diff --git a/routes/history/history_routes.py b/routes/history/history_routes.py
index 617ff7ed7..308062b36 100644
--- a/routes/history/history_routes.py
+++ b/routes/history/history_routes.py
@@ -471,10 +471,16 @@ def setup_history_routes(session_manager, upload_handler=None) -> APIRouter:
         _verify_session_owner(request, session_id)
         try:
             body = await request.json()
+            if not isinstance(body, dict):
+                raise HTTPException(400, "Request body must be a JSON object")
             role = body.get("role", "assistant")
             content = body.get("content", "")
+            if role not in ("user", "assistant"):
+                raise HTTPException(400, "role must be 'user' or 'assistant'")
             if not content:
                 raise HTTPException(400, "content is required")
+            if not isinstance(content, str):
+                raise HTTPException(400, "content must be a string")
             metadata = sanitize_client_message_metadata(body.get("metadata"))
             _reserve_message_uploads(request, content, metadata)
             msg = ChatMessage(role=role, content=content, metadata=metadata)
diff --git a/tests/test_history_add_message_validation.py b/tests/test_history_add_message_validation.py
new file mode 100644
index 000000000..2e28492ad
--- /dev/null
+++ b/tests/test_history_add_message_validation.py
@@ -0,0 +1,62 @@
+import asyncio
+
+import pytest
+from fastapi import HTTPException
+
+from tests.helpers.database import isolated_session_database
+
+
+@pytest.fixture
+def manager_database(tmp_path):
+    with isolated_session_database(tmp_path) as resources:
+        yield resources
+
+
+def add_message(sm, monkeypatch, body):
+    import routes.history.history_routes as history
+
+    monkeypatch.setattr(history, '_verify_session_owner', lambda *args, **kwargs: None)
+    monkeypatch.setattr(history, 'effective_user', lambda request: 'u')
+    monkeypatch.setattr(history, 'reserve_message_upload_references', lambda *args, **kwargs: None)
+    route = next(r for r in history.setup_history_routes(sm).routes
+                 if r.path == '/api/session/{session_id}/message')
+
+    class Request:
+        async def json(self):
+            return body
+
+    return asyncio.run(route.endpoint(Request(), 'sid'))
+
+
+@pytest.mark.parametrize('body', [
+    {'role': 'bogus', 'content': 'x'},
+    {'role': 'system', 'content': 'x'},
+    {'role': 'user', 'content': 123},
+    {'content': ['a']},
+    {'role': ['user'], 'content': 'x'},
+    [1],
+])
+def test_bad_role_or_content_is_a_client_error(manager_database, monkeypatch, body):
+    sm, _ = manager_database
+    sm.create_session(session_id='sid', name='t', endpoint_url='x', model='m', rag=False, owner='u')
+
+    with pytest.raises(HTTPException) as exc:
+        add_message(sm, monkeypatch, body)
+
+    assert exc.value.status_code == 400
+    assert sm.get_session('sid').history == []
+
+
+@pytest.mark.parametrize('body, role', [
+    ({'role': 'user', 'content': 'hi'}, 'user'),
+    ({'role': 'assistant', 'content': 'hello'}, 'assistant'),
+    ({'content': 'default role'}, 'assistant'),
+])
+def test_user_and_assistant_text_are_stored(manager_database, monkeypatch, body, role):
+    sm, _ = manager_database
+    sm.create_session(session_id='sid', name='t', endpoint_url='x', model='m', rag=False, owner='u')
+
+    assert add_message(sm, monkeypatch, body) == {'status': 'ok'}
+
+    stored = sm.get_session('sid').history
+    assert [(m.role, m.content) for m in stored] == [(role, body['content'])]