diff --git a/packages/adapters/acpx-local/src/server/execute.ts b/packages/adapters/acpx-local/src/server/execute.ts
index 211eb4f97fe..7d9080c6535 100644
--- a/packages/adapters/acpx-local/src/server/execute.ts
+++ b/packages/adapters/acpx-local/src/server/execute.ts
@@ -832,7 +832,10 @@ async function applySessionConfigOptions(input: {
   const options = sessionConfigOptions(input.prepared);
   if (options.length === 0) return;
   if (!input.runtime.setConfigOption) {
-    throw new Error("ACPX runtime does not expose session config controls.");
+    const message =
+      "ACPX runtime does not expose session config controls; upgrade ACPX or remove configured model, effort, and fast mode overrides.";
+    await input.onLog("stderr", `[paperclip] ${message}\n`);
+    throw new Error(message);
   }
   for (const option of options) {
     await input.runtime.setConfigOption({
diff --git a/packages/adapters/acpx-local/src/ui/build-config.ts b/packages/adapters/acpx-local/src/ui/build-config.ts
index 3df73ecc934..729d16c12ca 100644
--- a/packages/adapters/acpx-local/src/ui/build-config.ts
+++ b/packages/adapters/acpx-local/src/ui/build-config.ts
@@ -5,6 +5,7 @@ import {
   DEFAULT_ACPX_LOCAL_NON_INTERACTIVE_PERMISSIONS,
   DEFAULT_ACPX_LOCAL_PERMISSION_MODE,
   DEFAULT_ACPX_LOCAL_TIMEOUT_SEC,
+  DEFAULT_ACPX_LOCAL_WARM_HANDLE_IDLE_MS,
 } from "../index.js";
 
 function parseCommaArgs(value: string): string[] {
@@ -88,7 +89,7 @@ export function buildAcpxLocalConfig(v: CreateConfigValues): Record<string, unkn
     nonInteractivePermissions:
       schemaValues.nonInteractivePermissions || DEFAULT_ACPX_LOCAL_NON_INTERACTIVE_PERMISSIONS,
     timeoutSec: readNumber(schemaValues.timeoutSec, DEFAULT_ACPX_LOCAL_TIMEOUT_SEC),
-    warmHandleIdleMs: readNumber(schemaValues.warmHandleIdleMs, 0),
+    warmHandleIdleMs: readNumber(schemaValues.warmHandleIdleMs, DEFAULT_ACPX_LOCAL_WARM_HANDLE_IDLE_MS),
   };
 
   for (const key of [
diff --git a/server/src/__tests__/acpx-local-execute.test.ts b/server/src/__tests__/acpx-local-execute.test.ts
index 805a6272c3f..148e52a0519 100644
--- a/server/src/__tests__/acpx-local-execute.test.ts
+++ b/server/src/__tests__/acpx-local-execute.test.ts
@@ -273,6 +273,45 @@ describe("acpx_local execute", () => {
     }
   });
 
+  it("logs a clear error when configured session options need unsupported runtime controls", async () => {
+    const root = await fs.mkdtemp(path.join(os.tmpdir(), "paperclip-acpx-missing-config-controls-"));
+    try {
+      const runtime = new FakeRuntime({} as AcpRuntimeOptions);
+      Object.defineProperty(runtime, "setConfigOption", { value: undefined });
+      const logs: LogEntry[] = [];
+      const execute = createAcpxLocalExecutor({
+        createRuntime: () => runtime,
+      });
+      const result = await execute(buildContext(root, {
+        config: {
+          agent: "codex",
+          cwd: root,
+          stateDir: path.join(root, "state"),
+          promptTemplate: "Do the assigned work.",
+          model: "gpt-5.4",
+        },
+        onLog: async (stream, chunk) => logs.push({ stream, chunk }),
+      }));
+
+      expect(result.exitCode).toBe(1);
+      expect(result.errorMessage).toContain("does not expose session config controls");
+      expect(logs).toEqual(expect.arrayContaining([
+        expect.objectContaining({
+          stream: "stderr",
+          chunk: expect.stringContaining("upgrade ACPX or remove configured model"),
+        }),
+      ]));
+      expect(runtime.closeInputs).toEqual([
+        expect.objectContaining({
+          reason: "paperclip config cleanup",
+          discardPersistentState: false,
+        }),
+      ]);
+    } finally {
+      await fs.rm(root, { recursive: true, force: true });
+    }
+  });
+
   it("reuses a compatible warm session and starts fresh when cwd changes", async () => {
     const root = await fs.mkdtemp(path.join(os.tmpdir(), "paperclip-acpx-reuse-"));
     const other = path.join(root, "other");