diff --git a/packages/compact/compact-basic/src/index.ts b/packages/compact/compact-basic/src/index.ts
index 4c7bce00bc..f7fdadb3fa 100644
--- a/packages/compact/compact-basic/src/index.ts
+++ b/packages/compact/compact-basic/src/index.ts
@@ -291,6 +291,7 @@ export class BasicCompactService extends CompactService {
       }],
       system: SUMMARIZE_SYSTEM_PROMPT,
       maxTokens: this.config.maxTokens,
+      sessionId: agent.session.id,
     }
     // exactOptionalPropertyTypes: only set `signal` when present — assigning
     // `undefined` to an optional `signal?: AbortSignal` is a type error.
@@ -512,8 +513,6 @@ export class BasicCompactService extends CompactService {
 
   // ---- Internal helpers ----
 
-  /**
-
   /**
    * Frame the raw summary blocks into the content that lands on the surface:
    * a checkpoint preamble (so a resuming model reads it as a checkpoint, not a
diff --git a/packages/compact/compact-basic/tests/compact-basic.spec.ts b/packages/compact/compact-basic/tests/compact-basic.spec.ts
index 1134862793..10aae6d8be 100644
--- a/packages/compact/compact-basic/tests/compact-basic.spec.ts
+++ b/packages/compact/compact-basic/tests/compact-basic.spec.ts
@@ -975,6 +975,7 @@ describe('BasicCompactService.summarize (real ctx.llm.stream)', () => {
     expect(adapter.lastOptions!.system).toContain('compaction engine')
     expect(adapter.lastOptions!.system).toContain('## Next Step')
     expect(adapter.lastOptions!.maxTokens).toBe(512)
+    expect(adapter.lastOptions!.sessionId).toBe(SessionId('summary'))
     expect(adapter.lastOptions!.messages[0]!.content[0]).toMatchObject({ type: 'text' })
   })