fix(core): preserve chunk id and runId in BatchPartsProcessor when batching text-deltas (#14974)

Co-authored-by: Abhi Aiyer <abhiaiyer91@gmail.com>
Co-authored-by: Mastra Code (anthropic/claude-opus-4-7) <noreply@mastra.ai>

Author: 3 people

.changeset/fix-batch-parts-processor-id.md

@@ -0,0 +1,5 @@
+---
+"@mastra/core": patch
+---
+
+Fixed BatchPartsProcessor using a hardcoded id in batched text-delta chunks. The real message id and runId are now preserved from the original chunks, preventing AI SDK UIMessage stream from dropping batched deltas.

packages/core/src/processors/processors/batch-parts.test.ts

@@ -120,6 +120,60 @@ describe('BatchPartsProcessor', () => {
         payload: { text: 'ABCDE', id: 'text-1' },
       });
     });
+
+    it('should preserve id and runId from the first chunk when batching (regression #14890)', async () => {
+      processor = new BatchPartsProcessor({ batchSize: 3 });
+
+      const chunks: ChunkType[] = [
+        {
+          type: 'text-delta',
+          payload: { text: 'Hello', id: 'msg_abc123' },
+          runId: 'run_xyz789',
+          from: ChunkFrom.AGENT,
+        },
+        {
+          type: 'text-delta',
+          payload: { text: ' ', id: 'msg_abc123' },
+          runId: 'run_xyz789',
+          from: ChunkFrom.AGENT,
+        },
+        {
+          type: 'text-delta',
+          payload: { text: 'world', id: 'msg_abc123' },
+          runId: 'run_xyz789',
+          from: ChunkFrom.AGENT,
+        },
+      ];
+
+      const state: BatchPartsState = { batch: [], timeoutId: undefined, timeoutTriggered: false };
+
+      for (let i = 0; i < 2; i++) {
+        await processor.processOutputStream({
+          part: chunks[i]!,
+          streamParts: chunks.slice(0, i),
+          state,
+          abort: () => {
+            throw new Error('abort');
+          },
+        });
+      }
+
+      const result = await processor.processOutputStream({
+        part: chunks[2]!,
+        streamParts: chunks.slice(0, 2),
+        state,
+        abort: () => {
+          throw new Error('abort');
+        },
+      });
+
+      expect(result).toEqual({
+        type: 'text-delta',
+        runId: 'run_xyz789',
+        from: ChunkFrom.AGENT,
+        payload: { text: 'Hello world', id: 'msg_abc123' },
+      });
+    });
   });
 
   describe('non-text chunks', () => {

packages/core/src/processors/processors/batch-parts.ts

@@ -136,11 +136,12 @@ export class BatchPartsProcessor implements Processor<'batch-parts'> {
       // Combine all text deltas
       const combinedText = textChunks.map(part => (part.type === 'text-delta' ? part.payload.text : '')).join('');
 
-      // Create a new combined text part
+      // Create a new combined text part, preserving id and runId from the first chunk
+      const firstChunk = textChunks[0] as ChunkType & { type: 'text-delta' };
       const combinedChunk: ChunkType = {
         type: 'text-delta',
-        payload: { text: combinedText, id: 'text-1' },
-        runId: '1',
+        payload: { text: combinedText, id: firstChunk.payload.id },
+        runId: firstChunk.runId,
         from: ChunkFrom.AGENT,
       };