diff --git a/src/dev-ui/app/layouts/default.vue b/src/dev-ui/app/layouts/default.vue
index 5037058f0..8686326d0 100644
--- a/src/dev-ui/app/layouts/default.vue
+++ b/src/dev-ui/app/layouts/default.vue
@@ -430,7 +430,7 @@ const sidebarWidth = computed(() => (isCollapsed.value ? 'w-16' : 'w-64'))
               <TooltipTrigger as-child>
                 <DropdownMenuTrigger as-child>
                   <button
-                    class="flex w-full items-center justify-center rounded-md bg-sidebar-accent p-2 transition-colors hover:bg-sidebar-accent/80 focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-sidebar-ring"
+                    class="flex w-full items-center justify-center rounded-md bg-sidebar-accent p-2 transition-colors hover:bg-sidebar-accent/80 focus-visible:outline-none focus-visible:ring-[3px] focus-visible:ring-sidebar-ring/50"
                     :aria-label="tenantAriaLabel"
                   >
                     <Building2 class="size-4 shrink-0 text-sidebar-primary" aria-hidden="true" />
@@ -443,7 +443,7 @@ const sidebarWidth = computed(() => (isCollapsed.value ? 'w-16' : 'w-64'))
             </Tooltip>
             <DropdownMenuTrigger v-else as-child>
               <button
-                class="flex w-full items-center gap-2 rounded-md bg-sidebar-accent px-2 py-2 text-left transition-colors hover:bg-sidebar-accent/80 focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-sidebar-ring"
+                class="flex w-full items-center gap-2 rounded-md bg-sidebar-accent px-2 py-2 text-left transition-colors hover:bg-sidebar-accent/80 focus-visible:outline-none focus-visible:ring-[3px] focus-visible:ring-sidebar-ring/50"
                 :aria-label="tenantAriaLabel"
               >
                 <Building2 class="size-4 shrink-0 text-sidebar-primary" aria-hidden="true" />
@@ -678,7 +678,7 @@ const sidebarWidth = computed(() => (isCollapsed.value ? 'w-16' : 'w-64'))
             <DropdownMenu v-else>
               <DropdownMenuTrigger as-child>
                 <button
-                  class="flex w-full items-center gap-2 rounded-md bg-sidebar-accent px-2 py-2 text-left transition-colors hover:bg-sidebar-accent/80 focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-sidebar-ring"
+                  class="flex w-full items-center gap-2 rounded-md bg-sidebar-accent px-2 py-2 text-left transition-colors hover:bg-sidebar-accent/80 focus-visible:outline-none focus-visible:ring-[3px] focus-visible:ring-sidebar-ring/50"
                   :aria-label="tenantAriaLabel"
                 >
                   <Building2 class="size-4 shrink-0 text-sidebar-primary" aria-hidden="true" />
diff --git a/src/dev-ui/app/pages/integrate/mcp.vue b/src/dev-ui/app/pages/integrate/mcp.vue
index 77267d8d9..23c42eaee 100644
--- a/src/dev-ui/app/pages/integrate/mcp.vue
+++ b/src/dev-ui/app/pages/integrate/mcp.vue
@@ -602,7 +602,7 @@ async function copyHeaderValue(key: string, value: string) {
       <!-- Endpoint Details (collapsible) -->
       <Card>
         <button
-          class="flex w-full items-center justify-between px-6 py-4 text-left transition-colors hover:bg-muted/50 focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-ring focus-visible:ring-inset rounded-lg"
+          class="flex w-full items-center justify-between px-6 py-4 text-left transition-colors hover:bg-muted/50 focus-visible:outline-none focus-visible:ring-[3px] focus-visible:ring-ring/50 focus-visible:ring-inset rounded-lg"
           :aria-expanded="showDetails" aria-controls="endpoint-details-content" @click="showDetails = !showDetails">
           <div class="flex items-center gap-2">
             <Info class="size-4 text-muted-foreground" />
@@ -682,7 +682,7 @@ async function copyHeaderValue(key: string, value: string) {
       <!-- Available Tools (collapsible) -->
       <Card>
         <button
-          class="flex w-full items-center justify-between px-6 py-4 text-left transition-colors hover:bg-muted/50 focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-ring focus-visible:ring-inset rounded-lg"
+          class="flex w-full items-center justify-between px-6 py-4 text-left transition-colors hover:bg-muted/50 focus-visible:outline-none focus-visible:ring-[3px] focus-visible:ring-ring/50 focus-visible:ring-inset rounded-lg"
           :aria-expanded="showTools" aria-controls="tools-content" @click="showTools = !showTools">
           <div class="flex items-center gap-2">
             <Terminal class="size-4 text-muted-foreground" />
diff --git a/src/dev-ui/app/pages/tenants/index.vue b/src/dev-ui/app/pages/tenants/index.vue
index 839a7093e..b9c6f98c3 100644
--- a/src/dev-ui/app/pages/tenants/index.vue
+++ b/src/dev-ui/app/pages/tenants/index.vue
@@ -330,7 +330,7 @@ onMounted(fetchTenants)
             :key="tenant.id"
             role="listitem"
             tabindex="0"
-            class="flex items-center gap-2 px-4 py-2.5 transition-colors hover:bg-muted/50 cursor-pointer focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-ring focus-visible:ring-inset"
+            class="flex items-center gap-2 px-4 py-2.5 transition-colors hover:bg-muted/50 cursor-pointer focus-visible:outline-none focus-visible:ring-[3px] focus-visible:ring-ring/50 focus-visible:ring-inset"
             :class="[
               selectedTenant?.id === tenant.id ? 'bg-muted' : '',
             ]"
diff --git a/src/dev-ui/app/tests/focus-ring.test.ts b/src/dev-ui/app/tests/focus-ring.test.ts
new file mode 100644
index 000000000..f6ee30960
--- /dev/null
+++ b/src/dev-ui/app/tests/focus-ring.test.ts
@@ -0,0 +1,126 @@
+import { describe, it, expect } from 'vitest'
+import { readFileSync } from 'fs'
+import { resolve } from 'path'
+
+// ── Focus Ring Consistency Tests ──────────────────────────────────────────────
+//
+// Spec: "Interaction Principles — Scenario: Focus indicators"
+// "GIVEN an interactive element receiving focus
+//  THEN a 3px ring in the primary color at 50% opacity is shown
+//  AND native outlines are suppressed in favor of the ring"
+//
+// The shadcn/vue component library uses `focus-visible:ring-[3px]` consistently.
+// Several manually-written interactive elements were using `focus-visible:ring-2`
+// (Tailwind's preset 8px unit) instead of the spec-required 3px literal.
+//
+// These tests enforce that all manually-written interactive elements in:
+//   - app/layouts/default.vue (tenant selector buttons)
+//   - app/pages/integrate/mcp.vue (collapsible section buttons)
+//   - app/pages/tenants/index.vue (tenant list items)
+// ... use `focus-visible:ring-[3px]` (3px) and NOT `focus-visible:ring-2` (8px).
+//
+// Testing approach: Source-level inspection via readFileSync, matching the
+// pattern used by design-system.test.ts. Mounting the full Nuxt layout in a
+// unit test is impractical; source inspection is equivalent and more targeted.
+
+// ── Read source files ─────────────────────────────────────────────────────────
+
+const layoutsDir = resolve(__dirname, '../layouts')
+const pagesDir = resolve(__dirname, '../pages')
+
+const defaultVue = readFileSync(resolve(layoutsDir, 'default.vue'), 'utf-8')
+const mcpVue = readFileSync(resolve(pagesDir, 'integrate/mcp.vue'), 'utf-8')
+const tenantsVue = readFileSync(resolve(pagesDir, 'tenants/index.vue'), 'utf-8')
+
+// ── default.vue — Tenant selector buttons ─────────────────────────────────────
+
+describe('Focus Rings - default.vue: tenant selector buttons', () => {
+  it('does NOT use focus-visible:ring-2 anywhere in default.vue', () => {
+    // ring-2 is a Tailwind preset unit (8px), not the spec-required 3px
+    expect(defaultVue).not.toContain('focus-visible:ring-2')
+  })
+
+  it('uses focus-visible:ring-[3px] on the collapsed tenant icon button', () => {
+    // Line ~394: collapsed sidebar tenant trigger (icon-only)
+    // Must have ring-[3px] rather than ring-2
+    expect(defaultVue).toContain('focus-visible:ring-[3px]')
+  })
+
+  it('has ring-sidebar-ring/50 opacity on all sidebar focus rings in default.vue', () => {
+    // Every focus-visible:ring-sidebar-ring must include the /50 opacity modifier
+    // so the ring renders at 50% opacity as required by the spec
+    const withoutOpacity = [...defaultVue.matchAll(/focus-visible:ring-sidebar-ring(?!\/)/g)]
+    expect(withoutOpacity).toHaveLength(0)
+  })
+
+  it('retains outline-none on tenant selector buttons to suppress native outlines', () => {
+    // outline-none suppresses the browser default focus outline in favour of the ring
+    expect(defaultVue).toContain('focus-visible:outline-none')
+  })
+})
+
+// ── mcp.vue — Collapsible section buttons ─────────────────────────────────────
+
+describe('Focus Rings - mcp.vue: collapsible section buttons', () => {
+  it('does NOT use focus-visible:ring-2 anywhere in mcp.vue', () => {
+    expect(mcpVue).not.toContain('focus-visible:ring-2')
+  })
+
+  it('uses focus-visible:ring-[3px] on the Endpoint Details collapsible button', () => {
+    // Line ~605: "Endpoint Details" section toggle button
+    expect(mcpVue).toContain('focus-visible:ring-[3px]')
+  })
+
+  it('has ring-ring/50 opacity on all focus ring colors in mcp.vue', () => {
+    // focus-visible:ring-ring must always be focus-visible:ring-ring/50
+    // A bare `focus-visible:ring-ring` without the opacity modifier violates the spec
+    const bareRing = [...mcpVue.matchAll(/focus-visible:ring-ring(?!\/)/g)]
+    expect(bareRing).toHaveLength(0)
+  })
+
+  it('retains outline-none on collapsible buttons to suppress native outlines', () => {
+    expect(mcpVue).toContain('focus-visible:outline-none')
+  })
+})
+
+// ── tenants/index.vue — Tenant list items ─────────────────────────────────────
+
+describe('Focus Rings - tenants/index.vue: tenant list items', () => {
+  it('does NOT use focus-visible:ring-2 anywhere in tenants/index.vue', () => {
+    expect(tenantsVue).not.toContain('focus-visible:ring-2')
+  })
+
+  it('uses focus-visible:ring-[3px] on the tenant list item divs', () => {
+    // Line ~333: keyboard-focusable tenant row divs (role="listitem", tabindex="0")
+    expect(tenantsVue).toContain('focus-visible:ring-[3px]')
+  })
+
+  it('has ring-ring/50 opacity on all focus ring colors in tenants/index.vue', () => {
+    // focus-visible:ring-ring must always be focus-visible:ring-ring/50
+    const bareRing = [...tenantsVue.matchAll(/focus-visible:ring-ring(?!\/)/g)]
+    expect(bareRing).toHaveLength(0)
+  })
+
+  it('retains outline-none on tenant list items to suppress native outlines', () => {
+    expect(tenantsVue).toContain('focus-visible:outline-none')
+  })
+})
+
+// ── Cross-file regression guard ───────────────────────────────────────────────
+
+describe('Focus Rings - global: no new ring-2 regressions', () => {
+  it('default.vue uses ring-[3px] for all manual interactive elements (no ring-2)', () => {
+    const ring2Count = (defaultVue.match(/focus-visible:ring-2/g) ?? []).length
+    expect(ring2Count).toBe(0)
+  })
+
+  it('mcp.vue uses ring-[3px] for all manual interactive elements (no ring-2)', () => {
+    const ring2Count = (mcpVue.match(/focus-visible:ring-2/g) ?? []).length
+    expect(ring2Count).toBe(0)
+  })
+
+  it('tenants/index.vue uses ring-[3px] for all manual interactive elements (no ring-2)', () => {
+    const ring2Count = (tenantsVue.match(/focus-visible:ring-2/g) ?? []).length
+    expect(ring2Count).toBe(0)
+  })
+})