Summary

Issue Description

I am experiencing an issue where uv fails to recognize an index defined in uv.toml when that index is explicitly referenced by a dependency in pyproject.toml. It appears that uv attempts to validate the dependency source in pyproject.toml before fully loading or merging the index configuration from uv.toml.

Use Case & Context

We use an internal package registry that must be set as explicit due to security requirements. We maintain two separate configuration files to handle deployment constraints:

• uv.toml: Used for local development (contains pypi index requiring internet connection).
• uv.prod.toml: Used for production (disconnected from the internet, renamed to uv.toml by CI/CD Pipeline).

We rely on uv.toml for the index definition because pyproject.toml does not allow us to easily swap configurations based on the environment (local vs. prod) without modifying the version-controlled project file itself.

Reproduction Steps

1. Define an internal index in uv.toml with a specific name and mark it as explicit.
2. Add a dependency in pyproject.toml using tool.uv.sources and explicitly assign it to the index name defined in step 1.
3. Run uv sync.

Example Configuration

uv.toml:

[[index]]
name = "gitlab"
url = "https://gitlab.example.com/api/v4/projects/1/packages/pypi/simple"
explicit = true

pyproject.toml:

[project]
name = "uv-bug-report"
version = "0.1.0"
requires-python = ">=3.13"
dependencies = [
    "my-custom-package >= 0.1.0"
]

[tool.uv.sources]
my-custom-package = { index = "gitlab" }

Console output

> uv sync
  x Failed to build `uv-bug-report @ file:///C:/Users/rothadam/PycharmProjects/uv-bug-report`
  |-> Failed to parse entry: `my-custom-package`
  `-> Package `my-custom-package` references an undeclared index: `gitlab`

Platform

Windows 11 x86_64 Enterprise 25H2 (build 26200.7840)

Version

uv 0.10.3 (c75a0c6 2026-02-16)

Python version

Python 3.13.5