Fix MemoryContent.mime_type degenerating to str after dump/load roundtrip

python/packages/autogen-core/src/autogen_core/memory/_base_memory.py

@@ -2,7 +2,7 @@
 from enum import Enum
 from typing import Any, Dict, List, Union
 
-from pydantic import BaseModel, ConfigDict, field_serializer
+from pydantic import BaseModel, ConfigDict, field_serializer, field_validator
 
 from .._cancellation_token import CancellationToken
 from .._component_config import ComponentBase
@@ -44,6 +44,24 @@ def serialize_mime_type(self, mime_type: MemoryMimeType | str) -> str:
             return mime_type.value
         return mime_type
 
+    @field_validator("mime_type", mode="before")
+    @classmethod
+    def _coerce_mime_type(cls, v: object) -> object:
+        """Restore a serialized string back to its :class:`MemoryMimeType`.
+
+        ``dump_component()`` serializes an enum member to its string value, so a
+        ``load_component()`` roundtrip used to leave ``mime_type`` as a plain
+        ``str``, changing behaviour for code that dispatches on the enum. A
+        string that matches an enum value is coerced back; other custom MIME
+        type strings stay strings.
+        """
+        if isinstance(v, str):
+            try:
+                return MemoryMimeType(v)
+            except ValueError:
+                return v
+        return v
+
 
 class MemoryQueryResult(BaseModel):
     """Result of a memory :meth:`~autogen_core.memory.Memory.query` operation."""

python/packages/autogen-core/tests/test_memory.py

@@ -51,6 +51,29 @@ def test_memory_component_dump_config_to_base_model() -> None:
     assert len(config.config["memory_contents"]) == 1
 
 
+def test_memory_mime_type_roundtrip() -> None:
+    """A dump/load roundtrip restores the enum, not a plain str (#8293)."""
+    memory = ListMemory(
+        name="test_memory",
+        memory_contents=[MemoryContent(content="hello", mime_type=MemoryMimeType.MARKDOWN)],
+    )
+    config = memory.dump_component()
+
+    loaded = ListMemory.load_component(config)
+    assert loaded.content[0].mime_type is MemoryMimeType.MARKDOWN
+
+
+def test_memory_mime_type_custom_string_stays_str() -> None:
+    """A custom MIME type that is not an enum value stays a string."""
+    memory = ListMemory(
+        name="test_memory",
+        memory_contents=[MemoryContent(content="hello", mime_type="application/custom")],
+    )
+    loaded = ListMemory.load_component(memory.dump_component())
+    assert loaded.content[0].mime_type == "application/custom"
+    assert not isinstance(loaded.content[0].mime_type, MemoryMimeType)
+
+
 def test_memory_abc_implementation() -> None:
     """Test that Memory ABC is properly implemented."""