diff --git a/.changeset/clear-defaults-via-replace.md b/.changeset/clear-defaults-via-replace.md
new file mode 100644
index 0000000000..ab0944e210
--- /dev/null
+++ b/.changeset/clear-defaults-via-replace.md
@@ -0,0 +1,5 @@
+---
+"@moonshot-ai/kimi-code": patch
+---
+
+Fix the daemon leaving a stale default provider or default model behind when the default provider is removed or after OAuth logout.
diff --git a/packages/agent-core-v2/src/app/auth/authService.ts b/packages/agent-core-v2/src/app/auth/authService.ts
index 00e402f978..bd5306cd9c 100644
--- a/packages/agent-core-v2/src/app/auth/authService.ts
+++ b/packages/agent-core-v2/src/app/auth/authService.ts
@@ -510,8 +510,10 @@ export class OAuthService extends Disposable implements IOAuthService {
       await this.config.replace(SERVICES_SECTION, next.services);
     }
     if (cleanup.defaultModelCleared) {
-      await this.config.set(DEFAULT_MODEL_SECTION, undefined);
-      await this.config.set(THINKING_SECTION, undefined);
+      // `set()` cannot delete: its deepMerge resolves an undefined patch back
+      // to the existing base value. `replace()` removes the section instead.
+      await this.config.replace(DEFAULT_MODEL_SECTION, undefined);
+      await this.config.replace(THINKING_SECTION, undefined);
     }
   }
 
diff --git a/packages/agent-core-v2/src/app/provider/providerService.ts b/packages/agent-core-v2/src/app/provider/providerService.ts
index f02de8031f..4ccebff9fe 100644
--- a/packages/agent-core-v2/src/app/provider/providerService.ts
+++ b/packages/agent-core-v2/src/app/provider/providerService.ts
@@ -64,7 +64,9 @@ export class ProviderService extends Disposable implements IProviderService {
     const { [name]: _removed, ...rest } = current;
     await this.config.replace(PROVIDERS_SECTION, rest);
     if (this.config.get<string>(DEFAULT_PROVIDER_SECTION) === name) {
-      await this.config.set(DEFAULT_PROVIDER_SECTION, undefined);
+      // `set()` cannot delete: its deepMerge resolves an undefined patch back
+      // to the existing base value. `replace()` removes the section instead.
+      await this.config.replace(DEFAULT_PROVIDER_SECTION, undefined);
     }
   }
 }
diff --git a/packages/agent-core-v2/test/app/auth/auth.test.ts b/packages/agent-core-v2/test/app/auth/auth.test.ts
index 6081ef9243..af57d9b5b4 100644
--- a/packages/agent-core-v2/test/app/auth/auth.test.ts
+++ b/packages/agent-core-v2/test/app/auth/auth.test.ts
@@ -133,6 +133,14 @@ describe('OAuthService', () => {
         services = value as Record<string, unknown> | undefined;
         return;
       }
+      if (domain === 'defaultModel') {
+        defaultModel = value as string | undefined;
+        return;
+      }
+      if (domain === 'thinking') {
+        thinking = value as { enabled?: boolean; effort?: string } | undefined;
+        return;
+      }
       throw new Error(`unexpected config replace: ${domain}`);
     });
     events = [];
@@ -593,8 +601,11 @@ describe('OAuthService', () => {
         maxContextSize: 8192,
       },
     });
-    expect(configSet).toHaveBeenCalledWith('defaultModel', undefined);
-    expect(configSet).toHaveBeenCalledWith('thinking', undefined);
+    // Clearing must go through `replace` — `set()`'s deepMerge cannot delete.
+    expect(configReplace).toHaveBeenCalledWith('defaultModel', undefined);
+    expect(configReplace).toHaveBeenCalledWith('thinking', undefined);
+    expect(defaultModel).toBeUndefined();
+    expect(thinking).toBeUndefined();
   });
 
   it('logout removes managed web services while preserving unrelated services', async () => {
diff --git a/packages/agent-core-v2/test/app/provider/provider.test.ts b/packages/agent-core-v2/test/app/provider/provider.test.ts
index 98d3f301c4..da018d85c8 100644
--- a/packages/agent-core-v2/test/app/provider/provider.test.ts
+++ b/packages/agent-core-v2/test/app/provider/provider.test.ts
@@ -118,7 +118,8 @@ describe('ProviderService', () => {
     expect(configReplace).toHaveBeenCalledWith(PROVIDERS_SECTION, {
       p2: { type: 'kimi' },
     });
-    expect(configSet).toHaveBeenCalledWith('defaultProvider', undefined);
+    // Clearing must go through `replace` — `set()`'s deepMerge cannot delete.
+    expect(configReplace).toHaveBeenCalledWith('defaultProvider', undefined);
   });
 
   it('delete leaves defaultProvider when removing a different provider', async () => {
@@ -127,7 +128,7 @@ describe('ProviderService', () => {
     defaultProvider = 'p2';
     const svc = ix.get(IProviderService);
     await svc.delete('p1');
-    expect(configSet).not.toHaveBeenCalled();
+    expect(configReplace).toHaveBeenCalledTimes(1);
   });
 
   it('forwards providers section changes as onDidChangeProviders with a diff', () => {