fix(ai): keep first text delta after a tool call in the same turn

.changeset/fix-post-tool-first-text-delta.md

@@ -0,0 +1,5 @@
+---
+'@tanstack/ai': patch
+---
+
+Fix StreamProcessor dropping the first text delta of a post-tool segment. When an assistant turn opened with a tool call and then spoke, the message auto-created by `TOOL_CALL_START` left `hasToolCallsSinceTextStart` set through `TEXT_MESSAGE_START`, so the segment reset fired one delta late and folded away the first `TEXT_MESSAGE_CONTENT` chunk (e.g. losing the first word of the reply). The segment reset now runs on the first post-tool delta regardless of whether the prior segment was empty, and only the flush of a non-empty prior segment stays gated.

packages/ai/src/activities/chat/stream/processor.ts

@@ -1307,12 +1307,14 @@ export class StreamProcessor {
     // Detect if this is a NEW text segment (after tool calls) vs continuation
     const isNewSegment =
       state.hasToolCallsSinceTextStart &&
-      previousSegment.length > 0 &&
       this.isNewTextSegment(chunk, previousSegment)
 
     if (isNewSegment) {
       // Emit any accumulated text before starting new segment
-      if (previousSegment !== state.lastEmittedText) {
+      if (
+        previousSegment.length > 0 &&
+        previousSegment !== state.lastEmittedText
+      ) {
         this.emitTextUpdateForMessage(messageId)
       }
       // Reset SEGMENT text accumulation for the new text segment after tool calls

packages/ai/tests/stream-processor.test.ts

@@ -1396,6 +1396,39 @@ describe('StreamProcessor', () => {
       expect((textParts[0] as any).content).toBe('Before')
       expect((textParts[1] as any).content).toBe('After')
     })
+
+    // A turn that opens with a tool call and THEN speaks: the assistant
+    // message is auto-created by TOOL_CALL_START, so TEXT_MESSAGE_START hits
+    // the pending-message path and does not reset the post-tool segment flag.
+    // The first TEXT_MESSAGE_CONTENT delta must still survive in the assembled
+    // TextPart (spec: every delta appends) rather than being folded away on the
+    // next delta.
+    it('keeps the first post-tool text delta when the turn opens with a tool call', () => {
+      const processor = new StreamProcessor()
+      processor.prepareAssistantMessage()
+
+      processor.processChunk(ev.runStarted())
+      processor.processChunk(ev.toolStart('tc-1', 'search'))
+      processor.processChunk(ev.toolArgs('tc-1', '{}'))
+      processor.processChunk(ev.toolEnd('tc-1', 'search'))
+      processor.processChunk(ev.toolResult('tc-1', '{"ok":true}'))
+      processor.processChunk(ev.textStart())
+      processor.processChunk(ev.textContent('Hello '))
+      processor.processChunk(ev.textContent('from '))
+      processor.processChunk(ev.textContent('the '))
+      processor.processChunk(ev.textContent('model.'))
+      processor.processChunk(ev.textEnd())
+      processor.processChunk(ev.runFinished('stop'))
+      processor.finalizeStream()
+
+      const textParts = processor
+        .getMessages()
+        .flatMap((m) => m.parts)
+        .filter((p): p is TextPart => p.type === 'text')
+      expect(textParts.map((p) => p.content).join('')).toBe(
+        'Hello from the model.',
+      )
+    })
   })
 
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