diff --git a/pydantic_ai_slim/pydantic_ai/_agent_graph.py b/pydantic_ai_slim/pydantic_ai/_agent_graph.py
index d7a54c5c71..13dc4d6b15 100644
--- a/pydantic_ai_slim/pydantic_ai/_agent_graph.py
+++ b/pydantic_ai_slim/pydantic_ai/_agent_graph.py
@@ -1169,32 +1169,39 @@ async def _process_message_history(
 def _clean_message_history(messages: list[_messages.ModelMessage]) -> list[_messages.ModelMessage]:
     """Clean the message history by merging consecutive messages of the same type."""
     clean_messages: list[_messages.ModelMessage] = []
+    # Add parts to a set to ensure no duplication
     for message in messages:
         last_message = clean_messages[-1] if len(clean_messages) > 0 else None
 
         if isinstance(message, _messages.ModelRequest):
-            if (
-                last_message
-                and isinstance(last_message, _messages.ModelRequest)
-                # Requests can only be merged if they have the same instructions
-                and (
+            if last_message and isinstance(last_message, _messages.ModelRequest):
+                same_instructions = (
                     not last_message.instructions
                     or not message.instructions
                     or last_message.instructions == message.instructions
                 )
-            ):
-                parts = [*last_message.parts, *message.parts]
-                parts.sort(
-                    # Tool return parts always need to be at the start
-                    key=lambda x: 0 if isinstance(x, _messages.ToolReturnPart | _messages.RetryPromptPart) else 1
+                last_is_stub = all(
+                    isinstance(part, _messages.ToolReturnPart | _messages.RetryPromptPart)
+                    for part in last_message.parts
                 )
-                merged_message = _messages.ModelRequest(
-                    parts=parts,
-                    instructions=last_message.instructions or message.instructions,
+                message_is_stub = all(
+                    isinstance(part, _messages.ToolReturnPart | _messages.RetryPromptPart) for part in message.parts
                 )
-                clean_messages[-1] = merged_message
-            else:
-                clean_messages.append(message)
+
+                if same_instructions and (not last_is_stub or message_is_stub):
+                    parts = [*last_message.parts, *message.parts]
+                    parts.sort(
+                        # Tool return parts always need to be at the start
+                        key=lambda x: 0 if isinstance(x, _messages.ToolReturnPart | _messages.RetryPromptPart) else 1
+                    )
+                    merged_message = _messages.ModelRequest(
+                        parts=parts,
+                        instructions=last_message.instructions or message.instructions,
+                    )
+                    clean_messages[-1] = merged_message
+                    continue
+
+            clean_messages.append(message)
         elif isinstance(message, _messages.ModelResponse):  # pragma: no branch
             if (
                 last_message
diff --git a/tests/test_a2a.py b/tests/test_a2a.py
index 93e56f12c0..048f8a1e8f 100644
--- a/tests/test_a2a.py
+++ b/tests/test_a2a.py
@@ -623,10 +623,12 @@ def track_messages(messages: list[ModelMessage], info: AgentInfo) -> ModelRespon
                                 content='Final result processed.',
                                 tool_call_id=IsStr(),
                                 timestamp=IsDatetime(),
-                            ),
-                            UserPromptPart(content='Second message', timestamp=IsDatetime()),
+                            )
                         ],
                     ),
+                    ModelRequest(
+                        parts=[UserPromptPart(content='Second message', timestamp=IsDatetime())],
+                    ),
                 ]
             )
 
diff --git a/tests/test_history_processor.py b/tests/test_history_processor.py
index 54d38935d2..78660ef517 100644
--- a/tests/test_history_processor.py
+++ b/tests/test_history_processor.py
@@ -801,3 +801,29 @@ def __call__(self, _: RunContext, messages: list[ModelMessage]) -> list[ModelMes
         ]
     )
     assert result.new_messages() == result.all_messages()[-2:]
+
+
+def test_clean_message_history_keeps_tool_stub_separate():
+    """Regression guard for b26a6872f that merged tool-return stubs into the next user request."""
+
+    # TODO: imports should get moved to the top whenever we open P/R
+    from pydantic_ai._agent_graph import _clean_message_history  # pyright: ignore
+    from pydantic_ai.messages import ToolReturnPart
+
+    tool_stub = ModelRequest(
+        parts=[
+            ToolReturnPart(
+                tool_name='summarize',
+                content='summaries galore',
+                tool_call_id='call-1',
+            )
+        ]
+    )
+    user_request = ModelRequest(parts=[UserPromptPart(content='fresh prompt')])
+
+    cleaned = _clean_message_history([tool_stub, user_request])
+
+    assert len(cleaned[0].parts) == 1, 'tool-return part started as unique and should remain unique'
+    assert len(cleaned) == 2, 'tool-return stubs must remain separate from subsequent user prompts'
+    assert isinstance(cleaned[0].parts[0], ToolReturnPart)
+    assert isinstance(cleaned[1].parts[0], UserPromptPart)