Evaluate baml attribute jinja templates

[aaronvg]

Pull Request Template

Thanks for taking the time to fill out this pull request!

Issue Reference

Please link to any related issues

• This PR fixes/closes #[issue number]

Changes

This PR enables Jinja templating for @alias and @description attributes in BAML, allowing users to inject dynamic content (e.g., environment variables or simple expressions) into these attributes.

• Updated engine/baml-lib/baml-core/src/ir/walker.rs to resolve @alias and @description attribute strings using Jinja.
  • Introduced resolve_attribute_string to handle StringOr::Value as Jinja templates, StringOr::JinjaExpression for direct evaluation, and StringOr::EnvVar.
• Added vars_map() to engine/baml-lib/baml-types/src/value_expr.rs to expose environment variables to the Jinja rendering context.

Testing

Please describe how you tested these changes

• Unit tests added/updated
  • Added test_attribute_alias_jinja_expression_and_description_template in engine/baml-runtime/src/internal/prompt_renderer/render_output_format.rs.
• Manual testing performed
  • Tested locally using cargo test -p baml-runtime --lib.
• Tested in [environment]

Screenshots

If applicable, add screenshots to help explain your changes

[Add screenshots here...]

PR Checklist

Please ensure you've completed these items

• I have read and followed the contributing guidelines
• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

Additional Notes

To avoid building unrelated workspace members (and their Ruby dependencies), tests were run selectively using cargo test -p baml-runtime --lib.

[Add any additional notes here...]

---

Slack Thread

 

[cursor]

Cursor Agent can help with this pull request. Just @cursor in comments and I'll start working on changes in this branch.
_{Learn more about Cursor Agents}

[vercel]

The latest updates on your projects. Learn more about Vercel for GitHub.

1 Skipped Deployment

Project	Deployment	Preview	Comments	Updated (UTC)
promptfiddle	Skipped			Sep 17, 2025 10:41pm

[github-actions]

🌿 Preview your docs: https://boundary-preview-e08c3118-f7ee-48bf-84c5-3a16a644f256.docs.buildwithfern.com

[codecov]

Codecov Report

❌ Patch coverage is 95.77465% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
engine/baml-lib/baml-core/src/ir/walker.rs	90.32%	3 Missing ⚠️

📢 Thoughts on this report? Let us know!

[github-actions]

https://sage-backend-cp6dkyiv4-baml.vercel.app