fix(memory): allow base session settings overrides for subclasses

src/agents/memory/session_settings.py

@@ -51,10 +51,10 @@ def resolve(self, override: SessionSettings | dict[str, Any] | None) -> SessionS
         override = _coerce_session_settings(override, settings_type=type(self))
 
         changes = {
-            field.name: getattr(override, field.name)
+            field.name: getattr(override, field.name, None)
             for field in fields(self)
             if (override_fields is None or field.name in override_fields)
-            and getattr(override, field.name) is not None
+            and getattr(override, field.name, None) is not None
         }
 
         return replace(self, **changes)
@@ -76,6 +76,13 @@ def _coerce_session_settings(
     *,
     settings_type: type[SessionSettings],
 ) -> SessionSettings:
+    if isinstance(value, SessionSettings):
+        if type(value) is SessionSettings or isinstance(value, settings_type):
+            return value
+        raise TypeError(
+            f"Session settings override must be SessionSettings or an instance of "
+            f"{settings_type.__name__}; got {type(value).__name__}"
+        )
     return coerce_dataclass_config(value, settings_type, parameter_name="session")
 
 

tests/memory/test_session.py

@@ -962,6 +962,44 @@ async def test_session_settings_resolve():
     assert final_none.limit == 100
 
 
+def test_session_settings_resolve_accepts_base_override_for_subclass() -> None:
+    """A subclass can resolve settings produced by the base RunConfig type."""
+    from pydantic.dataclasses import dataclass
+
+    @dataclass
+    class TenantSessionSettings(SessionSettings):
+        tenant: str = "default"
+
+    base = TenantSessionSettings(limit=100, tenant="acme")
+    run_config = RunConfig(session_settings={"limit": 50})
+
+    final = base.resolve(run_config.session_settings)
+
+    assert isinstance(final, TenantSessionSettings)
+    assert final.limit == 50
+    assert final.tenant == "acme"
+    assert base.limit == 100
+
+
+def test_session_settings_resolve_rejects_incompatible_subclass_override() -> None:
+    """A subclass must not silently discard fields from a sibling subclass."""
+    from pydantic.dataclasses import dataclass
+
+    @dataclass
+    class TenantSessionSettings(SessionSettings):
+        tenant: str = "default"
+
+    @dataclass
+    class RegionSessionSettings(SessionSettings):
+        region: str = "default"
+
+    base = TenantSessionSettings(limit=100, tenant="acme")
+    override = RegionSessionSettings(limit=50, region="us-east-1")
+
+    with pytest.raises(TypeError, match="TenantSessionSettings"):
+        base.resolve(override)
+
+
 @pytest.mark.asyncio
 async def test_runner_with_session_settings_override():
     """Test that RunConfig can override session's default settings."""