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<@U06RKAMK0UC> I get `External sources are not supported in pyproject.toml` when I want to use Union's builder and I have pytorch in my pyproject.toml defined with `tool.uv.index`

*Reasoning Complete!* :robot_face: 

:mag: *Searched knowledge base*: I want to look up why you're seeing this error with Union's builder and how it relates to using external sources like pytorch in your pyproject.toml, so I can provide you with the most accurate solution.
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:writing_hand: *Generated answer*: I have found a clear explanation for the error and can provide you with the reason and available workarounds.

The error `External sources are not supported in pyproject.toml` occurs because Flytekit's ImageSpec builder (Union's builder) does not support the use of external package sources (such as custom indexes defined under `tool.uv.index` in `pyproject.toml`). The builder expects dependencies to be resolvable from standard sources and does not handle custom or external indexes specified in this way. This limitation is not explicitly documented in the main Flyte docs, but is implied by the error and the <https://github.com/flyteorg/flytekit/blob/master/flytekit/image_spec/default_builder.py|default builder's handling of lock files and requirements>.

To resolve this, use only standard PyPI dependencies in your `pyproject.toml` or requirements file, or specify extra indexes using the `pip_index` or `pip_extra_index_url` parameters in your ImageSpec, rather than through `tool.uv.index` in `pyproject.toml` (<https://github.com/flyteorg/flytekit/blob/master/flytekit/image_spec/default_builder.py|source>). Would you like more detail on how to configure your dependencies for Union's builder?

Sources:
- <https://github.com/flyteorg/flytekit/blob/master/flytekit/image_spec/default_builder.py|default builder's handling of lock files and requirements> :+1: 0 button :-1: 0 button :bulb: *Hint:* Mention @runllm in the thread for followups.