Handle prompt variables

[marcjaner]

The Java client documentation shows how to fetch prompts but doesn't include examples or documentation on how to compile prompts with variable values after fetching them.

Current Behavior

The current documentation demonstrates fetching prompts:

import com.langfuse.client.LangfuseClient;
import com.langfuse.client.resources.prompts.types.PromptMetaListResponse;

LangfuseClient client = LangfuseClient.builder()
    .url("https://cloud.langfuse.com")
    .credentials("pk-lf-...", "sk-lf-...")
    .build();

PromptMetaListResponse prompts = client.prompts().list();

However, there's no documentation on how to pass variable values to compile the prompt template after fetching it.

Expected Behavior

Similar to the Python and JS/TS SDKs, I would expect the Java client to support compiling prompts with variables. For example, in Python you can do:

prompt = langfuse.get_prompt("prompt-name")
compiled = prompt.compile(variable_name="value")

Use Case

I need to fetch a prompt with placeholders (e.g., {{user_name}}, {{context}}) and compile it with actual values before sending it to the LLM.

Questions

• Does the Java client support prompt variable compilation?
• If yes, could you provide documentation or examples?
• If not, are there plans to add this feature?

[Steffen911]

@marcjaner As of now, the langfuse-java client is a pure implementation of our API routes with no additional logic. This includes that there is no support for prompt compilation. You'll need to do this yourself.

Right now, there are no plans to expand the feature set here unless they are being contributed by the community. Any patches will need to be compatible with the automatic code generation, though.

[RyanDeleurme]

What we ended up doing is this in our util class:

    public static String compilePrompt(@Nonnull String prompt, @Nonnull Map<String, String> params) {
        String compiledPrompt = prompt;
        for (Map.Entry<String, String> entry : params.entrySet()) {
            String placeholder = "{{" + entry.getKey() + "}}";
            String value = entry.getValue() != null ? entry.getValue() : "";

            compiledPrompt = compiledPrompt.replace(placeholder, value);
        }

        return compiledPrompt;
    }

I might look into adding a static method to the ChatPrompt or TextPrompt, but also adding the above code is pretty minimal into any codebase. If you do not care about the other prompt properties, there is potential to add a getAndCompile method as well...

[ChoMinGi]

@Steffen911 mind if I take a stab at this?

I've been using langfuse-java for an HR service and have a few subprojects that all fetch prompts from Langfuse — every one of them ended up with its own {{variable}} replacement hack, so would love to just have this in the SDK.

I'm thinking I'd add a utility outside the Fern-generated code so regeneration doesn't blow it away. Basically match what the Python/JS SDKs do with compile(), both text and chat prompts, with tests.

Does that approach work for you, or did you have something else in mind? Also wondering if variable validation (missing/extra vars) is in scope or if that's overkill for a first pass.

ps. English isn't my first language so feel free to let me know if anything's unclear.

[Steffen911]

@ChoMinGi If you find a good way to do this outside of the fern auto update path, feel free to take a shot at it.
Alternatively, we could simply add a section in the readme that contains the guidance and examples provided in this ticket.
Let me know what you think!

[ChoMinGi]

Thanks @Steffen911 I'd lean toward the utility class over a readme section -- our subprojects were originally on the Python SDK where compile() just works, so having the same experience in Java would be the ideal outcome. I've been prototyping something locally, I'll clean it up and open a draft PR so you can check the direction.