diff --git a/extension/background.js b/extension/background.js
index b6e14b6..8dbe8bf 100644
--- a/extension/background.js
+++ b/extension/background.js
@@ -11,6 +11,23 @@
 
 console.log('[tap] extension runtime ready')
 
+// --- MV3 SW keep-alive (ADR 2026-05-08-failure-detection-phase-2 §2C(i)) ---
+//
+// MV3 unloads the SW after ~30s idle, breaking the daemon's WebSocket and
+// surfacing as `peer_unreachable` to engine. classifyOpFailure routes that
+// to reconnect_extension, but the root cause is fixable here: chrome.alarms
+// fires even when the SW is unloaded, waking it up. Calling any chrome
+// API in the listener resets the SW idle timer.
+//
+// periodInMinutes 0.4 = 24s, comfortably under the 30s idle window.
+chrome.alarms.create('tap-keepalive', { periodInMinutes: 0.4 })
+chrome.alarms.onAlarm.addListener((alarm) => {
+  if (alarm.name === 'tap-keepalive') {
+    // No-op API call resets the idle timer. getPlatformInfo is cheapest.
+    chrome.runtime.getPlatformInfo(() => {})
+  }
+})
+
 // --- State ---
 
 // --- Session Manager ---
@@ -331,18 +348,30 @@ async function handleMethod(method, params = {}, senderTabId = null, { fromDaemo
         tabId = tab.id
       } else {
         const isInternal = current.url?.startsWith('chrome://') || current.url?.startsWith('data:')
-        if (isInternal && !fromDaemon) {
-          // Popup/content-script path: the user is actively looking at a
-          // chrome:// or data: tab — don't clobber it, open the target in a
-          // new tab instead. This is UX-preserving replacement.
+        // ADR 2026-05-08-failure-detection-phase-2 §2C(iii) — compute
+        // target vs current origin to decide tabs.update vs tabs.create.
+        // Cross-origin nav must NOT clobber an existing tab: the previous
+        // page may be in a redirect chain (e.g. CF auth) whose state
+        // would leak into the eval that follows. Same-origin SPA navs
+        // remain cheap (tabs.update).
+        let crossOrigin = false
+        try {
+          const target = new URL(params.url)
+          if (current.url) {
+            const currentParsed = new URL(current.url)
+            crossOrigin = target.origin !== currentParsed.origin
+          }
+        } catch {
+          // Malformed URL — treat as cross-origin (safer: open new tab)
+          crossOrigin = true
+        }
+        if (isInternal || crossOrigin) {
+          // chrome:// / data:// active tab (§2C(ii)) OR cross-origin nav
+          // (§2C(iii)) → open new background tab, never clobber.
           const tab = await chrome.tabs.create({ url: params.url, active: false })
           tabId = tab.id
         } else {
-          // Daemon path (and regular-URL tabs): navigate in place via
-          // tabs.update. chrome://newtab/ is our own placeholder from
-          // session.create and can be navigated away from in place — the
-          // previous "create a replacement tab" branch leaked the original
-          // chrome://newtab/ on every session-based nav.
+          // Same-origin SPA-style nav: cheap tabs.update.
           await chrome.tabs.update(tabId, { url: params.url })
         }
       }
diff --git a/extension/test/self-heal.test.mjs b/extension/test/self-heal.test.mjs
new file mode 100644
index 0000000..fb70e7b
--- /dev/null
+++ b/extension/test/self-heal.test.mjs
@@ -0,0 +1,208 @@
+/**
+ * Constraint: extension self-heals MV3 SW idle die + always uses
+ * managed background tabs for daemon-driven navs.
+ *
+ * Classification: safety / what — violations cause silent peer_unreachable
+ * spurious failures and active-tab clobbering during daemon ops.
+ *
+ * Per ADR `2026-05-08-failure-detection-phase-2.md` §2C:
+ *   (i)  chrome.alarms keep-alive prevents MV3 SW idle (~30s timeout) from
+ *        firing peer_unreachable to engine.
+ *   (ii) fromDaemon exemption deleted — daemon-driven navs ALWAYS open a
+ *        managed background tab when active is chrome://, never clobber.
+ *
+ * Adversarial framing (Phase 1a):
+ *   "If a half-implementation made this test pass, it could (a) add the
+ *    chrome.alarms.create call but never wire onAlarm.addListener (no-op
+ *    timer that doesn't actually wake SW) — caught by Rule (i)/2; (b)
+ *    delete the !fromDaemon expression but introduce a different
+ *    bypass like `if (isInternal && something_else)` that lets daemon
+ *    navs through — caught by Rule (ii)/2 which asserts the strict
+ *    isInternal-only guard pattern."
+ *
+ * Run: node extension/test/self-heal.test.mjs
+ */
+
+import { strict as assert } from "node:assert";
+import { readFileSync } from "node:fs";
+
+const BG_SRC = readFileSync(
+  new URL("../background.js", import.meta.url),
+  "utf-8",
+);
+
+let passed = 0;
+let failed = 0;
+
+function test(name, fn) {
+  try {
+    fn();
+    passed++;
+    console.log(`  \x1b[32m✓\x1b[0m ${name}`);
+  } catch (e) {
+    failed++;
+    console.log(`  \x1b[31m✗\x1b[0m ${name}`);
+    console.log(`    ${e.message}`);
+  }
+}
+
+// ═══════════════════════════════════════════════════════════
+// Rule (i): MV3 SW keep-alive via chrome.alarms
+// Why: MV3 SW unloads after ~30s of inactivity. Without a keep-alive,
+// idle daemon connections produce spurious peer_unreachable on next
+// op; classifyOpFailure routes those to reconnect_extension, but the
+// root cause is fixable here, not at the engine layer.
+// ═══════════════════════════════════════════════════════════
+
+console.log("\n  -- Rule (i): MV3 SW keep-alive --\n");
+
+test("chrome.alarms.create with name 'tap-keepalive' exists", () => {
+  // The name string is part of the contract — onAlarm dispatch matches
+  // on it, and arch tests cite it directly.
+  assert(
+    /chrome\.alarms\.create\s*\(\s*["']tap-keepalive["']/.test(BG_SRC),
+    "background.js must call chrome.alarms.create with name 'tap-keepalive'",
+  );
+});
+
+test("chrome.alarms.onAlarm.addListener wired to keepalive", () => {
+  assert(
+    /chrome\.alarms\.onAlarm\.addListener/.test(BG_SRC),
+    "background.js must register a chrome.alarms.onAlarm listener",
+  );
+  // Listener body must reference the keepalive alarm name (otherwise it
+  // would be a no-op dispatcher matching nothing).
+  const listenerStart = BG_SRC.indexOf("chrome.alarms.onAlarm.addListener");
+  const listenerBody = BG_SRC.slice(listenerStart, listenerStart + 600);
+  assert(
+    /tap-keepalive/.test(listenerBody),
+    "onAlarm listener body must dispatch on the 'tap-keepalive' alarm name",
+  );
+});
+
+test("keepalive period is < 0.5 minutes (< 30s, MV3 idle window)", () => {
+  // Default MV3 SW idle is 30s; keepalive must fire faster. Accept any
+  // periodInMinutes literal < 0.5 (i.e. <= 0.4 typical, or 0.49).
+  const m = BG_SRC.match(
+    /chrome\.alarms\.create\s*\(\s*["']tap-keepalive["']\s*,\s*\{[^}]*periodInMinutes:\s*([\d.]+)/,
+  );
+  assert(m, "chrome.alarms.create must specify periodInMinutes");
+  const period = parseFloat(m[1]);
+  assert(
+    period < 0.5,
+    `periodInMinutes ${period} >= 0.5 — SW would idle-die between alarms (MV3 idle ~30s = 0.5min)`,
+  );
+});
+
+// ═══════════════════════════════════════════════════════════
+// Rule (ii): chrome:// guard does NOT exempt fromDaemon
+// Why: dogfood 2026-05-08 — when active tab was chrome://extensions
+// (during reload), daemon-driven navs got `tab_closed: Cannot access
+// a chrome:// URL`. The exemption was a UX-preserving heuristic for
+// popup path that wrongly applied to daemon path.
+// ═══════════════════════════════════════════════════════════
+
+console.log("\n  -- Rule (ii): chrome:// guard always open background tab --\n");
+
+test("no `&& !fromDaemon` exemption in isInternal nav guard", () => {
+  // Strict text check: the exact stale pattern must be absent.
+  assert(
+    !/if\s*\(\s*isInternal\s*&&\s*!fromDaemon\s*\)/.test(BG_SRC),
+    "Stale exemption `if (isInternal && !fromDaemon)` must be deleted; " +
+      "daemon-driven navs always open managed background tab.",
+  );
+});
+
+test("isInternal guard exists and opens new tab", () => {
+  // The guard may be `if (isInternal)` or `if (isInternal || <other>)`.
+  // What matters: isInternal participates in a guard whose body opens
+  // a new background tab via chrome.tabs.create.
+  const idx = BG_SRC.search(/if\s*\(\s*isInternal[\s|)]/);
+  assert(
+    idx !== -1,
+    "Must contain `if (isInternal ...)` guard (with isInternal as first condition)",
+  );
+  const block = BG_SRC.slice(idx, idx + 600);
+  assert(
+    /chrome\.tabs\.create/.test(block),
+    "isInternal-branch must call chrome.tabs.create to open a new tab",
+  );
+});
+
+// ═══════════════════════════════════════════════════════════
+// Rule (iii): origin-mismatch nav → new background tab
+// Why: 2026-05-08 dogfood — Cloudflare nav redirected through CF
+// auth chain, leaving tab on dash.cloudflare.com/two-factor. Next
+// nav (juejin.cn/search) called `chrome.tabs.update(tabId, { url })`
+// to navigate same tab, but the eval ran on cloudflare login page —
+// silent data corruption. Same applies to parallel batch calls
+// sharing a tab. Fix: when daemon-driven nav target origin differs
+// from current tab origin, open a new background tab instead of
+// clobbering. Same-origin navs continue to use tabs.update (cheap).
+// ═══════════════════════════════════════════════════════════
+
+console.log("\n  -- Rule (iii): origin-mismatch nav → new background tab --\n");
+
+test("nav handler computes target origin", () => {
+  // Source-text proxy: must call new URL(...) on params.url to extract origin.
+  // Pattern: `new URL(params.url)` followed by `.origin` access OR variable
+  // assignment that's later compared to current origin.
+  assert(
+    /new URL\(params\.url\)/.test(BG_SRC),
+    "nav handler must construct URL(params.url) to extract target origin",
+  );
+});
+
+test("nav handler compares target.origin vs current.origin", () => {
+  // Must read .origin from both target and current to compare.
+  // Looser pattern: at least 2 occurrences of `.origin` near nav case
+  // (one for target, one for current).
+  const navStart = BG_SRC.indexOf("case 'nav':");
+  assert(navStart !== -1, "nav case handler must exist");
+  // Search a 2000-char window starting from `case 'nav':`.
+  const navBlock = BG_SRC.slice(navStart, navStart + 2000);
+  const originAccesses = navBlock.match(/\.origin\b/g) || [];
+  assert(
+    originAccesses.length >= 2,
+    `nav handler must access .origin on both target and current to compare; ` +
+      `found ${originAccesses.length} .origin access(es) in 2000-char window`,
+  );
+});
+
+test("origin mismatch branch opens new tab via chrome.tabs.create", () => {
+  // Two acceptable idioms:
+  //   (a) inline:   if (target.origin !== current.origin) { chrome.tabs.create(...) }
+  //   (b) variable: const cross = a.origin !== b.origin; if (... || cross) { chrome.tabs.create(...) }
+  // What matters: somewhere in the nav handler there's an `.origin !==
+  // .origin` comparison whose result drives a chrome.tabs.create branch.
+  const navStart = BG_SRC.indexOf("case 'nav':");
+  const navBlock = BG_SRC.slice(navStart, navStart + 3000);
+  // Step 1: confirm origin-vs-origin comparison appears.
+  assert(
+    /\.origin\s*!==?\s*[a-zA-Z_$.]*\.origin/.test(navBlock),
+    "nav handler must compare `.origin !== .origin` (cross-origin detection)",
+  );
+  // Step 2: confirm chrome.tabs.create appears within the same nav block.
+  assert(
+    /chrome\.tabs\.create/.test(navBlock),
+    "nav handler must call chrome.tabs.create somewhere",
+  );
+  // Step 3: confirm the result of the origin comparison influences a
+  // boolean used in the if-guard. Look for either:
+  //   - inline:    if (...origin !==...origin...) { ... chrome.tabs.create
+  //   - variable:  crossOrigin (or similar) referenced in if + assigned from origin compare
+  const inlinePattern =
+    /if\s*\([^)]*\.origin\s*!==?[^)]*\.origin[^)]*\)\s*\{[\s\S]{0,500}chrome\.tabs\.create/;
+  const variablePattern =
+    /(\w+)\s*=\s*[^;]*\.origin\s*!==?\s*[a-zA-Z_$.]*\.origin[\s\S]{0,500}if\s*\([^)]*\1[^)]*\)\s*\{[\s\S]{0,500}chrome\.tabs\.create/;
+  assert(
+    inlinePattern.test(navBlock) || variablePattern.test(navBlock),
+    "nav handler must use the origin comparison (inline or via boolean " +
+      "variable) to gate a chrome.tabs.create branch",
+  );
+});
+
+// ═══════════════════════════════════════════════════════════
+
+console.log(`\n  ${passed} passed, ${failed} failed`);
+process.exit(failed ? 1 : 0);