diff --git a/openhands-sdk/openhands/sdk/conversation/impl/local_conversation.py b/openhands-sdk/openhands/sdk/conversation/impl/local_conversation.py
index d07a9b836e..1260ac7d47 100644
--- a/openhands-sdk/openhands/sdk/conversation/impl/local_conversation.py
+++ b/openhands-sdk/openhands/sdk/conversation/impl/local_conversation.py
@@ -341,15 +341,17 @@ def fork(
             but has its own identity and independent state going forward.
         """
         fork_id = conversation_id or uuid.uuid4()
-        if agent is not None:
-            fork_agent = agent
-        else:
-            # Round-trip via JSON to produce a deep copy that avoids
-            # thread-lock pickling issues with model_copy(deep=True).
-            agent_cls = type(self.agent)
-            fork_agent = agent_cls.model_validate(
-                self.agent.model_dump(context={"expose_secrets": True}),
-            )
+        # Always deep-copy the agent (supplied or source) so the fork owns
+        # its own object graph. Required because __init__ mutates
+        # agent.llm._prompt_cache_key in place (#2917): a shared/aliased
+        # agent would clobber the source conversation's cache key.
+        # Round-trip via JSON avoids thread-lock pickling issues with
+        # model_copy(deep=True).
+        source_agent = agent if agent is not None else self.agent
+        agent_cls = type(source_agent)
+        fork_agent = agent_cls.model_validate(
+            source_agent.model_dump(context={"expose_secrets": True}),
+        )
 
         # Hold the state lock while reading mutable state from the source
         # conversation to avoid torn reads if run() is executing concurrently.
diff --git a/tests/sdk/conversation/local/test_fork.py b/tests/sdk/conversation/local/test_fork.py
index 34d379ae5a..cb1f448ece 100644
--- a/tests/sdk/conversation/local/test_fork.py
+++ b/tests/sdk/conversation/local/test_fork.py
@@ -250,3 +250,29 @@ def test_fork_persisted_events_survive_reload():
         resumed_ids = [e.id for e in resumed.state.events]
         assert evt_id_1 in resumed_ids
         assert evt_id_2 in resumed_ids
+
+
+def test_fork_default_does_not_clobber_source_cache_key():
+    """Default fork() must leave the source's prompt_cache_key intact (#2917)."""
+    with tempfile.TemporaryDirectory() as tmpdir:
+        src = Conversation(agent=_agent(), persistence_dir=tmpdir, workspace=tmpdir)
+        src_key_before = src.agent.llm._prompt_cache_key
+
+        fork = src.fork()
+
+        assert src.agent.llm._prompt_cache_key == src_key_before == str(src.id)
+        assert fork.agent.llm._prompt_cache_key == str(fork.id)
+        assert fork.agent.llm._prompt_cache_key != src.agent.llm._prompt_cache_key
+
+
+def test_fork_with_aliased_agent_does_not_clobber_source_cache_key():
+    """fork(agent=source.agent) must not repin the source LLM's cache key (#2917)."""
+    with tempfile.TemporaryDirectory() as tmpdir:
+        src = Conversation(agent=_agent(), persistence_dir=tmpdir, workspace=tmpdir)
+        src_key_before = src.agent.llm._prompt_cache_key
+
+        fork = src.fork(agent=src.agent)
+
+        assert src.agent.llm._prompt_cache_key == src_key_before == str(src.id)
+        assert fork.agent.llm._prompt_cache_key == str(fork.id)
+        assert fork.agent.llm is not src.agent.llm