diff --git a/packages/core/src/core/client.ts b/packages/core/src/core/client.ts
index 0abeeca32e..b14eb8b5c4 100644
--- a/packages/core/src/core/client.ts
+++ b/packages/core/src/core/client.ts
@@ -286,7 +286,7 @@ export class GeminiClient {
       );
       const selectedSet = new Set(selectedNames);
       toolDeclarations = toolDeclarations.filter((t) =>
-        selectedSet.has(t.name!),
+        t.name ? selectedSet.has(t.name) : false,
       );
     }
 
diff --git a/packages/core/src/services/toolPreselectionService.test.ts b/packages/core/src/services/toolPreselectionService.test.ts
index 775704d60b..db79be6883 100644
--- a/packages/core/src/services/toolPreselectionService.test.ts
+++ b/packages/core/src/services/toolPreselectionService.test.ts
@@ -124,4 +124,49 @@ describe('ToolPreselectionService', () => {
       'tool6',
     ]);
   });
+
+  it('falls back to all tools if LLM returns invalid format', async () => {
+    const tools: FunctionDeclaration[] = [
+      { name: 'tool1', description: 'desc1' },
+      { name: 'tool2', description: 'desc2' },
+      { name: 'tool3', description: 'desc3' },
+      { name: 'tool4', description: 'desc4' },
+      { name: 'tool5', description: 'desc5' },
+      { name: 'tool6', description: 'desc6' },
+    ];
+
+    // Missing 'relevant_tools' key
+    mockLlmClient['generateJson'].mockResolvedValue({
+      something_else: ['tool1'],
+    });
+
+    const result = await service.selectTools(
+      'query',
+      tools,
+      new AbortController().signal,
+    );
+    expect(result).toEqual([
+      'tool1',
+      'tool2',
+      'tool3',
+      'tool4',
+      'tool5',
+      'tool6',
+    ]);
+  });
+
+  it('safely handles tools with undefined names', async () => {
+    const tools: FunctionDeclaration[] = [
+      { name: 'tool1', description: 'desc1' },
+      { name: undefined, description: 'desc2' },
+    ];
+
+    const result = await service.selectTools(
+      'query',
+      tools,
+      new AbortController().signal,
+      { maxTools: 10 }, // Force threshold bypass
+    );
+    expect(result).toEqual(['tool1']);
+  });
 });
diff --git a/packages/core/src/services/toolPreselectionService.ts b/packages/core/src/services/toolPreselectionService.ts
index b33d4f9694..36e146e9c4 100644
--- a/packages/core/src/services/toolPreselectionService.ts
+++ b/packages/core/src/services/toolPreselectionService.ts
@@ -43,7 +43,7 @@ export class ToolPreselectionService {
     // Threshold below which we don't bother with pre-selection.
     const threshold = options.maxTools ?? 5;
     if (tools.length <= threshold) {
-      return tools.map((t) => t.name!);
+      return tools.filter((t) => !!t.name).map((t) => t.name!);
     }
 
     const schema = {
@@ -84,8 +84,17 @@ ${toolsList}`;
         role: LlmRole.UTILITY_TOOL,
       });
 
-      // eslint-disable-next-line @typescript-eslint/no-unsafe-type-assertion
-      const selectedTools = result['relevant_tools'] as string[];
+      const selectedTools = result['relevant_tools'];
+
+      if (
+        !Array.isArray(selectedTools) ||
+        !selectedTools.every((item): item is string => typeof item === 'string')
+      ) {
+        throw new Error(
+          'Tool preselection returned invalid data format. Expected an array of strings.',
+        );
+      }
+
       debugLogger.debug(
         `ToolPreselectionService: Selected ${selectedTools.length} tools out of ${tools.length} for query: "${query.substring(0, 50)}..."`,
       );
@@ -93,7 +102,7 @@ ${toolsList}`;
     } catch (error) {
       debugLogger.error('ToolPreselectionService failed:', error);
       // Fallback: return all tools if pre-selection fails.
-      return tools.map((t) => t.name!);
+      return tools.filter((t) => !!t.name).map((t) => t.name!);
     }
   }
 }