diff --git a/src/utils/incrementalTokenCounter.test.ts b/src/utils/incrementalTokenCounter.test.ts
index 26c42d905a..203674d4eb 100644
--- a/src/utils/incrementalTokenCounter.test.ts
+++ b/src/utils/incrementalTokenCounter.test.ts
@@ -1,4 +1,5 @@
 import { describe, expect, it, beforeEach } from 'bun:test'
+import { roughTokenCountEstimationForMessages } from '../services/tokenEstimation.js'
 import { IncrementalTokenCounter, CounterFactory } from './incrementalTokenCounter.js'
 import type { Message } from '../types/message.js'
 
@@ -247,6 +248,32 @@ describe('IncrementalTokenCounter', () => {
       expect(count2).toBeGreaterThan(count1)
     })
   })
+
+  describe('attachment cache invalidation', () => {
+    it('recalculates when attachment-only messages differ', () => {
+      const counter = new IncrementalTokenCounter()
+      const smallAttachment = {
+        type: 'attachment',
+        attachment: {
+          type: 'opened_file_in_ide',
+          filename: 'a',
+        },
+      } as unknown as Message
+      const largeAttachment = {
+        type: 'attachment',
+        attachment: {
+          type: 'opened_file_in_ide',
+          filename: 'x'.repeat(2000),
+        },
+      } as unknown as Message
+
+      const smallCount = counter.getCount([smallAttachment])
+      const largeCount = counter.getCount([largeAttachment])
+
+      expect(largeCount).toBe(roughTokenCountEstimationForMessages([largeAttachment]))
+      expect(largeCount).toBeGreaterThan(smallCount)
+    })
+  })
 })
 
 describe('CounterFactory', () => {
@@ -270,4 +297,4 @@ describe('CounterFactory', () => {
       expect(counter).toBeInstanceOf(IncrementalTokenCounter)
     })
   })
-})
\ No newline at end of file
+})
diff --git a/src/utils/incrementalTokenCounter.ts b/src/utils/incrementalTokenCounter.ts
index c88ddaf4fa..690b099459 100644
--- a/src/utils/incrementalTokenCounter.ts
+++ b/src/utils/incrementalTokenCounter.ts
@@ -30,16 +30,16 @@ export interface CounterStats {
 function getMessageHash(messages: readonly Message[]): string {
   if (messages.length === 0) return 'empty'
 
-  const fullContent = messages.map(m => {
-    const c = typeof m.message?.content === 'string'
-      ? m.message.content
-      : Array.isArray(m.message?.content)
-        ? JSON.stringify(m.message.content)
-        : ''
-    return c
-  }).join('|')
+  const tokenRelevantInput = messages.map(m => ({
+    type: m.type,
+    content: m.message?.content ?? null,
+    attachment: m.attachment ?? null,
+  }))
 
-  return createHash('sha256').update(fullContent).digest('hex').slice(0, 16)
+  return createHash('sha256')
+    .update(JSON.stringify(tokenRelevantInput))
+    .digest('hex')
+    .slice(0, 16)
 }
 
 /**
@@ -254,4 +254,4 @@ export const CounterFactory = {
       estimationMultiplier: 1.2,
     })
   },
-}
\ No newline at end of file
+}