diff --git a/extensions/piagentui-server.ts b/extensions/piagentui-server.ts
index 5ef3c5c..62a3944 100644
--- a/extensions/piagentui-server.ts
+++ b/extensions/piagentui-server.ts
@@ -21,6 +21,7 @@ import {
   type StoredSttSettings,
   type StoredTtsSettings,
 } from './piagentui-settings-store.js'
+import { createStatCache } from './piagentui-stat-cache.js'
 import { applySttSettingsUpdate, applyTtsSettingsUpdate, isTtsResponseFormat } from './piagentui-speech-settings.js'
 import {
   buildExtensionCatalog,
@@ -518,48 +519,76 @@ export default function (pi: ExtensionAPI) {
       .slice(0, 120)
   }
 
+  type ParsedSessionFile = {
+    id: string
+    title: string
+    cwd: string | undefined
+    headerTimestamp: string | undefined
+    messageCount: number
+  }
+
+  // GET /api/sessions parses every session file; re-parse only files whose mtime/size changed.
+  const sessionFileCache = createStatCache<ParsedSessionFile | null>()
+
+  function parseSessionFile(filePath: string): ParsedSessionFile | null {
+    const content = fs.readFileSync(filePath, 'utf8')
+    const lines = content.trim().split('\n').filter(Boolean)
+    if (lines.length === 0) return null
+
+    const header = JSON.parse(lines[0])
+    if (header.type !== 'session' || !header.id) return null
+
+    let sessionInfoTitle = ''
+    let firstUserMessage = ''
+    let messageCount = 0
+
+    for (const line of lines.slice(1)) {
+      try {
+        const entry = JSON.parse(line)
+        if (entry.type === 'session_info') {
+          sessionInfoTitle = normalizeSessionTitle(entry.name || '')
+        } else if (entry.type === 'message') {
+          messageCount++
+          if (!firstUserMessage && entry.message?.role === 'user') {
+            firstUserMessage = normalizeSessionTitle(getMessageText(entry.message))
+          }
+        }
+      } catch {
+        // ignore malformed entries
+      }
+    }
+
+    return {
+      id: header.id,
+      title: sessionInfoTitle || firstUserMessage || getFallbackSessionTitle(header.id),
+      cwd: header.cwd,
+      headerTimestamp: header.timestamp,
+      messageCount,
+    }
+  }
+
   function readSessionMetadata(filePath: string): any | null {
     try {
-      const content = fs.readFileSync(filePath, 'utf8')
-      const lines = content.trim().split('\n').filter(Boolean)
-      if (lines.length === 0) return null
-
-      const header = JSON.parse(lines[0])
-      if (header.type !== 'session' || !header.id) return null
-
-      let sessionInfoTitle = ''
-      let firstUserMessage = ''
-      let messageCount = 0
-
-      for (const line of lines.slice(1)) {
-        try {
-          const entry = JSON.parse(line)
-          if (entry.type === 'session_info') {
-            sessionInfoTitle = normalizeSessionTitle(entry.name || '')
-          } else if (entry.type === 'message') {
-            messageCount++
-            if (!firstUserMessage && entry.message?.role === 'user') {
-              firstUserMessage = normalizeSessionTitle(getMessageText(entry.message))
-            }
-          }
-        } catch {
-          // ignore malformed entries
-        }
+      const stats = fs.statSync(filePath)
+      let cached = sessionFileCache.get(filePath, stats)
+      if (!cached) {
+        cached = { value: parseSessionFile(filePath) }
+        sessionFileCache.set(filePath, stats, cached.value)
       }
 
-      const stats = fs.statSync(filePath)
-      const createdTime = header.timestamp ? new Date(header.timestamp).getTime() : stats.birthtimeMs
-      const title = sessionInfoTitle || firstUserMessage || getFallbackSessionTitle(header.id)
+      const parsed = cached.value
+      if (!parsed) return null
 
+      const createdTime = parsed.headerTimestamp ? new Date(parsed.headerTimestamp).getTime() : stats.birthtimeMs
       return {
-        id: header.id,
-        title,
-        directory: header.cwd || latestCtx?.cwd || process.cwd(),
+        id: parsed.id,
+        title: parsed.title,
+        directory: parsed.cwd || latestCtx?.cwd || process.cwd(),
         time: {
           created: Number.isNaN(createdTime) ? stats.birthtimeMs : createdTime,
           updated: stats.mtimeMs,
         },
-        summary: { deletions: 0, files: 0, additions: messageCount },
+        summary: { deletions: 0, files: 0, additions: parsed.messageCount },
       }
     } catch {
       return null
diff --git a/extensions/piagentui-stat-cache.test.ts b/extensions/piagentui-stat-cache.test.ts
new file mode 100644
index 0000000..30ef7d5
--- /dev/null
+++ b/extensions/piagentui-stat-cache.test.ts
@@ -0,0 +1,50 @@
+import { describe, expect, it } from 'vitest'
+import { createStatCache } from './piagentui-stat-cache.js'
+
+const stamp = (mtimeMs: number, size = 10) => ({ mtimeMs, size })
+
+describe('createStatCache', () => {
+  it('misses for unknown paths and hits for the same stamp', () => {
+    const cache = createStatCache<string>()
+    expect(cache.get('a', stamp(1))).toBeUndefined()
+    cache.set('a', stamp(1), 'x')
+    expect(cache.get('a', stamp(1))).toEqual({ value: 'x' })
+  })
+
+  it('treats a changed mtime or size as a miss and drops the stale entry', () => {
+    const cache = createStatCache<string>()
+    cache.set('a', stamp(1, 10), 'x')
+    expect(cache.get('a', stamp(2, 10))).toBeUndefined()
+    expect(cache.size).toBe(0)
+
+    cache.set('a', stamp(1, 10), 'x')
+    expect(cache.get('a', stamp(1, 11))).toBeUndefined()
+  })
+
+  it('can cache null values', () => {
+    const cache = createStatCache<string | null>()
+    cache.set('a', stamp(1), null)
+    expect(cache.get('a', stamp(1))).toEqual({ value: null })
+  })
+
+  it('evicts the oldest entries beyond the limit', () => {
+    const cache = createStatCache<number>(2)
+    cache.set('a', stamp(1), 1)
+    cache.set('b', stamp(1), 2)
+    cache.set('c', stamp(1), 3)
+    expect(cache.size).toBe(2)
+    expect(cache.get('a', stamp(1))).toBeUndefined()
+    expect(cache.get('b', stamp(1))).toEqual({ value: 2 })
+    expect(cache.get('c', stamp(1))).toEqual({ value: 3 })
+  })
+
+  it('refreshing a key moves it to the newest position', () => {
+    const cache = createStatCache<number>(2)
+    cache.set('a', stamp(1), 1)
+    cache.set('b', stamp(1), 2)
+    cache.set('a', stamp(2), 11)
+    cache.set('c', stamp(1), 3)
+    expect(cache.get('b', stamp(1))).toBeUndefined()
+    expect(cache.get('a', stamp(2))).toEqual({ value: 11 })
+  })
+})
diff --git a/extensions/piagentui-stat-cache.ts b/extensions/piagentui-stat-cache.ts
new file mode 100644
index 0000000..d1a63ab
--- /dev/null
+++ b/extensions/piagentui-stat-cache.ts
@@ -0,0 +1,34 @@
+export type FileStamp = { mtimeMs: number; size: number }
+
+/**
+ * Small bounded cache of values derived from file contents, keyed by path and
+ * invalidated when the file's mtime or size changes. Oldest entries are evicted first.
+ */
+export function createStatCache<T>(maxEntries = 500) {
+  const entries = new Map<string, FileStamp & { value: T }>()
+
+  return {
+    /** Returns `{ value }` on a hit (the value itself may be null), `undefined` on a miss or stale entry. */
+    get(filePath: string, stamp: FileStamp): { value: T } | undefined {
+      const entry = entries.get(filePath)
+      if (!entry) return undefined
+      if (entry.mtimeMs !== stamp.mtimeMs || entry.size !== stamp.size) {
+        entries.delete(filePath)
+        return undefined
+      }
+      return { value: entry.value }
+    },
+    set(filePath: string, stamp: FileStamp, value: T): void {
+      entries.delete(filePath)
+      entries.set(filePath, { mtimeMs: stamp.mtimeMs, size: stamp.size, value })
+      while (entries.size > maxEntries) {
+        const oldest = entries.keys().next().value
+        if (oldest === undefined) break
+        entries.delete(oldest)
+      }
+    },
+    get size(): number {
+      return entries.size
+    },
+  }
+}