Fix github issue 2441 selected call

[aaronvg]

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Issue Reference

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• This PR fixes/closes [bug] Collector: when using fallback strategy with 2 clients , and if one of the call is failed, the selected_call will be populated with failed call. #2441

Changes

Please describe the changes proposed in this pull request

This PR addresses an issue where selected_call could incorrectly mark a failed LLM response as selected. The logic for determining the selected call in engine/baml-runtime/src/tracingv2/storage/storage.rs now explicitly checks that the LLM response does not contain an error_message. A new unit test has been added to ensure that if multiple calls occur, a successful call is correctly prioritized as the selected_call over a failed one.

Testing

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• Unit tests added/updated (See test_selected_call_prefers_success_over_failure)
• Manual testing performed
• Tested in [environment]

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Additional Notes

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Tests were not executed in the environment due to missing cargo.
To run tests locally:

cargo test -p baml-runtime -q

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Project	Deployment	Preview	Comments	Updated (UTC)
promptfiddle	Skipped			Sep 13, 2025 11:52pm

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[aaronvg]

@cursoragent you should choose the last call in the stack using the timestamp, unless it failed then try to look for the one that passed. The http_request_id is ULID with a timestamp. see define_id!(..)

[cursor]

I refined the LLM call selection logic in build_function_log to ensure only one call is marked 'selected' per function log, prioritizing the latest successful call for improved trace clarity.

• I modified build_function_log in engine/baml-runtime/src/tracingv2/storage/storage.rs. I now collect all LLM call candidates, then apply a new algorithm: select the latest successful call; if the latest failed, select the most recent successful call.
• I added test_selected_call_chooses_earlier_success_if_last_failed to validate this new selection behavior.

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