Spring AI SystemPromptTemplate removes double curly braces {{...}} when rendering prompts

Description

When using SystemPromptTemplate in Spring AI, any usage of double curly braces like {{ this is my line }} gets removed entirely in the final output, instead of escaping to { this is my line } or remaining as-is.

Sample code

String template = "{{ this is my line }}";
SystemPromptTemplate promptTemplate = new SystemPromptTemplate(template);
String result = promptTemplate.create(Map.of()).getContents();
System.out.println(result);

Expected Output

{ this is my line }

Actual Output

this is my line

Comment From: sunyuhan1998

Hi @SaurabhDesaiCeloxis , the SystemPromptTemplate uses StringTemplate4 as the rendering engine by default. The default syntax in StringTemplate4 uses < and > as delimiters for expressions, and if {} is used inside <...>, it represents an anonymous sub-template. However, in SystemPromptTemplate, the default delimiters for expressions have been declared as { and } (instead of the default < and >).

Now let's look at your question: Why does the template {{ this is my line }} render to this is my line? This is because during rendering, the outer {} is treated as the expression delimiter, and the inner {} is interpreted as an anonymous sub-template, which results in outputting this is my line.

We can achieve the same behavior using native StringTemplate4 by using the default delimiters for expressions:

ST st = new ST("<{this is my line}>");
String rendered = st.render();
System.out.println(rendered); // this is my line

In Spring AI, to solve your issue, you simply need to escape with a backslash \:

String template = "\\{ this is my line \\}";
SystemPromptTemplate promptTemplate = new SystemPromptTemplate(template);
String result = promptTemplate.create(Map.of()).getContents();
System.out.println(result); // { this is my line }