From bde8b9b55422d7e4da81b9964438c01a418297f1 Mon Sep 17 00:00:00 2001
From: Dmitry Voropaev <dy.voropaev@gmail.com>
Date: Thu, 29 Oct 2026 20:12:13 +0300
Subject: [PATCH] fix(prompts): accept fetched chat messages in create_prompt
 and fallback

ChatPromptClient exposes chat messages with type="message", and
create_prompt is typed to accept that shape through
ChatMessageWithPlaceholdersDict. The generated API models only allow
"chatmessage" for that field, so passing a fetched prompt's .prompt to
create_prompt raised a ValidationError for CreateChatPromptRequest
before any request was sent. get_prompt failed the same way when such
a list was given as a chat fallback, raising inside the fetch error
path instead of returning the fallback.

Map type="message" to "chatmessage" before building the request and
the fallback Prompt_Chat. Placeholders and messages without a type key
are passed through unchanged, and the caller's list is not modified.
---
 langfuse/_client/client.py | 17 ++++++++--
 tests/unit/test_prompt.py  | 64 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 79 insertions(+), 2 deletions(-)

diff --git a/langfuse/_client/client.py b/langfuse/_client/client.py
index e926e4571..04ec2e4d9 100644
--- a/langfuse/_client/client.py
+++ b/langfuse/_client/client.py
@@ -173,6 +173,17 @@ def _serialize_evaluations(evaluations: List[Evaluation]) -> List[Dict[str, Any]
     ]
 
 
+def _to_api_chat_messages(messages: List[Any]) -> List[Any]:
+    # ChatPromptClient.prompt and ChatMessageWithPlaceholdersDict use
+    # type="message", but the API models only accept "chatmessage".
+    return [
+        {**message, "type": "chatmessage"}
+        if isinstance(message, dict) and message.get("type") == "message"
+        else message
+        for message in messages
+    ]
+
+
 class Langfuse:
     """Main client for Langfuse tracing and platform features.
 
@@ -3872,7 +3883,9 @@ def get_prompt(
 
                     fallback_client_args: Dict[str, Any] = {
                         "name": name,
-                        "prompt": fallback,
+                        "prompt": _to_api_chat_messages(fallback)
+                        if isinstance(fallback, list)
+                        else fallback,
                         "type": type,
                         "version": version or 0,
                         "config": {},
@@ -4074,7 +4087,7 @@ def create_prompt(
                 request: Union[CreateChatPromptRequest, CreateTextPromptRequest] = (
                     CreateChatPromptRequest(
                         name=name,
-                        prompt=cast(Any, prompt),
+                        prompt=_to_api_chat_messages(prompt),
                         labels=labels,
                         tags=tags,
                         config=config or {},
diff --git a/tests/unit/test_prompt.py b/tests/unit/test_prompt.py
index eadfb8221..d18570ba6 100644
--- a/tests/unit/test_prompt.py
+++ b/tests/unit/test_prompt.py
@@ -748,3 +748,67 @@ def test_get_fresh_prompt_when_version_changes(langfuse: Langfuse):
     result_call_2 = langfuse.get_prompt(prompt_name, version=2)
     assert mock_server_call.call_count == 2
     assert result_call_2 == version_changed_prompt_client
+
+
+def test_create_chat_prompt_from_fetched_chat_prompt(langfuse: Langfuse):
+    fetched = ChatPromptClient(
+        Prompt_Chat(
+            name="source",
+            version=1,
+            prompt=[
+                {"type": "chatmessage", "role": "system", "content": "Hi {{name}}"},
+                {"type": "placeholder", "name": "history"},
+            ],
+            labels=[],
+            type="chat",
+            config={},
+            tags=[],
+        )
+    )
+    assert fetched.prompt[0]["type"] == "message"
+
+    mock_server_call = langfuse.api.prompts.create
+    mock_server_call.return_value = Prompt_Chat(
+        name="copy",
+        version=1,
+        prompt=[
+            {"type": "chatmessage", "role": "system", "content": "Hi {{name}}"},
+            {"type": "placeholder", "name": "history"},
+        ],
+        labels=[],
+        type="chat",
+        config={},
+        tags=[],
+    )
+
+    result = langfuse.create_prompt(name="copy", prompt=fetched.prompt, type="chat")
+
+    request = mock_server_call.call_args.kwargs["request"]
+    assert request.dict()["prompt"] == [
+        {"type": "chatmessage", "role": "system", "content": "Hi {{name}}"},
+        {"type": "placeholder", "name": "history"},
+    ]
+    assert fetched.prompt[0]["type"] == "message"
+    assert result.prompt == fetched.prompt
+
+
+def test_get_chat_prompt_fallback_with_message_type(langfuse: Langfuse):
+    langfuse.api.prompts.get.side_effect = Exception("Prompt not found")
+    fallback = [
+        {"type": "message", "role": "system", "content": "Hi {{name}}"},
+        {"type": "placeholder", "name": "history"},
+    ]
+
+    result = langfuse.get_prompt(
+        "chat_fallback_with_message_type",
+        type="chat",
+        fallback=fallback,
+        max_retries=0,
+    )
+
+    assert result.is_fallback
+    assert result.prompt == fallback
+    assert result.compile(name="Bob", history=[{"role": "user", "content": "yo"}]) == [
+        {"role": "system", "content": "Hi Bob"},
+        {"role": "user", "content": "yo"},
+    ]