diff --git a/src/agents/memory/session_settings.py b/src/agents/memory/session_settings.py
index eb42f617f2..003aa85e8d 100644
--- a/src/agents/memory/session_settings.py
+++ b/src/agents/memory/session_settings.py
@@ -48,13 +48,14 @@ def resolve(self, override: SessionSettings | dict[str, Any] | None) -> SessionS
             if isinstance(override, dict)
             else None
         )
-        override = _coerce_session_settings(override, settings_type=type(self))
+        if type(override) is not SessionSettings:
+            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)
diff --git a/tests/memory/test_session.py b/tests/memory/test_session.py
index 9667c5b23c..6f55df3b0a 100644
--- a/tests/memory/test_session.py
+++ b/tests/memory/test_session.py
@@ -8,6 +8,7 @@
 from typing import Any, cast
 
 import pytest
+from pydantic.dataclasses import dataclass
 
 from agents import Agent, RunConfig, Runner, SessionSettings, SQLiteSession, TResponseInputItem
 from agents.memory.sqlite_session import _await_mutation
@@ -962,6 +963,106 @@ async def test_session_settings_resolve():
     assert final_none.limit == 100
 
 
+@dataclass
+class _TenantSessionSettings(SessionSettings):
+    tenant: str = "default"
+
+
+@dataclass
+class _OtherSessionSettings(SessionSettings):
+    tenant: str = "other"
+
+
+def test_session_settings_resolve_accepts_base_override_on_subclass():
+    """A SessionSettings subclass accepts a base-class override and keeps its own fields."""
+    settings = _TenantSessionSettings(limit=1, tenant="acme")
+
+    resolved = settings.resolve(SessionSettings(limit=5))
+
+    assert isinstance(resolved, _TenantSessionSettings)
+    assert resolved.limit == 5
+    assert resolved.tenant == "acme"
+
+
+def test_session_settings_resolve_rejects_sibling_subclass_override():
+    """Unrelated settings subclasses cannot silently copy overlapping fields."""
+    settings = _TenantSessionSettings(limit=1, tenant="acme")
+
+    with pytest.raises(TypeError, match="must be a _TenantSessionSettings instance or a dict"):
+        settings.resolve(_OtherSessionSettings(limit=5, tenant="other"))
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize(
+    "override",
+    [SessionSettings(limit=5), {"limit": 5}],
+    ids=["instance", "dict"],
+)
+async def test_runner_session_settings_override_applies_to_subclassed_settings(
+    override: SessionSettings | dict[str, Any],
+):
+    """A base SessionSettings override from RunConfig applies to a session whose settings
+    are a SessionSettings subclass."""
+    session = SQLiteSession(
+        "subclass_override_test",
+        session_settings=_TenantSessionSettings(tenant="acme"),
+    )
+    try:
+        items: list[TResponseInputItem] = [
+            {"role": "user", "content": f"Turn {i}"} for i in range(10)
+        ]
+        await session.add_items(items)
+
+        model = ScriptedModel()
+        model.enqueue([get_text_message("Got it")])
+        agent = Agent(name="test", model=model)
+
+        await Runner.run(
+            agent,
+            "New question",
+            session=session,
+            run_config=RunConfig(session_settings=override),
+        )
+
+        # The override's limit applies: only the last 5 history items plus the new question.
+        history_items = [
+            item for item in model.calls[-1].input if item.get("content") != "New question"
+        ]
+        assert len(history_items) == 5
+    finally:
+        session.close()
+
+
+@pytest.mark.asyncio
+async def test_runner_session_settings_override_rejects_sibling_subclass():
+    """Runner rejects an unrelated settings subclass before invoking the model."""
+    session = SQLiteSession(
+        "sibling_subclass_override_test",
+        session_settings=_TenantSessionSettings(tenant="acme"),
+    )
+    try:
+        model = ScriptedModel()
+        model.enqueue([get_text_message("Got it")])
+        agent = Agent(name="test", model=model)
+
+        with pytest.raises(
+            TypeError,
+            match="must be a _TenantSessionSettings instance or a dict",
+        ):
+            await Runner.run(
+                agent,
+                "New question",
+                session=session,
+                run_config=RunConfig(
+                    session_settings=_OtherSessionSettings(limit=5, tenant="other")
+                ),
+            )
+
+        assert not model.calls
+    finally:
+        session.close()
+
+
 @pytest.mark.asyncio
 async def test_runner_with_session_settings_override():
     """Test that RunConfig can override session's default settings."""
diff --git a/tests/test_run_config.py b/tests/test_run_config.py
index d1e0ed2e98..4ef0402de8 100644
--- a/tests/test_run_config.py
+++ b/tests/test_run_config.py
@@ -1,9 +1,11 @@
 from __future__ import annotations
 
+from dataclasses import dataclass
 from pathlib import PureWindowsPath
 from typing import Any, cast
 
 import pytest
+from pydantic.dataclasses import dataclass as pydantic_dataclass
 
 from agents import (
     Agent,
@@ -40,6 +42,16 @@ def get_model(self, model_name: str | None) -> Model:
         return self.model_to_return
 
 
+@pydantic_dataclass
+class _DeclaredSessionSettings(SessionSettings):
+    tenant: str = "default"
+
+
+@dataclass
+class _DeclaredSessionRunConfig(RunConfig):
+    session_settings: _DeclaredSessionSettings | None = None
+
+
 def test_run_config_normalizes_first_party_dictionary_settings() -> None:
     config = RunConfig(
         model_settings={"reasoning": {"context": "all_turns"}, "temperature": 0.0},
@@ -99,6 +111,22 @@ def test_run_config_preserves_typed_configuration_instances() -> None:
     assert config.session_settings is session_settings
 
 
+def test_run_config_subclass_uses_declared_session_settings_type() -> None:
+    config = cast(Any, _DeclaredSessionRunConfig)(session_settings={"limit": 5, "tenant": "acme"})
+
+    assert isinstance(config.session_settings, _DeclaredSessionSettings)
+    assert config.session_settings.limit == 5
+    assert config.session_settings.tenant == "acme"
+
+
+def test_run_config_subclass_rejects_base_session_settings_instance() -> None:
+    with pytest.raises(
+        TypeError,
+        match="must be a _DeclaredSessionSettings instance or a dict",
+    ):
+        cast(Any, _DeclaredSessionRunConfig)(session_settings=SessionSettings(limit=5))
+
+
 def test_run_config_accepts_output_guardrail_blocked_message_customizers() -> None:
     def formatter(_args: OutputGuardrailBlockedMessageArgs[Any]) -> str:
         return "custom"