diff --git a/extension/src/service-worker/__tests__/session-timer.test.ts b/extension/src/service-worker/__tests__/session-timer.test.ts
index 39a56fc..93a04aa 100644
--- a/extension/src/service-worker/__tests__/session-timer.test.ts
+++ b/extension/src/service-worker/__tests__/session-timer.test.ts
@@ -1,5 +1,6 @@
 import { describe, expect, it, vi, beforeEach, afterEach } from 'vitest';
 import * as timer from '../session-timer';
+import { READ_ONLY_CONTENT_CALLABLE } from '../session-timer';
 
 describe('session-timer', () => {
   beforeEach(() => {
@@ -97,3 +98,29 @@ describe('session-timer', () => {
     expect(cb).not.toHaveBeenCalled();
   });
 });
+
+describe('READ_ONLY_CONTENT_CALLABLE — inversion exclusion set', () => {
+  // The SW handler invokes resetTimer() on every message whose type is NOT
+  // in this set. These cases encode the documented inversion contract from
+  // Plan C Phase 5: popup-only messages reset, content-callable writes
+  // reset, only passive content reads (currently just get_autofill_candidates)
+  // do NOT reset.
+
+  it('popup-only message would reset the timer (not in exclusion set)', () => {
+    // e.g. list_items — popup interaction is unambiguously active use
+    expect(READ_ONLY_CONTENT_CALLABLE.has('list_items')).toBe(false);
+  });
+
+  it('content-callable get_autofill_candidates does NOT reset (in exclusion set)', () => {
+    expect(READ_ONLY_CONTENT_CALLABLE.has('get_autofill_candidates')).toBe(true);
+  });
+
+  it('content-callable capture_save_login DOES reset (write op = active use)', () => {
+    expect(READ_ONLY_CONTENT_CALLABLE.has('capture_save_login')).toBe(false);
+  });
+
+  it('content-callable check_credential DOES reset', () => {
+    // Asking "is this credential already saved" is user-initiated.
+    expect(READ_ONLY_CONTENT_CALLABLE.has('check_credential')).toBe(false);
+  });
+});
diff --git a/extension/src/service-worker/index.ts b/extension/src/service-worker/index.ts
index e3dec89..cab9a83 100644
--- a/extension/src/service-worker/index.ts
+++ b/extension/src/service-worker/index.ts
@@ -2,12 +2,12 @@
 /// forwards every message into router/index.route().
 
 import type { Request, Response, SessionTimeoutConfig } from '../shared/messages';
-import { CONTENT_CALLABLE_TYPES } from '../shared/messages';
 import type { RouterState } from './router/index';
 import { route } from './router/index';
 import * as vault from './vault';
 import { clearCurrent } from './session';
 import * as sessionTimer from './session-timer';
+import { READ_ONLY_CONTENT_CALLABLE } from './session-timer';
 
 // @ts-ignore TS2307 — resolved by webpack alias / copy
 import initDefault, { initSync } from '../../wasm/relicario_wasm.js';
@@ -73,7 +73,10 @@ chrome.commands.onCommand.addListener((command) => {
 chrome.runtime.onMessage.addListener(
   (request: Request, sender: chrome.runtime.MessageSender, sendResponse: (r: Response) => void) => {
     (async () => {
-      if (!CONTENT_CALLABLE_TYPES.has(request.type as never)) {
+      // Plan C Phase 5: invert the reset rule. Reset on every message
+      // except a documented passive-read exclusion set, so an active
+      // autofiller / content-driven flow keeps the vault alive.
+      if (!READ_ONLY_CONTENT_CALLABLE.has(request.type)) {
         sessionTimer.resetTimer();
       }
       if (!state.wasm) {
diff --git a/extension/src/service-worker/session-timer.ts b/extension/src/service-worker/session-timer.ts
index 9948586..2a7800a 100644
--- a/extension/src/service-worker/session-timer.ts
+++ b/extension/src/service-worker/session-timer.ts
@@ -48,3 +48,17 @@ export function stopTimer(): void {
     timerId = null;
   }
 }
+
+/**
+ * Content-callable message types that should NOT reset the inactivity timer.
+ *
+ * Rationale: a content script reading available autofill candidates is a
+ * passive query — it shouldn't keep the vault alive indefinitely while the
+ * user isn't actually interacting with it.
+ *
+ * Today this is the only known passive read; if a future content message
+ * is also passive, add it here with a one-line justification.
+ */
+export const READ_ONLY_CONTENT_CALLABLE: ReadonlySet<string> = new Set([
+  'get_autofill_candidates',
+]);