fix(core): sanitize function_response.parts before compression

packages/core/src/services/chatCompressionService.test.ts

@@ -835,4 +835,67 @@ describe('ChatCompressionService', () => {
       );
     });
   });
+
+  it('should strip function_response.parts (Gemini 3 multimodal data) before sending to summarizer', async () => {
+    const history: Content[] = [
+      { role: 'user', parts: [{ text: 'msg1' }] },
+      {
+        role: 'user',
+        parts: [
+          {
+            functionResponse: {
+              name: 'screenshot_tool',
+              response: { output: 'Screenshot taken.' },
+              // Gemini 3 nests inlineData inside function_response.parts at runtime
+              parts: [
+                { inlineData: { mimeType: 'image/png', data: 'base64data' } },
+              ],
+              // eslint-disable-next-line @typescript-eslint/no-explicit-any
+            } as any,
+          },
+        ],
+      },
+      { role: 'model', parts: [{ text: 'resp2' }] },
+      { role: 'user', parts: [{ text: 'msg3' }] },
+      { role: 'model', parts: [{ text: 'resp4' }] },
+    ];
+
+    vi.mocked(mockChat.getHistory).mockReturnValue(history);
+    vi.mocked(mockChat.getLastPromptTokenCount).mockReturnValue(600000);
+
+    await service.compress(
+      mockChat,
+      mockPromptId,
+      true,
+      mockModel,
+      mockConfig,
+      false,
+    );
+
+    // Verify that both generateContent calls had the nested .parts stripped
+    const generateContentMock = vi.mocked(
+      mockConfig.getBaseLlmClient().generateContent,
+    );
+    expect(generateContentMock).toHaveBeenCalledTimes(2);
+
+    for (const call of generateContentMock.mock.calls) {
+      const contents = call[0].contents;
+      for (const content of contents) {
+        for (const part of content.parts ?? []) {
+          if (part.functionResponse) {
+            // The nested .parts field should have been removed
+            expect(
+              // eslint-disable-next-line @typescript-eslint/no-explicit-any
+              (part.functionResponse as any).parts,
+            ).toBeUndefined();
+            // But the rest of functionResponse should be preserved
+            expect(part.functionResponse.name).toBe('screenshot_tool');
+            expect(part.functionResponse.response).toEqual({
+              output: 'Screenshot taken.',
+            });
+          }
+        }
+      }
+    }
+  });
 });

packages/core/src/services/chatCompressionService.ts

@@ -228,6 +228,29 @@ async function truncateHistoryToBudget(
   return truncatedHistory;
 }
 
+/**
+ * Strips the Gemini-3-specific `function_response.parts` (nested multimodal
+ * data such as inlineData) from history entries. The compression summarizer
+ * model may not support this feature, so we sanitize before sending.
+ */
+function sanitizeHistoryForSummarizer(history: Content[]): Content[] {
+  return history.map((content) => ({
+    ...content,
+    parts: content.parts?.map((part) => {
+      if (part.functionResponse) {
+
+        const { parts: _nestedParts, ...cleanFR } =
+          part.functionResponse as // The `parts` field is added at runtime for Gemini 3 models and is
+          // not part of the official FunctionResponse type.
+          // eslint-disable-next-line @typescript-eslint/no-explicit-any
+          Record<string, any>;
+        return { functionResponse: cleanFR } as typeof part;
+      }
+      return part;
+    }),
+  }));
+}
+
 export class ChatCompressionService {
   async compress(
     chat: GeminiChat,
@@ -353,7 +376,7 @@ export class ChatCompressionService {
     const summaryResponse = await config.getBaseLlmClient().generateContent({
       modelConfigKey: { model: modelStringToModelConfigAlias(model) },
       contents: [
-        ...historyForSummarizer,
+        ...sanitizeHistoryForSummarizer(historyForSummarizer),
         {
           role: 'user',
           parts: [
@@ -378,7 +401,7 @@ export class ChatCompressionService {
       .generateContent({
         modelConfigKey: { model: modelStringToModelConfigAlias(model) },
         contents: [
-          ...historyForSummarizer,
+          ...sanitizeHistoryForSummarizer(historyForSummarizer),
           {
             role: 'model',
             parts: [{ text: summary }],