diff --git a/pydantic_ai_slim/pydantic_ai/messages.py b/pydantic_ai_slim/pydantic_ai/messages.py
index 988430d12a..c880c75a74 100644
--- a/pydantic_ai_slim/pydantic_ai/messages.py
+++ b/pydantic_ai_slim/pydantic_ai/messages.py
@@ -970,6 +970,9 @@ class ModelRequest:
     run_id: str | None = None
     """The unique identifier of the agent run in which this message originated."""
 
+    metadata: dict[str, Any] | None = None
+    """Additional data that can be accessed programmatically by the application but is not sent to the LLM."""
+
     @classmethod
     def user_text_prompt(cls, user_prompt: str, *, instructions: str | None = None) -> ModelRequest:
         """Create a `ModelRequest` with a single user prompt as text."""
@@ -1214,6 +1217,9 @@ class ModelResponse:
     run_id: str | None = None
     """The unique identifier of the agent run in which this message originated."""
 
+    metadata: dict[str, Any] | None = None
+    """Additional data that can be accessed programmatically by the application but is not sent to the LLM."""
+
     @property
     def text(self) -> str | None:
         """Get the text in the response."""
diff --git a/tests/test_agent.py b/tests/test_agent.py
index d8323f3c98..29b54643f0 100644
--- a/tests/test_agent.py
+++ b/tests/test_agent.py
@@ -3730,6 +3730,7 @@ def test_binary_content_serializable():
                 'instructions': None,
                 'kind': 'request',
                 'run_id': IsStr(),
+                'metadata': None,
             },
             {
                 'parts': [{'content': 'success (no tool calls)', 'id': None, 'part_kind': 'text'}],
@@ -3751,6 +3752,7 @@ def test_binary_content_serializable():
                 'kind': 'response',
                 'finish_reason': None,
                 'run_id': IsStr(),
+                'metadata': None,
             },
         ]
     )
@@ -3788,6 +3790,7 @@ def test_image_url_serializable_missing_media_type():
                 'instructions': None,
                 'kind': 'request',
                 'run_id': IsStr(),
+                'metadata': None,
             },
             {
                 'parts': [{'content': 'success (no tool calls)', 'id': None, 'part_kind': 'text'}],
@@ -3809,6 +3812,7 @@ def test_image_url_serializable_missing_media_type():
                 'kind': 'response',
                 'finish_reason': None,
                 'run_id': IsStr(),
+                'metadata': None,
             },
         ]
     )
@@ -3853,6 +3857,7 @@ def test_image_url_serializable():
                 'instructions': None,
                 'kind': 'request',
                 'run_id': IsStr(),
+                'metadata': None,
             },
             {
                 'parts': [{'content': 'success (no tool calls)', 'id': None, 'part_kind': 'text'}],
@@ -3874,6 +3879,7 @@ def test_image_url_serializable():
                 'kind': 'response',
                 'finish_reason': None,
                 'run_id': IsStr(),
+                'metadata': None,
             },
         ]
     )
@@ -4353,6 +4359,7 @@ def foo_tool(foo: Foo) -> int:
             'instructions': None,
             'kind': 'request',
             'run_id': IsStr(),
+            'metadata': None,
         }
     )
 
diff --git a/tests/test_messages.py b/tests/test_messages.py
index 9b6ad3fbb8..d971ac46b3 100644
--- a/tests/test_messages.py
+++ b/tests/test_messages.py
@@ -463,6 +463,7 @@ def test_file_part_serialization_roundtrip():
                 'provider_response_id': None,
                 'finish_reason': None,
                 'run_id': None,
+                'metadata': None,
             }
         ]
     )
@@ -475,11 +476,9 @@ def test_model_messages_type_adapter_preserves_run_id():
         ModelRequest(
             parts=[UserPromptPart(content='Hi there', timestamp=datetime.now(tz=timezone.utc))],
             run_id='run-123',
+            metadata={'key': 'value'},
         ),
-        ModelResponse(
-            parts=[TextPart(content='Hello!')],
-            run_id='run-123',
-        ),
+        ModelResponse(parts=[TextPart(content='Hello!')], run_id='run-123', metadata={'key': 'value'}),
     ]
 
     serialized = ModelMessagesTypeAdapter.dump_python(messages, mode='python')