diff --git a/packages/builtin-tools/src/tools/AgentTool/runAgent.ts b/packages/builtin-tools/src/tools/AgentTool/runAgent.ts
index 0d718f538..7857b7445 100644
--- a/packages/builtin-tools/src/tools/AgentTool/runAgent.ts
+++ b/packages/builtin-tools/src/tools/AgentTool/runAgent.ts
@@ -853,6 +853,11 @@ export async function* runAgent({
     if (feature('PROMPT_CACHE_BREAK_DETECTION')) {
       cleanupAgentTracking(agentId)
     }
+    if (agentToolUseContext.readFileState !== toolUseContext.readFileState) {
+      toolUseContext.readFileState.mergeFrom(
+        agentToolUseContext.readFileState,
+      )
+    }
     // Release cloned file state cache memory
     agentToolUseContext.readFileState.clear()
     // Release the cloned fork context messages
diff --git a/src/utils/__tests__/fileStateCache.test.ts b/src/utils/__tests__/fileStateCache.test.ts
index 0cfb3d21b..d7288ad94 100644
--- a/src/utils/__tests__/fileStateCache.test.ts
+++ b/src/utils/__tests__/fileStateCache.test.ts
@@ -146,3 +146,80 @@ describe('coerceToolContentToString', () => {
     expect(coerceToolContentToString(nested)).toBe('{"a":{"b":[1,2,3]}}')
   })
 })
+
+describe('FileStateCache.mergeFrom', () => {
+  test('merges new entries from other cache into target', () => {
+    const target = createFileStateCacheWithSizeLimit(100)
+    const other = createFileStateCacheWithSizeLimit(100)
+
+    target.set('/a.txt', makeEntry('old-a', { timestamp: 100 }))
+    other.set('/b.txt', makeEntry('new-b', { timestamp: 200 }))
+
+    target.mergeFrom(other)
+
+    expect(target.get('/a.txt')?.content).toBe('old-a')
+    expect(target.get('/b.txt')?.content).toBe('new-b')
+    expect(target.size).toBe(2)
+  })
+
+  test('overrides entries with newer timestamp from other', () => {
+    const target = createFileStateCacheWithSizeLimit(100)
+    const other = createFileStateCacheWithSizeLimit(100)
+
+    target.set('/a.txt', makeEntry('old-content', { timestamp: 100 }))
+    other.set('/a.txt', makeEntry('new-content', { timestamp: 200 }))
+
+    target.mergeFrom(other)
+
+    expect(target.get('/a.txt')?.content).toBe('new-content')
+  })
+
+  test('does not override entries with older timestamp', () => {
+    const target = createFileStateCacheWithSizeLimit(100)
+    const other = createFileStateCacheWithSizeLimit(100)
+
+    target.set('/a.txt', makeEntry('newer-content', { timestamp: 200 }))
+    other.set('/a.txt', makeEntry('older-content', { timestamp: 100 }))
+
+    target.mergeFrom(other)
+
+    expect(target.get('/a.txt')?.content).toBe('newer-content')
+  })
+
+  test('handles empty other cache (no-op)', () => {
+    const target = createFileStateCacheWithSizeLimit(100)
+    const other = createFileStateCacheWithSizeLimit(100)
+
+    target.set('/a.txt', makeEntry('content'))
+    target.mergeFrom(other)
+
+    expect(target.size).toBe(1)
+    expect(target.get('/a.txt')?.content).toBe('content')
+  })
+
+  test('merges into empty target cache', () => {
+    const target = createFileStateCacheWithSizeLimit(100)
+    const other = createFileStateCacheWithSizeLimit(100)
+
+    other.set('/a.txt', makeEntry('content-a'))
+    other.set('/b.txt', makeEntry('content-b'))
+    target.mergeFrom(other)
+
+    expect(target.size).toBe(2)
+    expect(target.get('/a.txt')?.content).toBe('content-a')
+    expect(target.get('/b.txt')?.content).toBe('content-b')
+  })
+
+  test('preserves existing entries when merging new ones', () => {
+    const target = createFileStateCacheWithSizeLimit(100)
+    const other = createFileStateCacheWithSizeLimit(100)
+
+    target.set('/keep.txt', makeEntry('keep-me'))
+    other.set('/new.txt', makeEntry('new-file'))
+
+    target.mergeFrom(other)
+
+    expect(target.has('/keep.txt')).toBe(true)
+    expect(target.has('/new.txt')).toBe(true)
+  })
+})
diff --git a/src/utils/fileStateCache.ts b/src/utils/fileStateCache.ts
index 165b3fa1d..822b75a71 100644
--- a/src/utils/fileStateCache.ts
+++ b/src/utils/fileStateCache.ts
@@ -101,6 +101,15 @@ export class FileStateCache {
   load(entries: ReturnType<LRUCache<string, FileState>['dump']>): void {
     this.cache.load(entries)
   }
+
+  mergeFrom(other: FileStateCache): void {
+    for (const [filePath, fileState] of other.entries()) {
+      const existing = this.get(filePath)
+      if (!existing || fileState.timestamp > existing.timestamp) {
+        this.set(filePath, fileState)
+      }
+    }
+  }
 }
 
 /**