diff --git a/packages/agent-core/src/providers/provider-manager.ts b/packages/agent-core/src/providers/provider-manager.ts
index 7c2bc14f1..50f51ea8f 100644
--- a/packages/agent-core/src/providers/provider-manager.ts
+++ b/packages/agent-core/src/providers/provider-manager.ts
@@ -113,7 +113,10 @@ export class ProviderManager {
   }
 
   resolveThinkingLevel(requestedThinking?: string): ThinkingEffort {
-    return resolveThinkingEffort(requestedThinking, this.state.config.thinking);
+    return resolveThinkingEffort(
+      requestedThinking ?? defaultThinkingToRuntime(this.state.config.defaultThinking),
+      this.state.config.thinking,
+    );
   }
 
   resolveSelectedModel(requestedModel: string | undefined): string | undefined {
@@ -137,3 +140,8 @@ function normalizeString(value: string | undefined): string | undefined {
   const trimmed = value.trim();
   return trimmed.length > 0 ? trimmed : undefined;
 }
+
+function defaultThinkingToRuntime(defaultThinking: boolean | undefined): string | undefined {
+  if (defaultThinking === undefined) return undefined;
+  return defaultThinking ? 'on' : 'off';
+}
diff --git a/packages/agent-core/src/rpc/core-impl.ts b/packages/agent-core/src/rpc/core-impl.ts
index 0368cdf5b..189ae276d 100644
--- a/packages/agent-core/src/rpc/core-impl.ts
+++ b/packages/agent-core/src/rpc/core-impl.ts
@@ -143,9 +143,7 @@ export class KimiCore implements PromisableMethods<CoreAPI> {
     const config = this.reloadProviderManager();
     const id = options.id ?? createSessionId();
     const modelName = this.providerManager.resolveSelectedModel(options.model);
-    const thinkingLevel = this.providerManager.resolveThinkingLevel(
-      options.thinking ?? defaultThinkingToRuntime(config.defaultThinking),
-    );
+    const thinkingLevel = this.providerManager.resolveThinkingLevel(options.thinking);
     const permissionMode = options.permission ?? config.defaultPermissionMode;
     const mcpConfig = await resolveSessionMcpConfig({
       cwd: workDir,
@@ -682,11 +680,6 @@ function requiredWorkDir(operation: string, value: string): string {
   return normalizeWorkDir(value);
 }
 
-function defaultThinkingToRuntime(defaultThinking: boolean | undefined): string | undefined {
-  if (defaultThinking === undefined) return undefined;
-  return defaultThinking ? 'on' : 'off';
-}
-
 function createSessionId(): string {
   return `session_${randomUUID()}`;
 }
diff --git a/packages/agent-core/test/harness/runtime-provider.test.ts b/packages/agent-core/test/harness/runtime-provider.test.ts
index 31264e800..4466c9c7d 100644
--- a/packages/agent-core/test/harness/runtime-provider.test.ts
+++ b/packages/agent-core/test/harness/runtime-provider.test.ts
@@ -585,16 +585,26 @@ describe('ProviderManager thinking level', () => {
     const manager = new ProviderManager({
       config: {
         providers: {},
+        defaultThinking: false,
         thinking: { effort: 'medium', mode: 'auto' },
       },
     });
     expect(manager.resolveThinkingLevel('on')).toBe('medium');
     expect(manager.resolveThinkingLevel('off')).toBe('off');
     expect(manager.resolveThinkingLevel('low')).toBe('low');
-    expect(manager.resolveThinkingLevel()).toBe('medium');
+    expect(manager.resolveThinkingLevel()).toBe('off');
+
+    const managerOnByDefault = new ProviderManager({
+      config: {
+        providers: {},
+        defaultThinking: true,
+        thinking: { effort: 'medium', mode: 'auto' },
+      },
+    });
+    expect(managerOnByDefault.resolveThinkingLevel()).toBe('medium');
 
     const managerWithoutEffort = new ProviderManager({
-      config: { providers: {}, thinking: { mode: 'auto' } },
+      config: { providers: {}, defaultThinking: true, thinking: { mode: 'auto' } },
     });
     expect(managerWithoutEffort.resolveThinkingLevel('on')).toBe('high');
     expect(managerWithoutEffort.resolveThinkingLevel()).toBe('high');