diff --git a/python/packages/core/agent_framework/_types.py b/python/packages/core/agent_framework/_types.py
index 14c3050952..6d6bc58068 100644
--- a/python/packages/core/agent_framework/_types.py
+++ b/python/packages/core/agent_framework/_types.py
@@ -1380,6 +1380,13 @@ def _add_text_content(self, other: Content) -> Content:
 
     def _add_text_reasoning_content(self, other: Content) -> Content:
         """Add two TextReasoningContent instances."""
+        # Ensure we do not silently merge contents with conflicting ids
+        if self.id and other.id and self.id != other.id:
+            raise AdditionItemMismatch(
+                f"Cannot add text_reasoning content with different ids: {self.id!r} != {other.id!r}"
+            )
+        combined_id = self.id or other.id
+
         # Concatenate text, handling None values
         self_text = self.text or ""  # type: ignore[attr-defined]
         other_text = other.text or ""  # type: ignore[attr-defined]
@@ -1390,6 +1397,7 @@ def _add_text_reasoning_content(self, other: Content) -> Content:
 
         return Content(
             "text_reasoning",
+            id=combined_id,
             text=combined_text,
             protected_data=protected_data,
             annotations=_combine_annotations(self.annotations, other.annotations),
@@ -1880,7 +1888,12 @@ def _coalesce_text_content(contents: list[Content], type_str: Literal["text", "t
             if first_new_content is None:
                 first_new_content = deepcopy(content)
             else:
-                first_new_content += content
+                try:
+                    first_new_content += content
+                except AdditionItemMismatch:
+                    # Different IDs means a new logical segment; flush the current one
+                    coalesced_contents.append(first_new_content)
+                    first_new_content = deepcopy(content)
         else:
             # skip this content, it is not of the right type
             # so write the existing one to the list and start a new one,
diff --git a/python/packages/core/tests/core/test_types.py b/python/packages/core/tests/core/test_types.py
index 8e5a0dc7d8..9d1b39f29e 100644
--- a/python/packages/core/tests/core/test_types.py
+++ b/python/packages/core/tests/core/test_types.py
@@ -43,7 +43,7 @@
     add_usage_details,
     validate_tool_mode,
 )
-from agent_framework.exceptions import ContentError
+from agent_framework.exceptions import AdditionItemMismatch, ContentError
 
 
 @fixture
@@ -1526,6 +1526,88 @@ def test_text_reasoning_content_iadd_coverage():
     assert t1.text == "Thinking 1 Thinking 2"
 
 
+def test_text_reasoning_content_add_preserves_id():
+    """Test that coalescing text_reasoning Content preserves the id field."""
+
+    t1 = Content.from_text_reasoning(id="rs_abc123", text="Thinking part 1")
+    t2 = Content.from_text_reasoning(id="rs_abc123", text=" part 2")
+
+    result = t1 + t2
+    assert result.text == "Thinking part 1 part 2"
+    assert result.id == "rs_abc123"
+
+
+def test_text_reasoning_content_add_id_fallback_to_other():
+    """Test that coalescing falls back to other's id when self has no id."""
+
+    t1 = Content.from_text_reasoning(text="Thinking part 1")
+    t2 = Content.from_text_reasoning(id="rs_abc123", text=" part 2")
+
+    result = t1 + t2
+    assert result.id == "rs_abc123"
+
+
+def test_text_reasoning_content_add_preserves_id_with_encrypted_content():
+    """Test that id and encrypted_content both survive coalescing for round-trip."""
+
+    t1 = Content.from_text_reasoning(
+        id="rs_abc123",
+        text="Thinking",
+        additional_properties={"encrypted_content": "enc_blob_data"},
+    )
+    t2 = Content.from_text_reasoning(id="rs_abc123", text=" more")
+
+    result = t1 + t2
+    assert result.text == "Thinking more"
+    assert result.id == "rs_abc123"
+    assert result.additional_properties.get("encrypted_content") == "enc_blob_data"
+
+
+def test_text_reasoning_content_add_conflicting_ids_raises():
+    """Test that coalescing text_reasoning Content with different ids raises AdditionItemMismatch."""
+
+    t1 = Content.from_text_reasoning(id="rs_abc123", text="Thinking part 1")
+    t2 = Content.from_text_reasoning(id="rs_xyz789", text=" part 2")
+
+    with pytest.raises(AdditionItemMismatch, match="different ids"):
+        t1 + t2
+
+
+def test_text_reasoning_content_add_neither_has_id():
+    """Test that coalescing text_reasoning Content when neither has an id results in None id."""
+
+    t1 = Content.from_text_reasoning(text="Thinking part 1")
+    t2 = Content.from_text_reasoning(text=" part 2")
+
+    result = t1 + t2
+    assert result.text == "Thinking part 1 part 2"
+    assert result.id is None
+
+
+def test_coalesce_text_reasoning_with_different_ids():
+    """Test that _coalesce_text_content keeps separate text_reasoning items when IDs differ.
+
+    Regression test: streaming responses can produce multiple text_reasoning
+    segments with distinct IDs. These must not be merged into one.
+    """
+    from agent_framework._types import _coalesce_text_content
+
+    contents = [
+        Content.from_text_reasoning(id="rs_aaa", text="Thinking A1"),
+        Content.from_text_reasoning(id="rs_aaa", text=" A2"),
+        Content.from_text_reasoning(id="rs_bbb", text="Thinking B1"),
+        Content.from_text_reasoning(id="rs_bbb", text=" B2"),
+    ]
+
+    _coalesce_text_content(contents, "text_reasoning")
+
+    assert len(contents) == 2
+    assert contents[0].id == "rs_aaa"
+    assert contents[0].text == "Thinking A1 A2"
+    assert contents[1].id == "rs_bbb"
+    assert contents[1].text == "Thinking B1 B2"
+
+
 def test_comprehensive_to_dict_exclude_options():
     """Test to_dict methods with various exclude options for better coverage."""