diff --git a/CHANGELOG.md b/CHANGELOG.md
index 28e8b9df8cb..42447bcc1c7 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -88,6 +88,7 @@ Docs: https://docs.openclaw.ai
 - CLI/gateway: reuse cached paired-device auth during `gateway probe` and report post-connect diagnostic failures as degraded reachability, so healthy local gateways are no longer marked unreachable after loopback auth or read timeouts. Fixes #48360. Thanks @RacecarGuy.
 - Channels/Discord: give Discord Gateway WebSocket handshakes a 30s timeout so stalled TLS/network transitions emit an error and Carbon can continue its reconnect loop instead of leaving the bot silent until restart. Refs #50046. Thanks @codexGW.
 - Channels/Telegram: suppress standalone failed edit/write warning payloads when a user-facing assistant error reply already covers the turn, while keeping unresolved mutating failures visible behind success-looking or suppressed-error replies. Fixes #39631; refs #73750; carries forward #39636 and #39717; leaves #39406 for configurable delivery policy. Thanks @Bartok9 and @Bortlesboat.
+- Control UI/agents: persist the Set Default action through `agents.list[].default` instead of writing the unsupported `agents.defaultId` field, so saved default-agent changes survive config validation. Fixes #65565; carries forward #72585. Thanks @luyao618.
 - NVIDIA/NIM: persist the `NVIDIA_API_KEY` provider marker and mark bundled NVIDIA Chat Completions models as string-content compatible, so NIM models load from `models.json` and OpenAI-compatible subagent calls send plain text content. Fixes #73013 and #50107; refs #73014. Thanks @bautrey, @iot2edge, @ifearghal, and @futhgar.
 - Channels/Discord: let text-only configs drop the `GuildVoiceStates` gateway intent and expose a bounded `/gateway/bot` metadata timeout with rate-limited fallback logs, reducing idle CPU and warning floods. Fixes #73709 and #73585. Thanks @sanchezm86 and @trac3r00.
 - Agents/sessions: mark same-turn `sessions_send` and A2A reply prompts with an inter-session `isUser=false` envelope before they reach the model, so foreign session output no longer lands as bare active user text. Fixes #73702; refs #73698, #73609, #73595, and #73622. Thanks @alvelda.
diff --git a/ui/src/ui/app-render.ts b/ui/src/ui/app-render.ts
index be195040720..0b2ee0cc82c 100644
--- a/ui/src/ui/app-render.ts
+++ b/ui/src/ui/app-render.ts
@@ -40,6 +40,7 @@ import {
   resetConfigPendingChanges,
   runUpdate,
   saveConfig,
+  stageDefaultAgentConfigEntry,
   stageConfigPreset,
   updateConfigFormValue,
   removeConfigFormValue,
@@ -2148,10 +2149,7 @@ export function renderApp(state: AppViewState) {
                   updateConfigFormValue(state, basePath, { primary, fallbacks: normalized });
                 },
                 onSetDefault: (agentId) => {
-                  if (!configValue) {
-                    return;
-                  }
-                  updateConfigFormValue(state, ["agents", "defaultId"], agentId);
+                  stageDefaultAgentConfigEntry(state, agentId);
                 },
               }),
             )
diff --git a/ui/src/ui/controllers/config.test.ts b/ui/src/ui/controllers/config.test.ts
index 2923586b8e5..dea41b310a7 100644
--- a/ui/src/ui/controllers/config.test.ts
+++ b/ui/src/ui/controllers/config.test.ts
@@ -8,6 +8,7 @@ import {
   resetConfigPendingChanges,
   runUpdate,
   saveConfig,
+  stageDefaultAgentConfigEntry,
   stageConfigPreset,
   updateConfigFormValue,
   type ConfigState,
@@ -477,6 +478,50 @@ describe("agent config helpers", () => {
       },
     });
   });
+
+  it("sets default via agents.list[].default instead of agents.defaultId", () => {
+    const state = createState();
+    state.configSnapshot = {
+      config: {
+        agents: {
+          list: [{ id: "alpha", default: true }, { id: "beta" }],
+        },
+      },
+      valid: true,
+      issues: [],
+      raw: "{\n}\n",
+    };
+
+    const updated = stageDefaultAgentConfigEntry(state, "beta");
+
+    expect(updated).toBe(true);
+    expect(state.configFormDirty).toBe(true);
+    expect(state.configForm).toEqual({
+      agents: {
+        list: [{ id: "alpha" }, { id: "beta", default: true }],
+      },
+    });
+  });
+
+  it("does not stage agents.defaultId when the target agent is absent", () => {
+    const state = createState();
+    state.configSnapshot = {
+      config: {
+        agents: {
+          list: [{ id: "alpha", default: true }],
+        },
+      },
+      valid: true,
+      issues: [],
+      raw: "{\n}\n",
+    };
+
+    const updated = stageDefaultAgentConfigEntry(state, "beta");
+
+    expect(updated).toBe(false);
+    expect(state.configFormDirty).toBe(false);
+    expect(state.configForm).toBeNull();
+  });
 });
 
 describe("applyConfig", () => {
diff --git a/ui/src/ui/controllers/config.ts b/ui/src/ui/controllers/config.ts
index 5434e5833de..015e843fbf3 100644
--- a/ui/src/ui/controllers/config.ts
+++ b/ui/src/ui/controllers/config.ts
@@ -359,6 +359,38 @@ export function ensureAgentConfigEntry(state: ConfigState, agentId: string): num
   return nextIndex;
 }
 
+export function stageDefaultAgentConfigEntry(state: ConfigState, agentId: string): boolean {
+  const normalizedAgentId = agentId.trim();
+  if (!normalizedAgentId) {
+    return false;
+  }
+  const source =
+    state.configForm ?? (state.configSnapshot?.config as Record<string, unknown> | null);
+  const targetIndex = findAgentConfigEntryIndex(source, normalizedAgentId);
+  if (targetIndex < 0) {
+    return false;
+  }
+  mutateConfigForm(state, (draft) => {
+    const list = (draft as { agents?: { list?: unknown[] } } | null)?.agents?.list;
+    if (!Array.isArray(list)) {
+      return;
+    }
+    for (let i = 0; i < list.length; i++) {
+      const entry = list[i];
+      if (!entry || typeof entry !== "object" || Array.isArray(entry)) {
+        continue;
+      }
+      const record = entry as Record<string, unknown>;
+      if (i === targetIndex) {
+        record.default = true;
+      } else {
+        delete record.default;
+      }
+    }
+  });
+  return true;
+}
+
 export async function openConfigFile(state: ConfigState): Promise<void> {
   if (!state.client || !state.connected) {
     return;