Implement text file input for tests

[hellovai]

Pull Request Template

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Issue Reference

Please link to any related issues

• This PR addresses a frequently requested feature to allow string inputs for tests to be loaded from external files.

Changes

Please describe the changes proposed in this pull request

This PR introduces the ability to load string parameter values for BAML tests directly from external text files.

Key changes include:

• New Syntax: args { param { file "path/to/file.txt" } } is now supported for string type parameters.
• Relative Path Resolution: File paths are resolved relative to the BAML file containing the test.
• Union Precedence: Implemented left-to-right union precedence to correctly distinguish file references from struct properties named file, aligning with existing media type handling.
• Error Handling: Improved error messages for missing files or invalid properties.
• Documentation: Updated test.mdx to reflect the new functionality.

This feature is particularly useful for managing long text inputs, improving test organization, and reusing test data.

Testing

Please describe how you tested these changes

• Unit tests added/updated:
  • test_file_as_string: Basic file reading with an absolute path.
  • test_file_as_string_relative_path: Correct resolution of relative file paths.
  • test_file_as_string_missing_file: Proper error handling for non-existent files.
• Manual testing performed: Verified the new syntax with an integration test.
• Tested in local development environment

Screenshots

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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

Add any other context about the PR here

This change addresses the need for a more flexible way to provide string inputs to BAML tests, especially for large or frequently updated content. The approach leverages existing patterns for handling media files to ensure consistency and avoid ambiguity with struct definitions.

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Project	Deployment	Preview	Comments	Updated (UTC)
promptfiddle	Error			Dec 6, 2025 7:40pm

[github-actions]

🌿 Preview your docs: https://boundary-preview-3357c89f-f668-4b79-82f3-5c3562f09114.docs.buildwithfern.com

[github-actions]

https://sage-backend-4kg14t1n5-baml.vercel.app

[hellovai]

we now support template_strings in tests so closing out.