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feat: add prompt variable compilation#28

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Steffen911 merged 2 commits intolangfuse:mainfrom
ChoMinGi:feat/prompt-compilation
Apr 23, 2026
Merged

feat: add prompt variable compilation#28
Steffen911 merged 2 commits intolangfuse:mainfrom
ChoMinGi:feat/prompt-compilation

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@ChoMinGi ChoMinGi commented Mar 23, 2026 •
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Summary

Adds PromptTemplate and PromptCompiler under com.langfuse.client.prompt — no changes to Fern-generated code.

PromptTemplate handles {{var}} replacement and variable extraction. PromptCompiler wraps it for TextPrompt and ChatPrompt types so the usage feels similar to the Python/JS SDKs.
Tests cover basic replacement, missing/extra vars, null handling, edge cases for both text and chat prompts.

Closes #13

Scope

  • Text prompt compilation
  • Chat prompt compilation (per-message variable substitution)
  • PlaceholderMessage expansion (inserting message lists at placeholder positions) — not included in this PR, planned as a follow-up

Test plan

  • 20 unit tests for PromptTemplate (basic replacement, missing/extra vars, null handling, whitespace, edge cases)
  • 5 unit tests for PromptCompiler (text prompt, chat prompt, placeholder skipping, empty prompt)

@ChoMinGi
feat: add prompt variable compilation
08b432a 
Add PromptTemplate and PromptCompiler utilities for {{variable}}
template compilation, matching the Python/JS SDK compile() behavior.
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CLAassistant commented Mar 23, 2026 •
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@ChoMinGi ChoMinGi marked this pull request as ready for review March 24, 2026 02:33
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Steffen911 commented Apr 22, 2026

@ChoMinGi Thank you a lot! Did you test whether a regeneration of the client using fern from the langfuse/langfuse repo would overwrite the additions you've made here? If not, could you test it and confirm that this doesn't lead to issues?

@Steffen911 Steffen911 self-requested a review April 22, 2026 14:36
@Steffen911 Steffen911 self-assigned this Apr 22, 2026
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ChoMinGi commented Apr 23, 2026

Hi @Steffen911, thanks for the review!

I confirmed that Fern regeneration does not affect the files added in this PR.
Changes are in com.langfuse.client.prompt, which is outside the Fern-generated paths (client/, client/core/, client/resources/) None of our files contain the auto-generated by Fern marker, so there's no overlap.
I also tested this locally by running the fernapi/fern-java-sdk:3.38.1 generator via Docker against the current IR — the prompt/ package was left untouched.

@Steffen911 Steffen911 merged commit 23ff4ca into langfuse:main Apr 23, 2026
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ChoMinGi commented Apr 23, 2026

Thanks for reviewing and merging this, @Steffen911!
Really appreciate you taking the time. Looking forward to contributing more to langfuse-java.

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Handle prompt variables

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