diff --git a/agent/chat_completion_helpers.py b/agent/chat_completion_helpers.py
index 48c7517827..06f340ae47 100644
--- a/agent/chat_completion_helpers.py
+++ b/agent/chat_completion_helpers.py
@@ -3148,6 +3148,11 @@ class _StreamingCall(StreamingWaitMonitor):
         base_timeout, read_timeout, conn_cap = self._stream_timeouts()
         content_parts: list = []
         reasoning_parts: list = []
+        # Live-display accumulator for detail-derived reasoning text: de-gluing must
+        # compare against what the display actually received, not ``reasoning_parts``
+        # (a provider that mirrors the same text in both fields would otherwise read
+        # as already-glued on the first chunk and get a spurious break).
+        detail_display_parts: list[str] = []
         # OpenAI structured refusal (``delta.refusal``): the explanation streams here and
         # ``delta.content`` stays empty, so an un-accumulated refusal looks like an empty
         # stream and burns the empty-response retries (the non-streaming fix is #46013).
@@ -3247,7 +3252,17 @@ class _StreamingCall(StreamingWaitMonitor):
             # Details may carry the full text while ordinary reasoning is only
             # a sparse fragment or a mirror. Deliver one representation per
             # chunk, without rewriting either persisted/replayed field.
-            display_reasoning = "".join(detail_text_parts) or reasoning_text
+            # Summary-part boundaries need the same repair the plain path applies:
+            # de-glue against the detail display's own accumulator (not
+            # ``reasoning_parts`` — with mirrored fields that would insert a
+            # spurious break on the first chunk), so the live box and the
+            # persisted ``reasoning_content`` agree.
+            detail_text = "".join(detail_text_parts)
+            if detail_text:
+                detail_text = separate_glued_reasoning_blocks(
+                    detail_display_parts[-1] if detail_display_parts else "", detail_text)
+                detail_display_parts.append(detail_text)
+            display_reasoning = detail_text or reasoning_text
             if display_reasoning:
                 self._emit_reasoning(display_reasoning)
             # Not routed to the live display: the transport promotes a sole-payload
diff --git a/tests/agent/test_streamed_reasoning_details.py b/tests/agent/test_streamed_reasoning_details.py
index b12fc8a47d..714e93517b 100644
--- a/tests/agent/test_streamed_reasoning_details.py
+++ b/tests/agent/test_streamed_reasoning_details.py
@@ -78,6 +78,52 @@ def test_streamed_details_land_on_final_message_and_persist(_mock_close, mock_cr
     assert persisted["reasoning_details"] == msg.reasoning_details
 
 
+@patch("run_agent.AIAgent._create_request_openai_client")
+@patch("run_agent.AIAgent._close_request_openai_client")
+def test_live_details_deglue_summary_part_boundaries(_mock_close, mock_create):
+    """The live display gets the same boundary repair as the persisted reasoning.
+
+    A reasoning-summary model streams one delta per completed summary part, each
+    a bare bold heading. Without the ``separate_glued_reasoning_blocks`` repair
+    on the detail path, the live box glues parts head-to-tail while
+    ``reasoning_content`` stays de-glued — display and history disagree.
+    """
+    agent = _agent()
+    client = MagicMock()
+    mock_create.return_value = client
+
+    def summary_chunk(summary, **kw):
+        chunk = _make_chunk(**kw)
+        chunk.choices[0].delta.reasoning = summary  # provider mirrors both fields
+        chunk.choices[0].delta.model_extra = {
+            "reasoning_details": [{"type": "reasoning.summary", "summary": summary}]}
+        return chunk
+
+    client.chat.completions.create.return_value = iter([
+        summary_chunk("**One**"),
+        summary_chunk("**Two**"),
+        _make_chunk(content="Answer", finish_reason="stop", model="test-model"),
+    ])
+    delivered = []
+    agent.reasoning_callback = delivered.append
+    response = agent._interruptible_streaming_api_call({})
+
+    assert "".join(delivered) == "**One**\n\n**Two**"
+    assert response.choices[0].message.reasoning_content == "**One**\n\n**Two**"
+
+    # Details-only variant: the delta carries no plain ``reasoning`` field at all.
+    client.chat.completions.create.return_value = iter([
+        _make_chunk(reasoning_details=[{"type": "reasoning.summary", "summary": "**One**"}]),
+        _make_chunk(reasoning_details=[{"type": "reasoning.summary", "summary": "**Two**"}]),
+        _make_chunk(content="Answer", finish_reason="stop", model="test-model"),
+    ])
+    delivered = []
+    agent.reasoning_callback = delivered.append
+    response = agent._interruptible_streaming_api_call({})
+
+    assert "".join(delivered) == "**One**\n\n**Two**"
+
+
 @patch("run_agent.AIAgent._create_request_openai_client")
 @patch("run_agent.AIAgent._close_request_openai_client")
 def test_no_details_leaves_attribute_absent(_mock_close, mock_create):