fix(core): extract variables from inside Mustache sections #8580
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- Modified mustacheTemplateToNodes to recursively process nested content inside sections
- Fixes issue where variables inside conditional blocks (e.g., {{#isTrue}}{{myvar}}{{/isTrue}}) were not being extracted as input variables
- Updated test expectations to reflect the new comprehensive variable extraction behavior
- Removed trailing space from test template to match expected output
This ensures that variables like 'myvar' inside Mustache conditional sections are properly extracted as input variables when they are provided as top-level input values.
fixes langchain-ai#8456
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hntrl
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Jul 31, 2025
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This PR fixes a bug in Mustache template parsing where variables inside conditional sections were not being extracted as input variables.
Problem:
When using Mustache templates with conditional sections like
"Hey {{name}}, {{#isTrue}}{{myvar}}{{/isTrue}}", the variablemyvarinside the conditional block was not being recognized as an input variable. TheinputVariablesarray would only contain["name", "isTrue"]instead of the expected["name", "isTrue", "myvar"].Root Cause:
The
mustacheTemplateToNodesfunction inlangchain-core/src/prompts/template.tswas not recursively processing nested content inside Mustache sections. It only extracted the section variable (isTrue) but ignored variables inside the section (myvar).Solution:
mustacheTemplateToNodesto recursively process nested content inside sectionsImpact:
This fix ensures that variables inside Mustache conditional sections are properly extracted as input variables when they are provided as top-level input values, improving the robustness of Mustache template parsing.
Files Changed:
langchain-core/src/prompts/template.ts- Core fix for recursive variable extractionlangchain-core/src/prompts/tests/chat.mustache.test.ts- Updated test expectationslangchain-core/src/prompts/tests/prompt.mustache.test.ts- Updated test expectationsFixes #8456