diff --git a/apps/server/src/orchestration-v2/Adapters/AcpAdapterV2.ts b/apps/server/src/orchestration-v2/Adapters/AcpAdapterV2.ts
index e37fe223529e..b3df4d7b3b12 100644
--- a/apps/server/src/orchestration-v2/Adapters/AcpAdapterV2.ts
+++ b/apps/server/src/orchestration-v2/Adapters/AcpAdapterV2.ts
@@ -6938,6 +6938,9 @@ export function makeAcpAdapterV2(options: AcpAdapterV2Options): ProviderAdapterV
                     ) {
                       context.promptSettled = true;
                       context.promptSettledStatus = status;
+                      // The agent finished this prompt's reply. Background work
+                      // holds the run open, not the text it already sent.
+                      yield* closeTextStreams(context);
                       return;
                     }
                     yield* finalizeTurn(context, status);
diff --git a/apps/server/src/orchestration-v2/testkit/fixtures/grok_background_subagent/output.ts b/apps/server/src/orchestration-v2/testkit/fixtures/grok_background_subagent/output.ts
index 8311aa92f574..77fcec010038 100644
--- a/apps/server/src/orchestration-v2/testkit/fixtures/grok_background_subagent/output.ts
+++ b/apps/server/src/orchestration-v2/testkit/fixtures/grok_background_subagent/output.ts
@@ -65,6 +65,23 @@ export function assertGrokBackgroundSubagentOutput(
   assert.isAbove(rootCompletedAt, subagentCompletedAt, "run 1 completed before the subagent");
   assert.include(runAssistantTexts(projection, rootRun?.id), "ROOT_DONE");
 
+  // Grok ends ROOT_DONE with its prompt; the subagent running on must not keep
+  // the reply streaming.
+  const rootDoneCompletedAt = result.domainEvents.findIndex(
+    (event) =>
+      event.type === "turn-item.updated" &&
+      event.payload.runId === rootRun?.id &&
+      event.payload.type === "assistant_message" &&
+      event.payload.text.trim() === "ROOT_DONE" &&
+      event.payload.status === "completed",
+  );
+  assert.isAtLeast(rootDoneCompletedAt, 0);
+  assert.isBelow(
+    rootDoneCompletedAt,
+    subagentCompletedAt,
+    "ROOT_DONE streamed until the subagent finished",
+  );
+
   // The subagent's own reply stays in its child thread.
   if (subagent?.childThreadId == null) {
     throw new Error("The background subagent is missing its child thread.");