diff --git a/src/metamemory/context/index.ts b/src/metamemory/context/index.ts
index e05d2f6..b7bf6d9 100644
--- a/src/metamemory/context/index.ts
+++ b/src/metamemory/context/index.ts
@@ -127,6 +127,24 @@ export class ContextAssembler {
         content: `Summary of earlier discussion:\n${thread.summary}`
       } as ResponseInputItem);
     }
+
+    // Add summaries of related threads
+    const relatedThreads = this.threadManager.getRelatedThreads(thread.name);
+    if (relatedThreads.length > 0) {
+      let relatedText = 'Related topic summaries:\n';
+      for (const rel of relatedThreads) {
+        if (rel.summary) {
+          relatedText += `Topic: ${rel.name}\nSummary: ${rel.summary}\n`;
+        }
+      }
+      if (relatedText.trim() !== 'Related topic summaries:') {
+        items.push({
+          type: 'message',
+          role: 'system',
+          content: relatedText.trimEnd()
+        } as ResponseInputItem);
+      }
+    }
     
     // Add recent messages from the thread
     const recentMessages = this.threadManager.getRecentMessages(thread, recentMessageCount);
diff --git a/test/metamemory-new.test.ts b/test/metamemory-new.test.ts
index eb38453..126dab5 100644
--- a/test/metamemory-new.test.ts
+++ b/test/metamemory-new.test.ts
@@ -4,6 +4,7 @@ import { TopicThreadManager } from '../src/metamemory/utils/topic-thread-manager
 import { InMemoryVectorSearch } from '../src/metamemory/utils/vector-search';
 import { MessageTagger, TaggerLLM } from '../src/metamemory/tagger';
 import { ThreadCompactor, SummarizerInterface } from '../src/metamemory/compactor';
+import { ContextAssembler } from '../src/metamemory/context';
 import type { 
   Message, 
   TaggedMessage, 
@@ -240,7 +241,45 @@ describe('MetaMemory System', () => {
       expect(results[0].relevanceScore).toBeGreaterThan(results[1].relevanceScore);
     });
   });
-  
+
+  describe('ContextAssembler', () => {
+    let manager: TopicThreadManager;
+    let assembler: ContextAssembler;
+
+    beforeEach(() => {
+      manager = new TopicThreadManager();
+      assembler = new ContextAssembler(manager);
+    });
+
+    it('includes summaries of related threads for active topics', async () => {
+      const active = manager.createThread('active_topic', 'active');
+      active.summary = 'Active summary';
+      manager.addMessageToThread('active_topic', {
+        id: 'a1',
+        role: 'user',
+        content: 'hello',
+        timestamp: Date.now()
+      });
+
+      const related = manager.createThread('related_topic', 'idle');
+      related.summary = 'Related summary';
+      manager.addThreadRelationship('active_topic', 'related_topic');
+
+      const spy = vi.spyOn(manager, 'getRelatedThreads');
+
+      const context = await assembler.buildContext([], {
+        maxTokens: 1000,
+        includeIdleSummaries: false,
+        includeArchivedSearch: false,
+        recentMessageCount: 5
+      });
+
+      expect(spy).toHaveBeenCalledWith('active_topic');
+      const combined = context.map(m => ('content' in m ? m.content : '')).join(' ');
+      expect(combined).toContain('Related summary');
+    });
+  });
+
   describe('Metamemory Integration', () => {
     let metamemory: Metamemory;