diff --git a/src/gradient_labs/_conversation_finish.py b/src/gradient_labs/_conversation_finish.py
index 19a1329..89e603e 100644
--- a/src/gradient_labs/_conversation_finish.py
+++ b/src/gradient_labs/_conversation_finish.py
@@ -17,6 +17,10 @@ class FinishParams:
     # reason optionally allows you to describe why this conversation is finishing.
     reason: Optional[str] = None
 
+    # reason_code optionally categorises why this conversation is finishing.
+    # Valid values are "customer-ended-chat" and "customer-unresponsive".
+    reason_code: Optional[str] = None
+
 
 def finish_conversation(
     *, client: HttpClient, conversation_id: str, params: FinishParams
@@ -26,6 +30,8 @@ def finish_conversation(
         body["timestamp"] = client.localize(params.timestamp)
     if params.reason:
         body["reason"] = params.reason
+    if params.reason_code:
+        body["reason_code"] = params.reason_code
 
     _ = client.put(
         f"conversations/{conversation_id}/finish",
diff --git a/tests/test_conversation_finish.py b/tests/test_conversation_finish.py
new file mode 100644
index 0000000..c0157da
--- /dev/null
+++ b/tests/test_conversation_finish.py
@@ -0,0 +1,56 @@
+from unittest.mock import MagicMock
+
+from gradient_labs import Client, FinishParams
+
+CONVERSATION_ID = "conv_01ham6bzcdeja9xzqhjf6daq30"
+
+
+def _client():
+    client = Client(api_key="test-key")
+    put = MagicMock(return_value=None)
+    client.http_client.put = put
+    return client, put
+
+
+def test_finish_conversation():
+    client, put = _client()
+
+    client.finish_conversation(
+        conversation_id=CONVERSATION_ID,
+        params=FinishParams(),
+    )
+
+    args, kwargs = put.call_args
+    assert args[0] == f"conversations/{CONVERSATION_ID}/finish"
+    assert kwargs["body"] == {}
+
+
+def test_finish_conversation_includes_reason_code():
+    client, put = _client()
+
+    client.finish_conversation(
+        conversation_id=CONVERSATION_ID,
+        params=FinishParams(
+            reason="Customer stopped replying",
+            reason_code="customer-unresponsive",
+        ),
+    )
+
+    _, kwargs = put.call_args
+    body = kwargs["body"]
+    assert body["reason"] == "Customer stopped replying"
+    assert body["reason_code"] == "customer-unresponsive"
+
+
+def test_finish_conversation_omits_reason_code_when_unset():
+    client, put = _client()
+
+    client.finish_conversation(
+        conversation_id=CONVERSATION_ID,
+        params=FinishParams(reason="Resolved"),
+    )
+
+    _, kwargs = put.call_args
+    body = kwargs["body"]
+    assert body["reason"] == "Resolved"
+    assert "reason_code" not in body