diff --git a/routes/history_routes.py b/routes/history_routes.py
index 378fab35f..59ed6674e 100644
--- a/routes/history_routes.py
+++ b/routes/history_routes.py
@@ -490,7 +490,13 @@ def setup_history_routes(session_manager) -> APIRouter:
             # Copy messages up to keep_count
             msgs_to_copy = source.history[:keep_count]
             for msg in msgs_to_copy:
-                new_session.add_message(ChatMessage(msg.role, msg.content, msg.metadata))
+                # Copy the metadata dict. Sharing it would let the fork's
+                # persistence (add_message -> _persist_message stamps
+                # _db_id/timestamp onto the dict) mutate the SOURCE session's
+                # in-memory messages, corrupting their _db_id and breaking
+                # edit/delete-by-id on the original conversation.
+                meta = dict(msg.metadata) if isinstance(msg.metadata, dict) else None
+                new_session.add_message(ChatMessage(msg.role, msg.content, meta))
             try:
                 from src.event_bus import fire_event
                 fire_event("session_created", getattr(source, 'owner', None))
diff --git a/tests/test_fork_session_metadata.py b/tests/test_fork_session_metadata.py
new file mode 100644
index 000000000..cd278da1d
--- /dev/null
+++ b/tests/test_fork_session_metadata.py
@@ -0,0 +1,84 @@
+"""Forking a session must not mutate the source session's messages.
+
+ChatMessage.metadata is a dict. add_message() -> _persist_message() stamps
+_db_id (and timestamp) onto that dict in place. The fork handler used to pass
+the source message's metadata dict by reference into the new session, so
+persisting the fork rewrote the SOURCE messages' _db_id — breaking
+edit/delete-by-id on the original conversation. The fork must copy the dict.
+"""
+import asyncio
+from types import SimpleNamespace
+
+from core.models import ChatMessage
+import routes.history_routes as mod
+
+
+class _FakeSession:
+    def __init__(self, name="", owner=None):
+        self.name = name
+        self.owner = owner
+        self.endpoint_url = ""
+        self.model = ""
+        self.history = []
+
+    def add_message(self, message):
+        # Mirror _persist_message: stamp the in-memory message's metadata.
+        if message.metadata is None:
+            message.metadata = {}
+        message.metadata["_db_id"] = f"new-{len(self.history)}"
+        self.history.append(message)
+
+
+class _FakeSessionManager:
+    def __init__(self, source):
+        self.sessions = {"src-id": source}
+        self.created = None
+
+    def create_session(self, session_id=None, name=None, endpoint_url=None,
+                       model=None, rag=False, owner=None):
+        self.created = _FakeSession(name=name, owner=owner)
+        return self.created
+
+    def save_sessions(self):
+        pass
+
+
+def _fork_handler(router):
+    for route in router.routes:
+        if "/fork" in getattr(route, "path", "") and "POST" in getattr(route, "methods", set()):
+            return route.endpoint
+    raise AssertionError("fork route not found")
+
+
+def test_fork_does_not_corrupt_source_message_metadata(monkeypatch):
+    monkeypatch.setattr(mod, "_verify_session_owner", lambda *a, **k: None)
+
+    source = _FakeSession(name="Original", owner="alice")
+    source.history = [
+        ChatMessage("user", "hi", {"_db_id": "src-0"}),
+        ChatMessage("assistant", "yo", {"_db_id": "src-1"}),
+    ]
+    sm = _FakeSessionManager(source)
+
+    req = SimpleNamespace()
+
+    async def _json():
+        return {"keep_count": 2}
+
+    req.json = _json
+
+    router = mod.setup_history_routes(sm)
+    fork = _fork_handler(router)
+    result = asyncio.run(fork(request=req, session_id="src-id"))
+
+    assert result["status"] == "ok"
+    assert result["kept"] == 2
+
+    # The forked session got its own metadata dicts...
+    new_session = sm.created
+    assert new_session.history[0].metadata is not source.history[0].metadata
+    assert new_session.history[1].metadata is not source.history[1].metadata
+
+    # ...and the source session's _db_id values are untouched.
+    assert source.history[0].metadata["_db_id"] == "src-0"
+    assert source.history[1].metadata["_db_id"] == "src-1"