diff --git a/packages/server/src/server/agent/providers/acp-history-replay.local.e2e.test.ts b/packages/server/src/server/agent/providers/acp-history-replay.local.e2e.test.ts
new file mode 100644
index 0000000000..71fe23a166
--- /dev/null
+++ b/packages/server/src/server/agent/providers/acp-history-replay.local.e2e.test.ts
@@ -0,0 +1,66 @@
+import { describe, expect, test } from "vitest";
+import { ACPAgentClient } from "./acp-agent.js";
+import { createTestLogger } from "../../../test-utils/test-logger.js";
+import type { AgentStreamEvent } from "../agent-sdk-types.js";
+
+// A mode request deterministically releases replay through a controlled stdio peer.
+// Post-result cases deliberately violate the protocol completion boundary.
+const peer = String.raw`
+import { createInterface } from 'node:readline';
+const beforeResult = process.argv[1] === 'before';
+const send = (value) => process.stdout.write(JSON.stringify({jsonrpc:'2.0', ...value})+'\n');
+const update = (kind, id, text) => send({method:'session/update', params:{sessionId:'saved',update:{sessionUpdate:kind,messageId:id,content:{type:'text',text}}}});
+const tail = () => {
+  update('agent_message_chunk','reply-1','first reply');
+  update('user_message_chunk','user-2','follow up');
+  update('agent_message_chunk','reply-2','second reply');
+};
+for await (const line of createInterface({input:process.stdin})) {
+  const request = JSON.parse(line);
+  if (request.method === 'initialize') {
+    send({id:request.id,result:{protocolVersion:1,agentCapabilities:{loadSession:true},authMethods:[]}});
+  } else if (request.method === 'session/load') {
+    update('user_message_chunk','user-1','first prompt');
+    if (beforeResult) tail();
+    send({id:request.id,result:{modes:{currentModeId:'default',availableModes:[{id:'default',name:'Default'}]}}});
+  } else if (request.method === 'session/set_mode') {
+    if (!beforeResult) tail();
+    send({id:request.id,result:{}});
+  } else if (request.id !== undefined) {
+    send({id:request.id,result:{}});
+  }
+}
+`;
+
+describe("ACP persisted history through a real stdio peer", () => {
+  for (const order of ["before", "after", "during-drain"] as const) {
+    test(`retains all turns with ${order} replay`, async () => {
+      const client = new ACPAgentClient({
+        provider: "acp",
+        logger: createTestLogger(),
+        defaultCommand: [process.execPath, "--input-type=module", "-e", peer, order],
+      });
+      const session = await client.resumeSession(
+        { provider: "acp", sessionId: "saved" },
+        { cwd: process.cwd() },
+      );
+      try {
+        const collect = async () => {
+          const events: AgentStreamEvent[] = [];
+          for await (const event of session.streamHistory()) events.push(event);
+          return events;
+        };
+        const pendingHistory = order === "during-drain" ? collect() : undefined;
+        await session.setMode("default");
+        const history = await (pendingHistory ?? collect());
+        expect(
+          history.flatMap((event) =>
+            event.type === "timeline" && "text" in event.item ? [event.item.text] : [],
+          ),
+        ).toEqual(["first prompt", "first reply", "follow up", "second reply"]);
+      } finally {
+        await session.close();
+      }
+    });
+  }
+});