diff --git a/agent/codex_runtime.py b/agent/codex_runtime.py
index c01084c4a4..f36f1c78fe 100644
--- a/agent/codex_runtime.py
+++ b/agent/codex_runtime.py
@@ -1027,6 +1027,10 @@ def _consume_codex_event_stream(
     first_delta_fired = False
     active_message_phase: str | None = None
     commentary_text_deltas: List[str] = []
+    # Last reasoning summary_index seen. The Responses stream delimits summary
+    # parts by this index and gives each part no separator of its own, so a
+    # change of index is where the blank line belongs.
+    active_summary_index: Any = None
     terminal_status: str = "completed"
     terminal_usage: Any = None
     terminal_response_id: str = None
@@ -1120,6 +1124,17 @@ def _consume_codex_event_stream(
         if "reasoning" in event_type and "delta" in event_type:
             reasoning_text = _event_field(event, "delta", "")
             if reasoning_text and on_reasoning_delta is not None:
+                # Summary parts stream one after another with no separator of
+                # their own; summary_index is the boundary the wire gives us.
+                summary_index = _event_field(event, "summary_index")
+                if (
+                    summary_index is not None
+                    and active_summary_index is not None
+                    and summary_index != active_summary_index
+                ):
+                    reasoning_text = f"\n\n{reasoning_text}"
+                if summary_index is not None:
+                    active_summary_index = summary_index
                 try:
                     on_reasoning_delta(reasoning_text)
                 except Exception:
diff --git a/tests/run_agent/test_run_agent_codex_responses.py b/tests/run_agent/test_run_agent_codex_responses.py
index 4b11b1fdb7..18dc13526b 100644
--- a/tests/run_agent/test_run_agent_codex_responses.py
+++ b/tests/run_agent/test_run_agent_codex_responses.py
@@ -1985,11 +1985,59 @@ def test_duplicate_detection_uses_commentary_when_hidden_reasoning_changes(monke
 
 
 
+def test_consume_codex_stream_separates_reasoning_summary_parts():
+    """summary_index is the part boundary; the wire sends no separator itself."""
+    from agent.codex_runtime import _consume_codex_event_stream
+
+    reasoning_streamed = []
+
+    _consume_codex_event_stream(
+        _FakeCreateStream([
+            SimpleNamespace(type="response.created"),
+            SimpleNamespace(
+                type="response.reasoning_summary_text.delta",
+                summary_index=0,
+                delta="**Investigating culprit PRs**",
+            ),
+            SimpleNamespace(
+                type="response.reasoning_summary_text.delta",
+                summary_index=1,
+                delta="**Inspecting message schema**",
+            ),
+            SimpleNamespace(
+                type="response.reasoning_summary_text.delta",
+                summary_index=1,
+                delta=" and tool_calls content",
+            ),
+            SimpleNamespace(type="response.completed", response=SimpleNamespace(status="completed")),
+        ]),
+        model="gpt-5-codex",
+        on_reasoning_delta=reasoning_streamed.append,
+    )
+
+    joined = "".join(reasoning_streamed)
+    assert "****" not in joined
+    assert joined == (
+        "**Investigating culprit PRs**"
+        "\n\n**Inspecting message schema** and tool_calls content"
+    )
 
 
+def test_consume_codex_stream_leaves_unindexed_reasoning_untouched():
+    """Streams with no summary_index (plain reasoning_text) must not gain breaks."""
+    from agent.codex_runtime import _consume_codex_event_stream
 
+    reasoning_streamed = []
 
+    _consume_codex_event_stream(
+        _FakeCreateStream([
+            SimpleNamespace(type="response.created"),
+            SimpleNamespace(type="response.reasoning_text.delta", delta="Need to "),
+            SimpleNamespace(type="response.reasoning_text.delta", delta="inspect files."),
+            SimpleNamespace(type="response.completed", response=SimpleNamespace(status="completed")),
+        ]),
+        model="gpt-5-codex",
+        on_reasoning_delta=reasoning_streamed.append,
+    )
 
-
-
-
+    assert "".join(reasoning_streamed) == "Need to inspect files."