();
+    assertShellMatches('tags',reference,await measure());
+
+    await mutate(page,{remove:['.breadcrumbs']});
+    const domMutated=await measure();
+    expect(domMutated.chrome.breadcrumbs).toBeNull();
+    // The missing section is reported by its own path (A6: absent sections become null), not by a measuring error.
+    expect(()=>assertShellMatches('tags',reference,domMutated)).toThrow(/(at |, )chrome\.breadcrumbs(,|$)/);
+});
diff --git a/tests/browser/workspace.spec.mjs b/tests/browser/workspace.spec.mjs
index 65df3fd..2c0e2be 100644
--- a/tests/browser/workspace.spec.mjs
+++ b/tests/browser/workspace.spec.mjs
@@ -663,11 +663,17 @@ test('the ComfyUI link points to the root of a non-standard proxy mount', async
 test('a resolver failure for the ComfyUI link never stops the shell from mounting', async ({ page }) => {
     await serveUnderProxyMount(page);
     // Wrap the shared client as it loads so only the topbar's canvas-root lookup throws (the sender reads the same root).
+    // Every engine's stack names the calling module's URL; each injected failure is counted so a renamed module or a
+    // changed call site fails this test on the count instead of silently testing nothing.
     await page.addInitScript(() => {
         let client;
+        window.__resolverFailures = { injected: 0 };
         Object.defineProperty(window, 'ApiClient', { configurable: true, get: () => client, set(value) {
             client = { ...value, resolveUrl: (url, ...rest) => {
-                if (url === '/prompt_manager/' && /topbar\.mjs/.test(new Error().stack)) throw new Error('resolver unavailable');
+                if (url === '/prompt_manager/' && /\/workspace\/topbar\.mjs\b/.test(String(new Error().stack))) {
+                    window.__resolverFailures.injected += 1;
+                    throw new Error('resolver unavailable');
+                }
                 return value.resolveUrl(url, ...rest);
             } };
         } });
@@ -677,6 +683,7 @@ test('a resolver failure for the ComfyUI link never stops the shell from mountin
     await page.goto(`${PROXY_MOUNT}${workspace}#/overview`);
     await expect(page.locator('#workspace-main h1')).toHaveText('Overview');
     await expect(sidebarLinks(page)).toHaveCount(routes.length);
+    expect(await page.evaluate(() => window.__resolverFailures.injected), 'the topbar lookup was the one that failed').toBe(1);
     expect(warnings.filter(text => text.includes('the ComfyUI link falls back to the page mount'))).toHaveLength(1);
     await expect(page.getByRole('link', { name: 'ComfyUI', exact: true })).toHaveAttribute('href', `${PROXY_MOUNT}/`);
     await sidebarLink(page, 'Prompt library').click();
diff --git a/tests/vendor/concept-css.test.mjs b/tests/vendor/concept-css.test.mjs
index 3ad8d45..1de1bce 100644
--- a/tests/vendor/concept-css.test.mjs
+++ b/tests/vendor/concept-css.test.mjs
@@ -57,7 +57,7 @@ test('overlay lint requires a decision marker on concept selectors', () => {
 
 test('overlay rules on concept selectors carry a decision marker', () => {
     const overlay = join(root, 'web/js/workspace/workspace.css');
-    if (!existsSync(overlay)) return;
+    assert.ok(existsSync(overlay), 'web/js/workspace/workspace.css is missing');
     const { violations } = lintOverlay(readFileSync(concept, 'utf8'), readFileSync(overlay, 'utf8'));
     assert.deepEqual(violations, []);
 });
diff --git a/web/js/workspace/library.css b/web/js/workspace/library.css
index a1b71bc..84c759f 100644
--- a/web/js/workspace/library.css
+++ b/web/js/workspace/library.css
@@ -123,7 +123,8 @@ main.library-main { --library-negative: #ff8c98; }
     .history-run-marker[aria-pressed="true"] { outline-color: Highlight; }
 }
 
-/* Panel actions follow the concept (style.css 294-300, 540-549); the page heading is the concept's own (Task A5). */
+/* Panel actions follow the concept (style.css 294-300, 540-549); the page heading is the concept's own (Task A5).
+   These .row-actions rules restyle the concept's own selector and are retired in PR E (library cards). */
 .library-main .row-actions { display: flex; gap: 8px; align-items: center; flex-wrap: wrap; }
 .ca-tools .library-main .row-actions button { min-block-size: 32px; font-size: 12px; max-inline-size: 100%; white-space: normal; }
 .library-main .library-panel-heading { display: flex; align-items: center; justify-content: space-between; gap: 8px; margin-block-end: 12px; }