diff --git a/tests/agents/test_reproducibility_agent.py b/tests/agents/test_reproducibility_agent.py
new file mode 100644
index 000000000..f8c18a145
--- /dev/null
+++ b/tests/agents/test_reproducibility_agent.py
@@ -0,0 +1,124 @@
+"""Unit tests for the trace replay logic of ReproAgent and ReproChatModel."""
+
+from dataclasses import dataclass, field
+
+import pytest
+from browsergym.experiments.agent import AgentInfo
+
+from agentlab.agents.generic_agent import reproducibility_agent
+from agentlab.agents.generic_agent.agent_configs import FLAGS_GPT_3_5
+from agentlab.agents.generic_agent.generic_agent import GenericAgent
+from agentlab.agents.generic_agent.reproducibility_agent import ReproAgent, ReproChatModel
+from agentlab.llm.chat_api import CheatMiniWoBLLMArgs
+from agentlab.llm.llm_utils import AIMessage, Discussion, HumanMessage, SystemMessage
+
+
+@dataclass
+class StubStepInfo:
+    agent_info: dict = field(default_factory=dict)
+    action: str = None
+
+
+@dataclass
+class StubExpResult:
+    step_info: StubStepInfo
+    summary_info: dict = field(default_factory=lambda: {"err_msg": "some error"})
+
+    def get_step_info(self, step: int) -> StubStepInfo:
+        return self.step_info
+
+
+def make_agent(monkeypatch, step_info: StubStepInfo) -> ReproAgent:
+    monkeypatch.setattr(
+        reproducibility_agent, "ExpResult", lambda repro_dir: StubExpResult(step_info)
+    )
+    return ReproAgent(
+        chat_model_args=CheatMiniWoBLLMArgs(),
+        flags=FLAGS_GPT_3_5,
+        repro_dir="unused",
+    )
+
+
+def patch_parent_get_action(monkeypatch):
+    """Replay a 2-message discussion through self.chat_llm, like GenericAgent does."""
+
+    def fake_get_action(self, obs):
+        messages = Discussion([SystemMessage("system"), HumanMessage("user")])
+        response = self.chat_llm(messages)
+        self.actions.append(response["content"])
+        return response["content"], AgentInfo(chat_messages=messages)
+
+    monkeypatch.setattr(GenericAgent, "get_action", fake_get_action)
+
+
+def recorded_discussion(answer: str = None) -> Discussion:
+    messages = [SystemMessage("system"), HumanMessage("user")]
+    if answer is not None:
+        messages.append(AIMessage(answer))
+    return Discussion(messages)
+
+
+def test_get_action_without_chat_messages(monkeypatch):
+    agent = make_agent(monkeypatch, StubStepInfo(agent_info={"chat_messages": None}))
+
+    action, agent_info = agent.get_action(obs={})
+
+    assert action is None
+    assert "Agent had no chat messages" in agent_info.markdown_page
+    assert "some error" in agent_info.markdown_page
+
+
+def test_get_action_rebuilds_missing_assistant_message(monkeypatch):
+    old_messages = recorded_discussion()
+    step_info = StubStepInfo(agent_info={"chat_messages": old_messages}, action='click("42")')
+    agent = make_agent(monkeypatch, step_info)
+    patch_parent_get_action(monkeypatch)
+
+    action, _ = agent.get_action(obs={})
+
+    assert len(old_messages) == 3
+    assert old_messages[2]["role"] == "assistant"
+    assert old_messages[2]["content"] == '<action>click("42")</action>'
+    assert isinstance(agent.chat_llm, ReproChatModel)
+    assert action == '<action>click("42")</action>'
+
+
+def test_get_action_without_recorded_action_keeps_messages(monkeypatch):
+    old_messages = recorded_discussion()
+    step_info = StubStepInfo(agent_info={"chat_messages": old_messages}, action=None)
+    agent = make_agent(monkeypatch, step_info)
+    patch_parent_get_action(monkeypatch)
+
+    action, _ = agent.get_action(obs={})
+
+    assert len(old_messages) == 2
+    assert action == "<action>None</action>"  # fallback of ReproChatModel
+
+
+def test_get_action_replays_recorded_answer(monkeypatch):
+    old_messages = recorded_discussion('<action>click("7")</action>')
+    step_info = StubStepInfo(agent_info={"chat_messages": old_messages}, action='click("7")')
+    agent = make_agent(monkeypatch, step_info)
+    patch_parent_get_action(monkeypatch)
+
+    action, agent_info = agent.get_action(obs={})
+
+    assert len(old_messages) == 3  # no reconstruction when the answer was recorded
+    assert action == '<action>click("7")</action>'
+    assert agent_info.stats["lines_added"] == 0
+    assert agent_info.stats["lines_removed"] == 0
+
+
+@pytest.mark.parametrize("n_new_messages", [2, 3])
+def test_repro_chat_model(n_new_messages):
+    old_messages = recorded_discussion("<action>noop()</action>")
+    chat_model = ReproChatModel(old_messages, delay=0)
+
+    response = chat_model(Discussion(list(old_messages)[:n_new_messages]))
+
+    if n_new_messages == 2:
+        assert response is old_messages[2]
+        assert len(chat_model.new_messages) == 3
+    else:  # the recorded answer is missing, fall back to a no-op action
+        assert response["content"] == "<action>None</action>"
+    assert chat_model.get_stats() == {}