diff --git a/packages/adapter-utils/src/acpx-engine/ephemeral-session-environment.test.ts b/packages/adapter-utils/src/acpx-engine/ephemeral-session-environment.test.ts
index 40d13e7b403..ef0e834aa8b 100644
--- a/packages/adapter-utils/src/acpx-engine/ephemeral-session-environment.test.ts
+++ b/packages/adapter-utils/src/acpx-engine/ephemeral-session-environment.test.ts
@@ -162,4 +162,45 @@ describe("ephemeral ACPX session environment", () => {
     const input = freeze(record()); await expect(wrapper.save(input)).rejects.toBe(failure);
     expect(input.acpx?.session_options?.env).toEqual(currentEnvironment);
   });
+
+  it("never overlaps two saves of one record, even across wrappers, and keeps their order", async () => {
+    let active = 0, maxActive = 0;
+    const order: string[] = [];
+    const failure = new Error("fixture rename failure");
+    const backing = stubStore(undefined);
+    backing.save.mockImplementation(async (saved) => {
+      active += 1; maxActive = Math.max(maxActive, active);
+      await new Promise(resolve => setTimeout(resolve, 5));
+      order.push(saved.lastUsedAt);
+      active -= 1;
+      if (saved.lastUsedAt === "checkpoint") throw failure;
+    });
+    const turnStore = createEphemeralSessionEnvironmentStore(backing, currentEnvironment);
+    const closeStore = createEphemeralSessionEnvironmentStore(backing, currentEnvironment);
+    const results = await Promise.allSettled([
+      turnStore.save({ ...record(), lastUsedAt: "checkpoint" }),
+      turnStore.save({ ...record(), lastUsedAt: "turn-end" }),
+      closeStore.save({ ...record(), lastUsedAt: "close" }),
+    ]);
+    expect(maxActive).toBe(1);
+    expect(order).toEqual(["checkpoint", "turn-end", "close"]);
+    expect(results.map(result => result.status)).toEqual(["rejected", "fulfilled", "fulfilled"]);
+    expect((results[0] as PromiseRejectedResult).reason).toBe(failure);
+  });
+
+  it("does not serialize saves of different records", async () => {
+    let active = 0, maxActive = 0;
+    const backing = stubStore(undefined);
+    backing.save.mockImplementation(async () => {
+      active += 1; maxActive = Math.max(maxActive, active);
+      await new Promise(resolve => setTimeout(resolve, 5));
+      active -= 1;
+    });
+    const wrapper = createEphemeralSessionEnvironmentStore(backing, currentEnvironment);
+    await Promise.all([
+      wrapper.save({ ...record(), acpxRecordId: "fixture-record-a" }),
+      wrapper.save({ ...record(), acpxRecordId: "fixture-record-b" }),
+    ]);
+    expect(maxActive).toBe(2);
+  });
 });
diff --git a/packages/adapter-utils/src/acpx-engine/ephemeral-session-environment.ts b/packages/adapter-utils/src/acpx-engine/ephemeral-session-environment.ts
index 3bd59942ec9..a4d068f9f6c 100644
--- a/packages/adapter-utils/src/acpx-engine/ephemeral-session-environment.ts
+++ b/packages/adapter-utils/src/acpx-engine/ephemeral-session-environment.ts
@@ -1,5 +1,22 @@
 import type { AcpSessionStore } from "acpx/runtime";
 
+// ACPX saves one record from several writers (live checkpoint, turn end,
+// finalize, close) with no lock. Each save renames a temp file onto the record
+// file, and on Windows two overlapping renames onto one file fail with EPERM.
+// Chain every save of one record so they never overlap.
+const pendingSaves = new Map<string, Promise<void>>();
+
+function saveInOrder(recordId: string, save: () => Promise<void>): Promise<void> {
+  const previous = pendingSaves.get(recordId) ?? Promise.resolve();
+  const current = previous.catch(() => undefined).then(save);
+  const settled = current.catch(() => undefined);
+  pendingSaves.set(recordId, settled);
+  void settled.then(() => {
+    if (pendingSaves.get(recordId) === settled) pendingSaves.delete(recordId);
+  });
+  return current;
+}
+
 /** Keep the current run's environment in memory, outside persisted conversation state. */
 export function createEphemeralSessionEnvironmentStore(
   persistedStore: AcpSessionStore,
@@ -26,7 +43,7 @@ export function createEphemeralSessionEnvironmentStore(
         const { env: _environment, ...sessionOptions } = record.acpx.session_options;
         persisted.acpx = { ...record.acpx, session_options: sessionOptions };
       }
-      return persistedStore.save(persisted);
+      return saveInOrder(record.acpxRecordId, () => persistedStore.save(persisted));
     },
   };
 }