diff --git a/folder-index-state.js b/folder-index-state.js
new file mode 100644
index 0000000..7e52deb
--- /dev/null
+++ b/folder-index-state.js
@@ -0,0 +1,29 @@
+const fs = require('fs');
+const path = require('path');
+
+function getFolderIndexMtimeMs(folderPath) {
+  let indexMtimeMs = 0;
+
+  try {
+    indexMtimeMs = fs.statSync(folderPath).mtimeMs;
+  } catch {
+    return 0;
+  }
+
+  try {
+    // Session files are appended in place, which updates the file mtime but
+    // often leaves the containing directory mtime unchanged.
+    const entries = fs.readdirSync(folderPath, { withFileTypes: true });
+    for (const entry of entries) {
+      if (!entry.isFile() || !entry.name.endsWith('.jsonl')) continue;
+      try {
+        const fileMtimeMs = fs.statSync(path.join(folderPath, entry.name)).mtimeMs;
+        if (fileMtimeMs > indexMtimeMs) indexMtimeMs = fileMtimeMs;
+      } catch {}
+    }
+  } catch {}
+
+  return indexMtimeMs;
+}
+
+module.exports = { getFolderIndexMtimeMs };
diff --git a/main.js b/main.js
index 444aae9..9e24429 100644
--- a/main.js
+++ b/main.js
@@ -5,6 +5,7 @@ const fs = require('fs');
 const os = require('os');
 const pty = require('node-pty');
 const log = require('electron-log');
+const { getFolderIndexMtimeMs } = require('./folder-index-state');
 log.transports.file.level = app.isPackaged ? 'info' : 'debug';
 log.transports.console.level = app.isPackaged ? 'info' : 'debug';
 
@@ -347,10 +348,7 @@ function refreshFolder(folder) {
 
   const projectPath = deriveProjectPath(folderPath, folder);
   if (!projectPath) {
-    // Still record mtime so backgroundRefresh doesn't keep retrying
-    let mtimeMs = 0;
-    try { mtimeMs = fs.statSync(folderPath).mtimeMs; } catch {}
-    setFolderMeta(folder, null, mtimeMs);
+    setFolderMeta(folder, null, getFolderIndexMtimeMs(folderPath));
     return;
   }
 
@@ -407,9 +405,7 @@ function refreshFolder(folder) {
   }
 
   // Update folder mtime
-  let mtimeMs = 0;
-  try { mtimeMs = fs.statSync(folderPath).mtimeMs; } catch {}
-  setFolderMeta(folder, projectPath, mtimeMs);
+  setFolderMeta(folder, projectPath, getFolderIndexMtimeMs(folderPath));
 }
 
 /** Populate entire cache from filesystem (cold start) */
@@ -505,8 +501,7 @@ function backgroundRefresh() {
     // Check for new/changed folders
     for (const folder of folders) {
       const folderPath = path.join(PROJECTS_DIR, folder);
-      let currentMtime = 0;
-      try { currentMtime = fs.statSync(folderPath).mtimeMs; } catch {}
+      const currentMtime = getFolderIndexMtimeMs(folderPath);
 
       const cached = metaMap.get(folder);
       if (!cached || cached.indexMtimeMs !== currentMtime) {
@@ -576,7 +571,7 @@ function populateCacheViaWorker() {
 
     // Write results to DB on main thread (fast)
     let sessionCount = 0;
-    for (const { folder, projectPath, sessions, mtimeMs } of msg.results) {
+    for (const { folder, projectPath, sessions, indexMtimeMs } of msg.results) {
       deleteCachedFolder(folder);
       deleteSearchFolder(folder);
       if (sessions.length > 0) {
@@ -590,7 +585,7 @@ function populateCacheViaWorker() {
           if (s.customTitle) setName(s.sessionId, s.customTitle);
         }
       }
-      setFolderMeta(folder, projectPath, mtimeMs);
+      setFolderMeta(folder, projectPath, indexMtimeMs);
     }
 
     populatingCache = false;
diff --git a/package.json b/package.json
index 4559bc7..584312e 100644
--- a/package.json
+++ b/package.json
@@ -7,6 +7,7 @@
   "main": "main.js",
   "scripts": {
     "start": "npm run bundle:codemirror && electron .",
+    "test": "node --test",
     "electron": "electron .",
     "bundle:codemirror": "esbuild public/codemirror-setup.js --bundle --outfile=public/codemirror-bundle.js --format=iife --platform=browser --minify",
     "generate-icons": "node scripts/generate-icons.js && node scripts/generate-dmg-background.js",
diff --git a/test/folder-index-state.test.js b/test/folder-index-state.test.js
new file mode 100644
index 0000000..aa070b1
--- /dev/null
+++ b/test/folder-index-state.test.js
@@ -0,0 +1,28 @@
+const test = require('node:test');
+const assert = require('node:assert/strict');
+const fs = require('fs');
+const os = require('os');
+const path = require('path');
+
+const { getFolderIndexMtimeMs } = require('../folder-index-state');
+
+test('folder index timestamp advances when an existing session file is appended', async () => {
+  const tmpDir = fs.mkdtempSync(path.join(os.tmpdir(), 'switchboard-folder-index-'));
+
+  try {
+    const sessionPath = path.join(tmpDir, 'session.jsonl');
+    fs.writeFileSync(sessionPath, '{"type":"user","message":"first"}\n', 'utf8');
+
+    const before = getFolderIndexMtimeMs(tmpDir);
+
+    await new Promise(resolve => setTimeout(resolve, 1100));
+
+    fs.appendFileSync(sessionPath, '{"type":"assistant","message":"second"}\n', 'utf8');
+
+    const after = getFolderIndexMtimeMs(tmpDir);
+
+    assert.ok(after > before, `expected index mtime to increase (${before} -> ${after})`);
+  } finally {
+    fs.rmSync(tmpDir, { recursive: true, force: true });
+  }
+});
diff --git a/workers/scan-projects.js b/workers/scan-projects.js
index 2caeb47..e29c22e 100644
--- a/workers/scan-projects.js
+++ b/workers/scan-projects.js
@@ -1,6 +1,7 @@
 const { parentPort, workerData } = require('worker_threads');
 const fs = require('fs');
 const path = require('path');
+const { getFolderIndexMtimeMs } = require('../folder-index-state');
 
 const PROJECTS_DIR = workerData.projectsDir;
 
@@ -49,8 +50,7 @@ function readFolderFromFilesystem(folder) {
   const projectPath = deriveProjectPath(folderPath, folder);
   if (!projectPath) return null;
   const sessions = [];
-  let mtimeMs = 0;
-  try { mtimeMs = fs.statSync(folderPath).mtimeMs; } catch {}
+  const indexMtimeMs = getFolderIndexMtimeMs(folderPath);
 
   try {
     const jsonlFiles = fs.readdirSync(folderPath).filter(f => f.endsWith('.jsonl'));
@@ -99,7 +99,7 @@ function readFolderFromFilesystem(folder) {
     }
   } catch {}
 
-  return { folder, projectPath, sessions, mtimeMs };
+  return { folder, projectPath, sessions, indexMtimeMs };
 }
 
 // Scan all folders