diff --git a/.changeset/fix-post-tool-first-text-delta.md b/.changeset/fix-post-tool-first-text-delta.md
new file mode 100644
index 0000000000..05c3c95aee
--- /dev/null
+++ b/.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.
diff --git a/packages/ai/src/activities/chat/stream/processor.ts b/packages/ai/src/activities/chat/stream/processor.ts
index 8b6f5bc371..66b8bbc0db 100644
--- a/packages/ai/src/activities/chat/stream/processor.ts
+++ b/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
diff --git a/packages/ai/tests/stream-processor.test.ts b/packages/ai/tests/stream-processor.test.ts
index 61a37de65b..6f95dd1b15 100644
--- a/packages/ai/tests/stream-processor.test.ts
+++ b/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.',
+      )
+    })
   })
 
   // ==========================================================================
diff --git a/testing/e2e/fixtures/text-tool-text/tool-text.json b/testing/e2e/fixtures/text-tool-text/tool-text.json
new file mode 100644
index 0000000000..a976eb8d38
--- /dev/null
+++ b/testing/e2e/fixtures/text-tool-text/tool-text.json
@@ -0,0 +1,27 @@
+{
+  "fixtures": [
+    {
+      "match": {
+        "userMessage": "[tool-text] list the guitars in stock",
+        "sequenceIndex": 0
+      },
+      "response": {
+        "toolCalls": [
+          {
+            "name": "getGuitars",
+            "arguments": "{}"
+          }
+        ]
+      }
+    },
+    {
+      "match": {
+        "userMessage": "[tool-text] list the guitars in stock",
+        "sequenceIndex": 1
+      },
+      "response": {
+        "content": "Absolutely, here's our inventory: Fender Stratocaster, Gibson Les Paul, and Martin D-28."
+      }
+    }
+  ]
+}
diff --git a/testing/e2e/tests/text-tool-text.spec.ts b/testing/e2e/tests/text-tool-text.spec.ts
index 953fead6a4..291fc1351f 100644
--- a/testing/e2e/tests/text-tool-text.spec.ts
+++ b/testing/e2e/tests/text-tool-text.spec.ts
@@ -38,5 +38,33 @@ for (const provider of providersFor('text-tool-text')) {
       expect(combined).toContain('Let me check')
       expect(combined).toContain('Fender Stratocaster')
     })
+
+    // No leading text, so the tool call creates the message and the post-tool
+    // TEXT_MESSAGE_START skips the segment reset — the path that used to drop
+    // the reply's first word.
+    test('keeps the first word of the text that follows an opening tool call', async ({
+      page,
+      testId,
+      aimockPort,
+    }) => {
+      await page.goto(
+        featureUrl(provider, 'text-tool-text', testId, aimockPort),
+      )
+
+      await sendMessage(page, '[tool-text] list the guitars in stock')
+      await waitForResponse(page)
+
+      const toolCalls = await getToolCalls(page)
+      expect(toolCalls[0].name).toBe('getGuitars')
+
+      await waitForAssistantText(page, 'Martin D-28')
+
+      const allText = await page
+        .getByTestId('assistant-message')
+        .allInnerTexts()
+      const combined = allText.join(' ')
+      expect(combined).toContain('Absolutely, here')
+      expect(combined).toContain('Martin D-28')
+    })
   })
 }