Summary

• Handle chat prompt messages that omit the type field (chatmessage / placeholder) in API responses.
• Add a custom deserializer to infer message type from fields (role/content → chatmessage, name → placeholder).
• Preserve existing behavior for explicit type values and unknown payloads.

Context

Langfuse API returns chat prompt messages with {role, content} but without the type field.
The SDK expects type for ChatMessageWithPlaceholders and falls back to _UnknownValue, which makes isChatmessage() false and causes chat prompt content to be lost.
Using JsonTypeInfo.Id.DEDUCTION does not work because chatmessage and placeholder are structurally ambiguous for Jackson deduction.

Changes

• ChatMessageWithPlaceholders now uses a custom Jackson deserializer:
  • If type is present, parse by type.
  • If role/content exists, parse as ChatMessage.
  • If name exists, parse as PlaceholderMessage.
  • Otherwise, fall back to _UnknownValue with raw payload.

Files

• src/main/java/com/langfuse/client/resources/prompts/types/ChatMessageWithPlaceholders.java

Test Plan

• Built locally with mvn -DskipTests install.
• Verified with a real prompt response: chat prompt messages deserialize correctly and getChat().getPrompt() contains system/user messages.

Curl Example

Example (sanitized)

curl 'https://us.cloud.langfuse.com/api/public/v2/prompts/my-classify-prompt?label=dev' \
-H 'Authorization: Basic cxxxx==' \
-H 'X-Fern-Language: JAVA' \
-H 'X-Fern-SDK-Name: com.langfuse.fern:langfuse-sdk' \
-H 'X-Fern-SDK-Version: 0.0.2841' \
-H 'Accept: application/json' \
-H 'Host: us.cloud.langfuse.com' \
-H 'Connection: Keep-Alive' \
-H 'User-Agent: okhttp/4.11.0' \
-H 'Content-Type: application/json' \
--proxy http://localhost:9090

{
  "id": "069af3bc-xxx",
  "createdAt": "2026-01-08T05:53:05.355Z",
  "updatedAt": "2026-01-08T05:53:11.482Z",
  "projectId": "xxx",
  "createdBy": "xxxx",
  "prompt": [
    {
      "role": "system",
      "content": "- Role:..."
    },
    {
      "role": "user",
      "content": "my name is {name}..."
    }
  ],
  "name": "my-classify-prompt",
  "version": 4,
  "type": "chat",
  "isActive": true,
  "config": {},
  "tags": [],
  "labels": [
    "production",
    "latest",
    "dev",
    "test"
  ],
  "commitMessage": "first ",
  "resolutionGraph": null
}