diff --git a/CHANGELOG.md b/CHANGELOG.md
index 5c5233f9142..c61e93db7b6 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -14,6 +14,7 @@ Docs: https://docs.openclaw.ai
 - Channels/Discord: bound message read/search REST calls, route those actions through Gateway execution, and fall back to `CommandTargetSessionKey` for inbound hook session keys so Discord reads do not hang and hooks still fire when `SessionKey` is empty. Fixes #73431. (#73521) Thanks @amknight.
 - Plugins/media: auto-enable provider plugins referenced by `agents.defaults.imageGenerationModel`, `videoGenerationModel`, and `musicGenerationModel` primary/fallback refs, so configured Google and MiniMax media providers do not stay disabled behind a restrictive plugin allowlist. Thanks @vincentkoc.
 - Memory-core/dreaming: retry managed dreaming cron registration after startup when the cron service is not reachable yet, so the scheduled Memory Dreaming Promotion sweep recovers without waiting for heartbeat traffic. Fixes #72841. Thanks @amknight.
+- Acpx/runtime: validate the runtime session mode at the `AcpxRuntime.ensureSession` wrapper boundary so callers that pass anything other than `persistent` or `oneshot` get a clear `ACP_INVALID_RUNTIME_OPTION` error instead of silently round-tripping through the encoded handle as a default `persistent` mode and later throwing `SessionResumeRequiredError`. Investigation context: #73071. (#73548) Thanks @amknight.
 
 ## 2026.4.27
 
diff --git a/extensions/acpx/src/runtime.test.ts b/extensions/acpx/src/runtime.test.ts
index 97e2ab4a0b9..3d74c9216b3 100644
--- a/extensions/acpx/src/runtime.test.ts
+++ b/extensions/acpx/src/runtime.test.ts
@@ -1,5 +1,5 @@
 import { beforeEach, describe, expect, it, vi } from "vitest";
-import type { AcpRuntime } from "../runtime-api.js";
+import { AcpRuntimeError, type AcpRuntime } from "../runtime-api.js";
 import { AcpxRuntime, __testing } from "./runtime.js";
 
 type TestSessionStore = {
@@ -85,6 +85,43 @@ describe("AcpxRuntime fresh reset wrapper", () => {
     vi.restoreAllMocks();
   });
 
+  it("rejects unsupported runtime session modes with a clear AcpRuntimeError (issue #73071)", async () => {
+    const baseStore: TestSessionStore = {
+      load: vi.fn(async () => undefined),
+      save: vi.fn(async () => {}),
+    };
+    const { runtime, delegate } = makeRuntime(baseStore);
+    const ensureSpy = vi.spyOn(delegate, "ensureSession").mockResolvedValue({
+      sessionKey: "agent:claude:acp:test",
+      backend: "acpx",
+      runtimeSessionName: "claude",
+    });
+
+    for (const badMode of ["run", "session", "", undefined, null, 0]) {
+      await expect(
+        runtime.ensureSession({
+          sessionKey: "agent:claude:acp:test",
+          agent: "claude",
+          mode: badMode as never,
+        }),
+      ).rejects.toMatchObject({
+        name: "AcpRuntimeError",
+        code: "ACP_INVALID_RUNTIME_OPTION",
+        message: expect.stringContaining("Unsupported ACP runtime session mode"),
+      });
+    }
+
+    expect(ensureSpy).not.toHaveBeenCalled();
+  });
+
+  it("exposes assertSupportedRuntimeSessionMode as a typed guard", () => {
+    expect(() => __testing.assertSupportedRuntimeSessionMode("persistent")).not.toThrow();
+    expect(() => __testing.assertSupportedRuntimeSessionMode("oneshot")).not.toThrow();
+    expect(() => __testing.assertSupportedRuntimeSessionMode("run" as never)).toThrow(
+      AcpRuntimeError,
+    );
+  });
+
   it("normalizes OpenClaw Codex model ids for ACP startup", async () => {
     const baseStore: TestSessionStore = {
       load: vi.fn(async () => undefined),
diff --git a/extensions/acpx/src/runtime.ts b/extensions/acpx/src/runtime.ts
index 4bc2dcc0287..252eef132ea 100644
--- a/extensions/acpx/src/runtime.ts
+++ b/extensions/acpx/src/runtime.ts
@@ -231,6 +231,25 @@ function failUnsupportedCodexAcpModel(rawModel: string, detail?: string): never
   );
 }
 
+// acpx's `decodeAcpxRuntimeHandleState` only accepts `persistent` and `oneshot`; any other
+// value silently round-trips through the encoded handle as `persistent` and later throws
+// `SessionResumeRequiredError` on agent restart. Fail fast at this boundary instead.
+// See openclaw/openclaw#73071.
+const SUPPORTED_RUNTIME_SESSION_MODES = new Set(["persistent", "oneshot"] as const);
+
+function assertSupportedRuntimeSessionMode(
+  mode: unknown,
+): asserts mode is "persistent" | "oneshot" {
+  if (typeof mode === "string" && SUPPORTED_RUNTIME_SESSION_MODES.has(mode as never)) {
+    return;
+  }
+  const supported = Array.from(SUPPORTED_RUNTIME_SESSION_MODES).join(", ");
+  throw new AcpRuntimeError(
+    "ACP_INVALID_RUNTIME_OPTION",
+    `Unsupported ACP runtime session mode ${JSON.stringify(mode)}. Expected one of: ${supported}.`,
+  );
+}
+
 function failUnsupportedCodexAcpThinking(rawThinking: string): never {
   throw new AcpRuntimeError(
     "ACP_INVALID_RUNTIME_OPTION",
@@ -460,6 +479,7 @@ export class AcpxRuntime implements AcpRuntime {
   async ensureSession(
     input: Parameters<AcpRuntime["ensureSession"]>[0],
   ): Promise<AcpRuntimeHandle> {
+    assertSupportedRuntimeSessionMode(input.mode);
     const command = resolveAgentCommandForName({
       agentName: input.agent,
       agentRegistry: this.agentRegistry,
@@ -584,6 +604,7 @@ export {
 
 export const __testing = {
   appendCodexAcpConfigOverrides,
+  assertSupportedRuntimeSessionMode,
   codexAcpSessionModelId,
   isCodexAcpCommand,
   normalizeCodexAcpModelOverride,