diff --git a/web/src/shellResponsiveCss.test.ts b/web/src/shellResponsiveCss.test.ts
new file mode 100644
index 0000000..5ae45f7
--- /dev/null
+++ b/web/src/shellResponsiveCss.test.ts
@@ -0,0 +1,75 @@
+import { readFileSync } from 'node:fs';
+import { resolve } from 'node:path';
+import { describe, expect, it } from 'vitest';
+
+const css = readFileSync(resolve(__dirname, 'index.css'), 'utf8');
+
+const mediaBlocks = (() => {
+  const blocks: Array<{ query: string; body: string }> = [];
+  const mediaPattern = /@media\s*([^{]+)\{/g;
+  let match: RegExpExecArray | null;
+
+  while ((match = mediaPattern.exec(css))) {
+    let depth = 1;
+    let cursor = mediaPattern.lastIndex;
+    for (; cursor < css.length; cursor += 1) {
+      if (css[cursor] === '{') depth += 1;
+      if (css[cursor] === '}') depth -= 1;
+      if (depth === 0) break;
+    }
+    blocks.push({
+      query: match[1].trim(),
+      body: css.slice(mediaPattern.lastIndex, cursor),
+    });
+    mediaPattern.lastIndex = cursor + 1;
+  }
+
+  return blocks;
+})();
+
+const blocksFor = (queryPart: string) =>
+  mediaBlocks.filter((block) => block.query.includes(queryPart));
+
+const ruleBodies = (source: string, selector: string) => {
+  const escapedSelector = selector.replace(/[.*+?^${}()|[\]\\]/g, '\\$&');
+  return [...source.matchAll(new RegExp(`${escapedSelector}\\s*\\{([^}]*)\\}`, 'g'))]
+    .map((match) => match[1]);
+};
+
+describe('responsive shell CSS contract', () => {
+  it('keeps desktop navigation as a left rail by default', () => {
+    expect(css).toMatch(/\.dashboard-main\s*\{[\s\S]*?margin-left:\s*248px;/);
+    expect(css).toMatch(/\.trading-sidebar\s*\{[\s\S]*?width:\s*248px;/);
+    expect(css).toMatch(/\.dashboard-right-panel\s*\{[\s\S]*?width:\s*340px;/);
+  });
+
+  it('keeps tablet navigation on the side instead of switching to footer nav', () => {
+    const tabletBlocks = blocksFor('min-width: 561px');
+    expect(tabletBlocks.length).toBeGreaterThan(0);
+
+    const tabletCss = tabletBlocks.map((block) => block.body).join('\n');
+    const tabletSidebarRules = ruleBodies(tabletCss, '.trading-sidebar').join('\n');
+    expect(tabletCss).toContain('margin-left: 88px');
+    expect(tabletSidebarRules).toContain('width: 88px');
+    expect(tabletSidebarRules).toContain('flex-direction: column');
+    expect(tabletSidebarRules).toContain('border-right: 1px solid var(--border)');
+    expect(tabletSidebarRules).not.toContain('width: 100%');
+    expect(tabletSidebarRules).not.toContain('border-top: 1px solid var(--border)');
+  });
+
+  it('uses bottom navigation only at phone width', () => {
+    const phoneCss = blocksFor('max-width: 560px').map((block) => block.body).join('\n');
+    const phoneSidebarRules = ruleBodies(phoneCss, '.trading-sidebar').join('\n');
+    expect(phoneCss).toContain('margin-left: 0');
+    expect(phoneSidebarRules).toContain('width: 100%');
+    expect(phoneSidebarRules).toContain('flex-direction: row');
+    expect(phoneSidebarRules).toContain('border-top: 1px solid var(--border)');
+
+    const wideResponsiveCss = mediaBlocks
+      .filter((block) => !block.query.includes('max-width: 560px'))
+      .map((block) => block.body)
+      .join('\n');
+    const wideSidebarRules = ruleBodies(wideResponsiveCss, '.trading-sidebar').join('\n');
+    expect(wideSidebarRules).not.toContain('width: 100%');
+  });
+});