How to conditionally include a variable in a @UserMessage prompt in LangChain4j (skip if null)?

Question

I'm using LangChain4j to define prompts using @UserMessage annotations, like this:

@UserMessage( "The alert is described as follows: {{description}}\n\n" + "HTTP response evidence:\n---\n{{evidence}}\n---\n" + "Additional context:\n---\n{{otherinfo}}") Confidence review(@V("description") String description, @V("evidence") String evidence, @V("otherinfo") String otherinfo);

The problem: if otherinfo is null, the placeholder {{otherinfo}} is still rendered as the string "null" in the final prompt, which confuses the LLM. I want the otherinfo section to be excluded entirely if it's null or empty.

Is there a built-in way in LangChain4j to make prompt sections conditional based on variable values (e.g., skip {{otherinfo}} if it's null)? Or do I need to construct the prompt manually outside the annotation system?

Any advice or best practices appreciated.

Anmol Mishra · Accepted Answer · 2025-07-18 20:25:25Z

You're right to look for a cleaner approach here. As of now, @UserMessage in LangChain4j doesn’t support conditional inclusion of prompt sections based on null or empty values. If a variable like otherinfo is null, it will be rendered as the string "null", which can confuse the LLM and degrade prompt quality.

Recommended approach is -

StringBuilder prompt = new StringBuilder(); prompt.append("The alert is described as follows: ").append(description).append("\n\n") .append("HTTP response evidence:\n---\n").append(evidence).append("\n---\n"); if (otherinfo != null && !otherinfo.isBlank()) { prompt.append("Additional context:\n---\n").append(otherinfo); } String finalPrompt = prompt.toString();

This way the other info is included only if it contains the content.

If you're building more complex prompts, a templating engine like Mustache or Thymeleaf can handle conditionals cleanly.

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