Add `UV_INIT_BUILD_BACKEND` (#14821)

Closes https://github.com/astral-sh/uv/issues/14820

crates/uv-cli/src/lib.rs

@@ -2891,7 +2891,7 @@ pub struct InitArgs {
     /// Initialize a build-backend of choice for the project.
     ///
     /// Implicitly sets `--package`.
-    #[arg(long, value_enum, conflicts_with_all=["script", "no_package"])]
+    #[arg(long, value_enum, conflicts_with_all=["script", "no_package"], env = EnvVars::UV_INIT_BUILD_BACKEND)]
     pub build_backend: Option<ProjectBuildBackend>,
 
     /// Invalid option name for build backend.

crates/uv-static/src/env_vars.rs

@@ -257,6 +257,10 @@ impl EnvVars {
     /// Specifies the "bin" directory for installing tool executables.
     pub const UV_TOOL_BIN_DIR: &'static str = "UV_TOOL_BIN_DIR";
 
+    /// Equivalent to the `--build-backend` argument for `uv init`. Determines the default backend
+    /// to use when creating a new project.
+    pub const UV_INIT_BUILD_BACKEND: &'static str = "UV_INIT_BUILD_BACKEND";
+
     /// Specifies the path to the directory to use for a project virtual environment.
     ///
     /// See the [project documentation](../concepts/projects/config.md#project-environment-path)

docs/reference/cli.md

@@ -302,7 +302,7 @@ uv init [OPTIONS] [PATH]
 <p>Disables creating extra files like <code>README.md</code>, the <code>src/</code> tree, <code>.python-version</code> files, etc.</p>
 </dd><dt id="uv-init--build-backend"><a href="#uv-init--build-backend"><code>--build-backend</code></a> <i>build-backend</i></dt><dd><p>Initialize a build-backend of choice for the project.</p>
 <p>Implicitly sets <code>--package</code>.</p>
-<p>Possible values:</p>
+<p>May also be set with the <code>UV_INIT_BUILD_BACKEND</code> environment variable.</p><p>Possible values:</p>
 <ul>
 <li><code>hatch</code>:  Use <a href="https://pypi.org/project/hatchling">hatchling</a> as the project build backend</li>
 <li><code>flit</code>:  Use <a href="https://pypi.org/project/flit-core">flit-core</a> as the project build backend</li>

docs/reference/environment.md

@@ -147,6 +147,11 @@ Provides the HTTP Basic authentication username for a named index.
 The `name` parameter is the name of the index. For example, given an index named `foo`,
 the environment variable key would be `UV_INDEX_FOO_USERNAME`.
 
+### `UV_INIT_BUILD_BACKEND`
+
+Equivalent to the `--build-backend` argument for `uv init`. Determines the default backend
+to use when creating a new project.
+
 ### `UV_INSECURE_HOST`
 
 Equivalent to the `--allow-insecure-host` argument.