diff --git a/src/tau_ai/openai_codex.py b/src/tau_ai/openai_codex.py
index aa4bf042b..6a678c7a8 100644
--- a/src/tau_ai/openai_codex.py
+++ b/src/tau_ai/openai_codex.py
@@ -543,6 +543,12 @@ async def _codex_provider_events(
                 thinking_parts.append(delta)
                 yield ProviderThinkingDeltaEvent(delta=delta)
 
+        elif event_type == "response.reasoning_summary_part.done":
+            if thinking_parts:
+                separator = "\n\n"
+                thinking_parts.append(separator)
+                yield ProviderThinkingDeltaEvent(delta=separator)
+
         elif event_type in {
             "response.output_item.done",
             "response.output_item.completed",
diff --git a/src/tau_ai/openai_compatible.py b/src/tau_ai/openai_compatible.py
index dd1e5d0e8..bad24df4e 100644
--- a/src/tau_ai/openai_compatible.py
+++ b/src/tau_ai/openai_compatible.py
@@ -482,6 +482,12 @@ def feed(self, event: str) -> tuple[list[ProviderEvent], bool]:
                 self._thinking_parts.append(delta)
                 return [ProviderThinkingDeltaEvent(delta=delta)], False
 
+        elif chunk_type == "response.reasoning_summary_part.done":
+            if self._thinking_parts:
+                separator = "\n\n"
+                self._thinking_parts.append(separator)
+                return [ProviderThinkingDeltaEvent(delta=separator)], False
+
         elif chunk_type == "response.output_item.added":
             item = chunk.get("item")
             _register_reasoning_item(self._reasoning_items, item)
diff --git a/tests/test_tau_ai.py b/tests/test_tau_ai.py
index 78f314431..3a9025844 100644
--- a/tests/test_tau_ai.py
+++ b/tests/test_tau_ai.py
@@ -1371,6 +1371,59 @@ def handler(_request: httpx.Request) -> httpx.Response:
     assert events[-1].message.text == "Done"
 
 
+@pytest.mark.anyio
+async def test_openai_codex_provider_preserves_reasoning_summary_part_boundaries() -> None:
+    async def credentials() -> OpenAICodexCredentials:
+        return OpenAICodexCredentials(access_token="access-token", account_id="account-1")
+
+    def handler(_request: httpx.Request) -> httpx.Response:
+        return httpx.Response(
+            200,
+            text=(
+                'data: {"type":"response.reasoning_summary_text.delta",'
+                '"delta":"**First step**"}\n\n'
+                'data: {"type":"response.reasoning_summary_part.done"}\n\n'
+                'data: {"type":"response.reasoning_summary_text.delta",'
+                '"delta":"**Second step**"}\n\n'
+                'data: {"type":"response.reasoning_summary_part.done"}\n\n'
+                'data: {"type":"response.output_text.delta","delta":"Done"}\n\n'
+                'data: {"type":"response.completed","response":{"status":"completed"}}\n\n'
+            ),
+            headers={"content-type": "text/event-stream"},
+        )
+
+    async with httpx.AsyncClient(transport=httpx.MockTransport(handler)) as client:
+        provider = OpenAICodexProvider(
+            OpenAICodexConfig(
+                credential_resolver=credentials,
+                base_url="https://chatgpt.test/backend-api",
+            ),
+            client=client,
+        )
+
+        events = await _collect(
+            provider.stream_response(
+                model="gpt-5.5",
+                system="You are Tau.",
+                messages=[UserMessage(content="Say done")],
+                tools=[],
+            )
+        )
+
+    thinking_events = [event for event in events if isinstance(event, ThinkingDeltaEvent)]
+    assert [event.delta for event in thinking_events] == [
+        "**First step**",
+        "\n\n",
+        "**Second step**",
+        "\n\n",
+    ]
+    end = events[-1]
+    assert isinstance(end, AssistantDoneEvent)
+    thinking = end.message.content[0]
+    assert isinstance(thinking, ThinkingContent)
+    assert thinking.thinking == "**First step**\n\n**Second step**\n\n"
+
+
 @pytest.mark.anyio
 async def test_openai_codex_provider_streams_tool_calls() -> None:
     async def credentials() -> OpenAICodexCredentials:
@@ -2053,6 +2106,57 @@ def handler(_request: httpx.Request) -> httpx.Response:
     assert thinking.delta == "Considering"
 
 
+@pytest.mark.anyio
+async def test_responses_api_preserves_reasoning_summary_part_boundaries() -> None:
+    def handler(_request: httpx.Request) -> httpx.Response:
+        return httpx.Response(
+            200,
+            text=(
+                'data: {"type":"response.reasoning_summary_text.delta",'
+                '"delta":"**First step**"}\n\n'
+                'data: {"type":"response.reasoning_summary_part.done"}\n\n'
+                'data: {"type":"response.reasoning_summary_text.delta",'
+                '"delta":"**Second step**"}\n\n'
+                'data: {"type":"response.reasoning_summary_part.done"}\n\n'
+                'data: {"type":"response.output_text.delta","delta":"Answer"}\n\n'
+                'data: {"type":"response.completed","response":{"status":"completed"}}\n\n'
+            ),
+            headers={"content-type": "text/event-stream"},
+        )
+
+    async with httpx.AsyncClient(transport=httpx.MockTransport(handler)) as client:
+        provider = OpenAICompatibleProvider(
+            OpenAICompatibleConfig(
+                api_key="test-key",
+                base_url="https://example.test/v1",
+                reasoning_effort="high",
+            ),
+            client=client,
+        )
+
+        events = await _collect(
+            provider.stream_response(
+                model="gpt-5.5",
+                system="You are Tau.",
+                messages=[UserMessage(content="think")],
+                tools=[],
+            )
+        )
+
+    thinking_events = [event for event in events if isinstance(event, ThinkingDeltaEvent)]
+    assert [event.delta for event in thinking_events] == [
+        "**First step**",
+        "\n\n",
+        "**Second step**",
+        "\n\n",
+    ]
+    end = events[-1]
+    assert isinstance(end, AssistantDoneEvent)
+    thinking = end.message.content[0]
+    assert isinstance(thinking, ThinkingContent)
+    assert thinking.thinking == "**First step**\n\n**Second step**\n\n"
+
+
 @pytest.mark.anyio
 async def test_responses_api_omits_reasoning_when_effort_is_none() -> None:
     requests: list[httpx.Request] = []