Feature Request: Partial tool version override in `mise_toml`

[timonrieger]

Use case: Running test suites with matrix strategies where one tool (e.g., Python) varies by job, while other tools (e.g., uv) should remain consistent.

Current behavior: The mise_toml input overwrites the entire mise.toml file, requiring duplication of all tool definitions.

Workaround (current): Using --env ci with a separate mise.ci.toml file:

# mise.ci.toml
[tools]
uv = "0.9.21"  # Constant tools only

- uses: jdx/mise-action@v3
  with:
    install_args: "python@${{ matrix.python-version }} --env ci"

This works because mise merges [tools] sections additively, so mise.ci.toml provides base tools and install-args adds the matrix-specific Python version. However, this requires maintaining a separate config file and is less intuitive than a direct override mechanism.

Desired behavior: Allow mise_toml to merge with or override only the specified tools in the repository's mise.toml, rather than replacing the entire configuration.

Example:

Repository mise.toml:

[tools]
python = "3.14.2"
uv = "0.9.21"

Current approach (requires duplication):

- uses: jdx/mise-action@v3
  with:
    mise_toml: |
      [tools]
      python = "${{ matrix.python-version }}"
      uv = "0.9.21"  # Must duplicate

Desired approach (inherit other tools):

- uses: jdx/mise-action@v3
  with:
    mise_toml: |
      [tools]
      python = "${{ matrix.python-version }}"
    # uv = "0.9.21" automatically inherited from repo mise.toml

Questions for maintainers:

1. Could mise_toml be implemented to merge with existing configs rather than overwrite?
2. Would a new input like mise_toml_override or mise_toml_merge: true be preferable?
3. Are there other patterns or ideas for handling partial tool overrides in CI workflows?

This would make matrix-based CI workflows cleaner and reduce maintenance burden when tool versions change.

[risu729]

For tool versions, the MISE_PYTHON_VERSION environment variable overrides the version in the configuration.

[timonrieger]

For tool versions, the MISE_PYTHON_VERSION environment variable overrides the version in the configuration.

Thanks!

The MISE_PYTHON_VERSION env var works, but not when combined with uv because uv reads .python-version and uses that to select the Python version for creating the virtual environment.

Solution: Set UV_PYTHON explicitly to override uv's python version selection.