Fix(core): Do not retry if last chunk is empty with finishReason previous chunks are good (google-gemini#7859)

Author: SandyTao520

packages/core/src/core/geminiChat.test.ts

@@ -459,6 +459,58 @@ describe('GeminiChat', () => {
         })(),
       ).rejects.toThrow(EmptyStreamError);
     });
+
+    it('should succeed if the stream ends with an invalid part but has a finishReason and contained a valid part', async () => {
+      // 1. Mock a stream that sends a valid chunk, then an invalid one, but has a finish reason.
+      const streamWithInvalidEnd = (async function* () {
+        yield {
+          candidates: [
+            {
+              content: {
+                role: 'model',
+                parts: [{ text: 'Initial valid content...' }],
+              },
+            },
+          ],
+        } as unknown as GenerateContentResponse;
+        // This second chunk is invalid, but the response has a finishReason.
+        yield {
+          candidates: [
+            {
+              content: {
+                role: 'model',
+                parts: [{ text: '' }], // Invalid part
+              },
+              finishReason: 'STOP',
+            },
+          ],
+        } as unknown as GenerateContentResponse;
+      })();
+
+      vi.mocked(mockModelsModule.generateContentStream).mockResolvedValue(
+        streamWithInvalidEnd,
+      );
+
+      // 2. Action & Assert: The stream should complete without throwing an error.
+      const stream = await chat.sendMessageStream(
+        { message: 'test message' },
+        'prompt-id-valid-then-invalid-end',
+      );
+      await expect(
+        (async () => {
+          for await (const _ of stream) {
+            /* consume stream */
+          }
+        })(),
+      ).resolves.not.toThrow();
+
+      // 3. Verify history was recorded correctly with only the valid part.
+      const history = chat.getHistory();
+      expect(history.length).toBe(2); // user turn + model turn
+      const modelTurn = history[1]!;
+      expect(modelTurn?.parts?.length).toBe(1);
+      expect(modelTurn?.parts![0]!.text).toBe('Initial valid content...');
+    });
     it('should not consolidate text into a part that also contains a functionCall', async () => {
       // 1. Mock the API to stream a malformed part followed by a valid text part.
       const multiChunkStream = (async function* () {

packages/core/src/core/geminiChat.ts

@@ -612,6 +612,7 @@ export class GeminiChat {
   ): AsyncGenerator<GenerateContentResponse> {
     const modelResponseParts: Part[] = [];
     let hasReceivedAnyChunk = false;
+    let hasReceivedValidChunk = false;
     let hasToolCall = false;
     let lastChunk: GenerateContentResponse | null = null;
     let lastChunkIsInvalid = false;
@@ -621,6 +622,7 @@ export class GeminiChat {
       lastChunk = chunk;
 
       if (isValidResponse(chunk)) {
+        hasReceivedValidChunk = true;
         lastChunkIsInvalid = false;
         const content = chunk.candidates?.[0]?.content;
         if (content?.parts) {
@@ -658,15 +660,17 @@ export class GeminiChat {
       (candidate) => candidate.finishReason,
     );
 
-    // --- FIX: The entire validation block was restructured for clarity and correctness ---
     // Stream validation logic: A stream is considered successful if:
     // 1. There's a tool call (tool calls can end without explicit finish reasons), OR
-    // 2. Both conditions are met: last chunk is valid AND any candidate has a finish reason
+    // 2. There's a finish reason AND the last chunk is valid (or we haven't received any valid chunks)
     //
-    // We throw an error only when there's no tool call AND either:
-    // - The last chunk is invalid, OR
-    // - No candidate in the last chunk has a finish reason
-    if (!hasToolCall && (lastChunkIsInvalid || !hasFinishReason)) {
+    // We throw an error only when there's no tool call AND:
+    // - No finish reason, OR
+    // - Last chunk is invalid after receiving valid content
+    if (
+      !hasToolCall &&
+      (!hasFinishReason || (lastChunkIsInvalid && !hasReceivedValidChunk))
+    ) {
       throw new EmptyStreamError(
         'Model stream ended with an invalid chunk or missing finish reason.',
       );