fix(core): handle partial llm_request in BeforeModel hook override

When a BeforeModel hook returns only a model override without messages
(e.g. { model: 'gemini-2.5-flash' }), fromHookLLMRequest crashed with
TypeError because it called .map() on undefined messages.

Guard hookRequest.messages before mapping, falling back to
baseRequest?.contents to preserve the conversation. Use nullish
coalescing for model so a partial hook response does not overwrite
with undefined.

Fixes #21847

Author: krishdef7

packages/core/src/hooks/hookTranslator.test.ts

@@ -121,6 +121,56 @@ describe('HookTranslator', () => {
         },
       ]);
     });
+
+    it('should apply model override when hook returns only model field', () => {
+      const baseRequest: GenerateContentParameters = {
+        model: 'gemini-2.5-flash-lite',
+        contents: [
+          {
+            role: 'user',
+            parts: [{ text: 'Hello' }],
+          },
+        ],
+      } as unknown as GenerateContentParameters;
+
+      // Simulate a hook that only overrides the model — no messages field
+      const hookRequest = {
+        model: 'gemini-2.5-flash',
+      } as unknown as LLMRequest;
+
+      const sdkRequest = translator.fromHookLLMRequest(
+        hookRequest,
+        baseRequest,
+      );
+
+      // Model should be overridden
+      expect(sdkRequest.model).toBe('gemini-2.5-flash');
+      // Original conversation contents should be preserved
+      expect(sdkRequest.contents).toEqual(baseRequest.contents);
+    });
+
+    it('should preserve base request contents when hook messages is undefined', () => {
+      const baseRequest: GenerateContentParameters = {
+        model: 'gemini-1.5-flash',
+        contents: [
+          { role: 'user', parts: [{ text: 'original message' }] },
+          { role: 'model', parts: [{ text: 'original reply' }] },
+        ],
+      } as unknown as GenerateContentParameters;
+
+      const hookRequest = {
+        model: 'gemini-1.5-pro',
+        // messages intentionally omitted
+      } as unknown as LLMRequest;
+
+      const sdkRequest = translator.fromHookLLMRequest(
+        hookRequest,
+        baseRequest,
+      );
+
+      expect(sdkRequest.model).toBe('gemini-1.5-pro');
+      expect(sdkRequest.contents).toEqual(baseRequest.contents);
+    });
   });
 
   describe('LLM Response Translation', () => {

packages/core/src/hooks/hookTranslator.ts

@@ -225,23 +225,30 @@ export class HookTranslatorGenAIv1 extends HookTranslator {
     hookRequest: LLMRequest,
     baseRequest?: GenerateContentParameters,
   ): GenerateContentParameters {
-    // Convert hook messages back to SDK Content format
-    const contents = hookRequest.messages.map((message) => ({
-      role: message.role === 'model' ? 'model' : message.role,
-      parts: [
-        {
-          text:
-            typeof message.content === 'string'
-              ? message.content
-              : String(message.content),
-        },
-      ],
-    }));
+    // Convert hook messages back to SDK Content format.
+    // If the hook returned a partial request without messages (e.g. only
+    // overriding `model`), fall back to the base request's contents so the
+    // conversation is preserved.
+    const contents = hookRequest.messages
+      ? hookRequest.messages.map((message) => ({
+          role: message.role === 'model' ? 'model' : message.role,
+          parts: [
+            {
+              text:
+                typeof message.content === 'string'
+                  ? message.content
+                  : String(message.content),
+            },
+          ],
+        }))
+      : (baseRequest?.contents ?? []);
 
-    // Build the result with proper typing
+    // Build the result with proper typing.
+    // Use nullish coalescing so a hook that only sets `model` still works --
+    // fall back to the base request's model rather than overwriting with undefined.
     const result: GenerateContentParameters = {
       ...baseRequest,
-      model: hookRequest.model,
+      model: hookRequest.model ?? baseRequest?.model ?? '',
       contents,
     };