diff --git a/AGENTS.md b/AGENTS.md
index 4646a738..9864cadf 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -145,6 +145,10 @@ project:
 - Use `rstest` fixtures for shared setup.
 - Replace duplicated tests with `#[rstest(...)]` parameterised cases.
 - Prefer `mockall` for mocks/stubs.
+- Mock non-deterministic dependencies (e.g., environment variables and the
+  system clock) using dependency injection with the `mockable` crate (traits
+  like `Env` and `Clock`) where appropriate. See
+  `docs/reliable-testing-in-rust-via-dependency-injection.md` for guidance.
 - Prefer `.expect()` over `.unwrap()`.
 - Use `concat!()` to combine long string literals rather than escaping newlines
   with a backslash.
diff --git a/Cargo.lock b/Cargo.lock
index 26605112..d7f8b449 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -92,7 +92,7 @@ dependencies = [
  "bstr",
  "doc-comment",
  "libc",
- "predicates",
+ "predicates 3.1.3",
  "predicates-core",
  "predicates-tree",
  "wait-timeout",
@@ -106,7 +106,7 @@ checksum = "e539d3fca749fcee5236ab05e93a52867dd549cc157c8cb7f99595f3cedffdb5"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn",
+ "syn 2.0.104",
 ]
 
 [[package]]
@@ -206,7 +206,7 @@ dependencies = [
  "heck 0.5.0",
  "proc-macro2",
  "quote",
- "syn",
+ "syn 2.0.104",
 ]
 
 [[package]]
@@ -298,7 +298,7 @@ dependencies = [
  "globwalk",
  "humantime",
  "inventory",
- "itertools",
+ "itertools 0.12.1",
  "lazy-regex",
  "linked-hash-map",
  "once_cell",
@@ -316,11 +316,11 @@ checksum = "01091e28d1f566c8b31b67948399d2efd6c0a8f6228a9785519ed7b73f7f0aef"
 dependencies = [
  "cucumber-expressions",
  "inflections",
- "itertools",
+ "itertools 0.12.1",
  "proc-macro2",
  "quote",
  "regex",
- "syn",
+ "syn 2.0.104",
  "synthez",
 ]
 
@@ -346,7 +346,7 @@ checksum = "6edb4b64a43d977b8e99788fe3a04d483834fba1215a7e02caa415b626497f7f"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn",
+ "syn 2.0.104",
 ]
 
 [[package]]
@@ -371,6 +371,12 @@ version = "0.3.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "fea41bba32d969b513997752735605054bc0dfa92b4c56bf1189f2e174be7a10"
 
+[[package]]
+name = "downcast"
+version = "0.11.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1435fa1053d8b2fbbe9be7e97eca7f33d37b28409959813daefc1446a14247f1"
+
 [[package]]
 name = "drain_filter_polyfill"
 version = "0.1.3"
@@ -411,6 +417,21 @@ version = "2.3.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "37909eebbb50d72f9059c3b6d82c0463f2ff062c9e95845c43a6c9c0355411be"
 
+[[package]]
+name = "float-cmp"
+version = "0.9.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "98de4bbd547a563b716d8dfa9aad1cb19bfab00f4fa09a6a4ed21dbcf44ce9c4"
+dependencies = [
+ "num-traits",
+]
+
+[[package]]
+name = "fragile"
+version = "2.0.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "28dd6caf6059519a65843af8fe2a3ae298b14b80179855aeb4adc2c1934ee619"
+
 [[package]]
 name = "futures"
 version = "0.3.31"
@@ -467,7 +488,7 @@ checksum = "162ee34ebcb7c64a8abebc059ce0fee27c2262618d7b60ed8faf72fef13c3650"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn",
+ "syn 2.0.104",
 ]
 
 [[package]]
@@ -539,7 +560,7 @@ dependencies = [
  "quote",
  "serde",
  "serde_json",
- "syn",
+ "syn 2.0.104",
  "textwrap",
  "thiserror",
  "typed-builder",
@@ -675,6 +696,15 @@ version = "1.70.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "7943c866cc5cd64cbc25b2e01621d07fa8eb2a1a23160ee81ce38704e97b8ecf"
 
+[[package]]
+name = "itertools"
+version = "0.10.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b0fd2260e829bddf4cb6ea802289de2f86d6a7a690192fbe91b3f46e0f2c8473"
+dependencies = [
+ "either",
+]
+
 [[package]]
 name = "itertools"
 version = "0.12.1"
@@ -710,7 +740,7 @@ dependencies = [
  "proc-macro2",
  "quote",
  "regex",
- "syn",
+ "syn 2.0.104",
 ]
 
 [[package]]
@@ -804,6 +834,43 @@ dependencies = [
  "windows-sys 0.59.0",
 ]
 
+[[package]]
+name = "mockable"
+version = "0.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ce05993e13a317aef6ba5be77bb84b7e5b6bf67a7ba9a5168754c6a7aa3921da"
+dependencies = [
+ "mockall",
+ "tracing",
+]
+
+[[package]]
+name = "mockall"
+version = "0.11.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "4c84490118f2ee2d74570d114f3d0493cbf02790df303d2707606c3e14e07c96"
+dependencies = [
+ "cfg-if",
+ "downcast",
+ "fragile",
+ "lazy_static",
+ "mockall_derive",
+ "predicates 2.1.5",
+ "predicates-tree",
+]
+
+[[package]]
+name = "mockall_derive"
+version = "0.11.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "22ce75669015c4f47b289fd4d4f56e894e4c96003ffdf3ac51313126f94c6cbb"
+dependencies = [
+ "cfg-if",
+ "proc-macro2",
+ "quote",
+ "syn 1.0.109",
+]
+
 [[package]]
 name = "netsuke"
 version = "0.1.0"
@@ -813,9 +880,10 @@ dependencies = [
  "clap",
  "cucumber",
  "insta",
- "itertools",
+ "itertools 0.12.1",
  "itoa",
  "minijinja",
+ "mockable",
  "rstest",
  "semver",
  "serde",
@@ -852,6 +920,12 @@ dependencies = [
  "nom",
 ]
 
+[[package]]
+name = "normalize-line-endings"
+version = "0.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "61807f77802ff30975e01f4f071c8ba10c022052f98b3294119f3e615d13e5be"
+
 [[package]]
 name = "nu-ansi-term"
 version = "0.46.0"
@@ -862,6 +936,15 @@ dependencies = [
  "winapi",
 ]
 
+[[package]]
+name = "num-traits"
+version = "0.2.19"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "071dfc062690e90b734c0b2273ce72ad0ffa95f0c74596bc250dcfd960262841"
+dependencies = [
+ "autocfg",
+]
+
 [[package]]
 name = "object"
 version = "0.36.7"
@@ -956,7 +1039,7 @@ checksum = "6e918e4ff8c4549eb882f14b3a4bc8c8bc93de829416eacf579f1207a8fbf861"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn",
+ "syn 2.0.104",
 ]
 
 [[package]]
@@ -971,6 +1054,20 @@ version = "0.1.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "8b870d8c151b6f2fb93e84a13146138f05d02ed11c7e7c54f8826aaaf7c9f184"
 
+[[package]]
+name = "predicates"
+version = "2.1.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "59230a63c37f3e18569bdb90e4a89cbf5bf8b06fea0b84e65ea10cc4df47addd"
+dependencies = [
+ "difflib",
+ "float-cmp",
+ "itertools 0.10.5",
+ "normalize-line-endings",
+ "predicates-core",
+ "regex",
+]
+
 [[package]]
 name = "predicates"
 version = "3.1.3"
@@ -1097,7 +1194,7 @@ dependencies = [
  "regex",
  "relative-path",
  "rustc_version",
- "syn",
+ "syn 2.0.104",
  "unicode-ident",
 ]
 
@@ -1186,7 +1283,7 @@ dependencies = [
  "heck 0.4.1",
  "proc-macro2",
  "quote",
- "syn",
+ "syn 2.0.104",
 ]
 
 [[package]]
@@ -1215,7 +1312,7 @@ checksum = "5b0276cf7f2c73365f7157c8123c21cd9a50fbbd844757af28ca1f5925fc2a00"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn",
+ "syn 2.0.104",
 ]
 
 [[package]]
@@ -1278,7 +1375,7 @@ checksum = "5d69265a08751de7844521fd15003ae0a888e035773ba05695c5c759a6f89eef"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn",
+ "syn 2.0.104",
 ]
 
 [[package]]
@@ -1327,7 +1424,7 @@ checksum = "0eb01866308440fc64d6c44d9e86c5cc17adfe33c4d6eed55da9145044d0ffc1"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn",
+ "syn 2.0.104",
 ]
 
 [[package]]
@@ -1342,6 +1439,17 @@ version = "0.11.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "7da8b5736845d9f2fcb837ea5d9e2628564b3b043a70948a3f0b778838c5fb4f"
 
+[[package]]
+name = "syn"
+version = "1.0.109"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "72b64191b275b66ffe2469e8af2c1cfe3bafa67b529ead792a6d0160888b4237"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "unicode-ident",
+]
+
 [[package]]
 name = "syn"
 version = "2.0.104"
@@ -1359,7 +1467,7 @@ version = "0.3.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "a3d2c2202510a1e186e63e596d9318c91a8cbe85cd1a56a7be0c333e5f59ec8d"
 dependencies = [
- "syn",
+ "syn 2.0.104",
  "synthez-codegen",
  "synthez-core",
 ]
@@ -1370,7 +1478,7 @@ version = "0.3.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "f724aa6d44b7162f3158a57bccd871a77b39a4aef737e01bcdff41f4772c7746"
 dependencies = [
- "syn",
+ "syn 2.0.104",
  "synthez-core",
 ]
 
@@ -1383,7 +1491,7 @@ dependencies = [
  "proc-macro2",
  "quote",
  "sealed",
- "syn",
+ "syn 2.0.104",
 ]
 
 [[package]]
@@ -1443,7 +1551,7 @@ checksum = "4fee6c4efc90059e10f81e6d42c60a18f76588c3d74cb83a0b242a2b6c7504c1"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn",
+ "syn 2.0.104",
 ]
 
 [[package]]
@@ -1478,7 +1586,7 @@ checksum = "6e06d43f1345a3bcd39f6a56dbb7dcab2ba47e68e8ac134855e7e2bdbaf8cab8"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn",
+ "syn 2.0.104",
 ]
 
 [[package]]
@@ -1500,7 +1608,7 @@ checksum = "81383ab64e72a7a8b8e13130c49e3dab29def6d0c7d76a03087b3cf71c5c6903"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn",
+ "syn 2.0.104",
 ]
 
 [[package]]
@@ -1555,7 +1663,7 @@ checksum = "29a3151c41d0b13e3d011f98adc24434560ef06673a155a6c7f66b9879eecce2"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn",
+ "syn 2.0.104",
 ]
 
 [[package]]
diff --git a/Cargo.toml b/Cargo.toml
index a5a97648..048b393d 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -65,8 +65,9 @@ rstest = "0.18.0"
 cucumber = "0.20.0"
 tokio = { version = "1", features = ["macros", "rt-multi-thread"], default-features = false }
 insta = { version = "1", features = ["yaml"] }
-serial_test = "3"
 assert_cmd = "2.0.17"
+mockable = { version = "0.3", features = ["mock"] }
+serial_test = "3"
 
 [[test]]
 name = "cucumber"
diff --git a/src/runner.rs b/src/runner.rs
index ff170246..197c4968 100644
--- a/src/runner.rs
+++ b/src/runner.rs
@@ -21,6 +21,34 @@ pub const NINJA_PROGRAM: &str = "ninja";
 /// Environment variable override for the Ninja executable.
 pub const NINJA_ENV: &str = "NETSUKE_NINJA";
 
+// Public helpers for doctests only. This exposes internal helpers as a stable
+// testing surface without exporting them in release builds.
+#[doc(hidden)]
+pub mod doc {
+    #[allow(unused_imports, reason = "doctest-only wrapper module")]
+    use super::*;
+
+    // Public wrappers to expose crate-private helpers to doctests.
+    #[must_use]
+    pub fn contains_sensitive_keyword(arg: &CommandArg) -> bool {
+        super::contains_sensitive_keyword(arg)
+    }
+    #[must_use]
+    pub fn is_sensitive_arg(arg: &CommandArg) -> bool {
+        super::is_sensitive_arg(arg)
+    }
+    #[must_use]
+    pub fn redact_argument(arg: &CommandArg) -> CommandArg {
+        super::redact_argument(arg)
+    }
+    #[must_use]
+    pub fn redact_sensitive_args(args: &[CommandArg]) -> Vec<CommandArg> {
+        super::redact_sensitive_args(args)
+    }
+
+    pub use super::CommandArg;
+}
+
 #[derive(Debug, Clone)]
 pub struct NinjaContent(String);
 impl NinjaContent {
@@ -163,6 +191,12 @@ fn create_temp_ninja_file(content: &NinjaContent) -> Result<NamedTempFile> {
 /// write_ninja_file(Path::new("out.ninja"), &content).unwrap();
 /// ```
 fn write_ninja_file(path: &Path, content: &NinjaContent) -> Result<()> {
+    // Ensure the parent directory exists; guard against empty components so we
+    // do not attempt to create the current directory on some platforms.
+    if let Some(parent) = path.parent().filter(|p| !p.as_os_str().is_empty()) {
+        fs::create_dir_all(parent)
+            .with_context(|| format!("failed to create parent directory {}", parent.display()))?;
+    }
     fs::write(path, content.as_str())
         .with_context(|| format!("failed to write Ninja file to {}", path.display()))?;
     info!("Generated Ninja file at {}", path.display());
@@ -225,10 +259,11 @@ fn resolve_ninja_program() -> PathBuf {
 ///
 /// # Examples
 /// ```
+/// # use netsuke::runner::doc::{CommandArg, contains_sensitive_keyword};
 /// assert!(contains_sensitive_keyword(&CommandArg::new("token=abc".into())));
 /// assert!(!contains_sensitive_keyword(&CommandArg::new("path=/tmp".into())));
 /// ```
-fn contains_sensitive_keyword(arg: &CommandArg) -> bool {
+pub(crate) fn contains_sensitive_keyword(arg: &CommandArg) -> bool {
     let lower = arg.as_str().to_lowercase();
     lower.contains("password") || lower.contains("token") || lower.contains("secret")
 }
@@ -237,10 +272,11 @@ fn contains_sensitive_keyword(arg: &CommandArg) -> bool {
 ///
 /// # Examples
 /// ```
+/// # use netsuke::runner::doc::{CommandArg, is_sensitive_arg};
 /// assert!(is_sensitive_arg(&CommandArg::new("password=123".into())));
 /// assert!(!is_sensitive_arg(&CommandArg::new("file=readme".into())));
 /// ```
-fn is_sensitive_arg(arg: &CommandArg) -> bool {
+pub(crate) fn is_sensitive_arg(arg: &CommandArg) -> bool {
     contains_sensitive_keyword(arg)
 }
 
@@ -250,12 +286,13 @@ fn is_sensitive_arg(arg: &CommandArg) -> bool {
 ///
 /// # Examples
 /// ```
+/// # use netsuke::runner::doc::{CommandArg, redact_argument};
 /// let arg = CommandArg::new("token=abc".into());
 /// assert_eq!(redact_argument(&arg).as_str(), "token=***REDACTED***");
 /// let arg = CommandArg::new("path=/tmp".into());
 /// assert_eq!(redact_argument(&arg).as_str(), "path=/tmp");
 /// ```
-fn redact_argument(arg: &CommandArg) -> CommandArg {
+pub(crate) fn redact_argument(arg: &CommandArg) -> CommandArg {
     if is_sensitive_arg(arg) {
         let redacted = arg.as_str().split_once('=').map_or_else(
             || "***REDACTED***".to_string(),
@@ -271,6 +308,7 @@ fn redact_argument(arg: &CommandArg) -> CommandArg {
 ///
 /// # Examples
 /// ```
+/// # use netsuke::runner::doc::{CommandArg, redact_sensitive_args};
 /// let args = vec![
 ///     CommandArg::new("ninja".into()),
 ///     CommandArg::new("token=abc".into()),
@@ -278,7 +316,7 @@ fn redact_argument(arg: &CommandArg) -> CommandArg {
 /// let redacted = redact_sensitive_args(&args);
 /// assert_eq!(redacted[1].as_str(), "token=***REDACTED***");
 /// ```
-fn redact_sensitive_args(args: &[CommandArg]) -> Vec<CommandArg> {
+pub(crate) fn redact_sensitive_args(args: &[CommandArg]) -> Vec<CommandArg> {
     args.iter().map(redact_argument).collect()
 }
 
diff --git a/tests/runner_tests.rs b/tests/runner_tests.rs
index 5d4ace19..b2a5b69b 100644
--- a/tests/runner_tests.rs
+++ b/tests/runner_tests.rs
@@ -1,11 +1,84 @@
 use netsuke::cli::{BuildArgs, Cli, Commands};
 use netsuke::runner::{BuildTargets, NINJA_ENV, run, run_ninja};
-use rstest::rstest;
+use rstest::{fixture, rstest};
 use serial_test::serial;
+use std::ffi::OsString;
 use std::path::{Path, PathBuf};
 
 mod support;
 
+/// Guard that restores PATH to its original value when dropped.
+///
+/// Using a simple guard avoids heap allocation and guarantees teardown on
+/// early returns or panics.
+struct PathGuard {
+    original: OsString,
+}
+
+impl PathGuard {
+    fn new(original: OsString) -> Self {
+        Self { original }
+    }
+}
+
+impl Drop for PathGuard {
+    fn drop(&mut self) {
+        // Nightly marks set_var unsafe.
+        unsafe { std::env::set_var("PATH", &self.original) };
+    }
+}
+
+/// Fixture: Put a fake `ninja` (that checks for a build file) on PATH.
+///
+/// Returns: (tempdir holding ninja, `ninja_path`, PATH guard)
+#[fixture]
+fn ninja_in_path() -> (tempfile::TempDir, PathBuf, PathGuard) {
+    let (ninja_dir, ninja_path) = support::fake_ninja_check_build_file();
+
+    // Save PATH and prepend our fake ninja directory.
+    let original_path = std::env::var_os("PATH").unwrap_or_default();
+    let mut paths: Vec<_> = std::env::split_paths(&original_path).collect();
+    paths.insert(0, ninja_dir.path().to_path_buf());
+    let new_path = std::env::join_paths(paths).expect("join paths");
+    // Nightly marks set_var unsafe.
+    unsafe { std::env::set_var("PATH", &new_path) };
+
+    let guard = PathGuard::new(original_path);
+    (ninja_dir, ninja_path, guard)
+}
+
+/// Fixture: Put a fake `ninja` with a specific exit code on PATH.
+///
+/// The default exit code is 0, but can be customised via `#[with(...)]`.
+///
+/// Returns: (tempdir holding ninja, `ninja_path`, PATH guard)
+#[fixture]
+fn ninja_with_exit_code(#[default(0)] exit_code: i32) -> (tempfile::TempDir, PathBuf, PathGuard) {
+    let (ninja_dir, ninja_path) = support::fake_ninja(exit_code);
+
+    // Save PATH and prepend our fake ninja directory.
+    let original_path = std::env::var_os("PATH").unwrap_or_default();
+    let mut paths: Vec<_> = std::env::split_paths(&original_path).collect();
+    paths.insert(0, ninja_dir.path().to_path_buf());
+    let new_path = std::env::join_paths(paths).expect("join paths");
+    // Nightly marks set_var unsafe.
+    unsafe { std::env::set_var("PATH", &new_path) };
+
+    let guard = PathGuard::new(original_path);
+    (ninja_dir, ninja_path, guard)
+}
+
+/// Fixture: Create a temporary project with a Netsukefile from minimal.yml.
+///
+/// Returns: (tempdir for project, path to Netsukefile)
+#[fixture]
+fn test_manifest() -> (tempfile::TempDir, PathBuf) {
+    let temp = tempfile::tempdir().expect("temp dir");
+    let manifest_path = temp.path().join("Netsukefile");
+    std::fs::copy("tests/data/minimal.yml", &manifest_path).expect("copy manifest");
+    (temp, manifest_path)
+}
+
 #[test]
 fn run_exits_with_manifest_error_on_invalid_version() {
     let temp = tempfile::tempdir().expect("temp dir");
@@ -54,18 +127,8 @@ fn run_ninja_not_found() {
 #[rstest]
 #[serial]
 fn run_executes_ninja_without_persisting_file() {
-    let (ninja_dir, ninja_path) = support::fake_ninja_check_build_file();
-    let original_path = std::env::var_os("PATH").unwrap_or_default();
-    let mut paths: Vec<_> = std::env::split_paths(&original_path).collect();
-    paths.insert(0, ninja_dir.path().to_path_buf());
-    let new_path = std::env::join_paths(paths).expect("join paths");
-    unsafe {
-        std::env::set_var("PATH", &new_path);
-    } // Nightly marks set_var unsafe.
-
-    let temp = tempfile::tempdir().expect("temp dir");
-    let manifest_path = temp.path().join("Netsukefile");
-    std::fs::copy("tests/data/minimal.yml", &manifest_path).expect("copy manifest");
+    let (_ninja_dir, ninja_path, _guard) = ninja_in_path();
+    let (temp, manifest_path) = test_manifest();
     let cli = Cli {
         file: manifest_path.clone(),
         directory: Some(temp.path().to_path_buf()),
@@ -83,28 +146,16 @@ fn run_executes_ninja_without_persisting_file() {
     // Ensure no ninja file remains in project directory
     assert!(!temp.path().join("build.ninja").exists());
 
-    unsafe {
-        std::env::set_var("PATH", original_path);
-    } // Nightly marks set_var unsafe.
+    // Drop the fake ninja artifacts. PATH is restored by guard drop.
     drop(ninja_path);
 }
 
 #[cfg(unix)]
-#[test]
 #[serial]
+#[rstest]
 fn run_build_with_emit_keeps_file() {
-    let (ninja_dir, ninja_path) = support::fake_ninja_check_build_file();
-    let original_path = std::env::var_os("PATH").unwrap_or_default();
-    let mut paths: Vec<_> = std::env::split_paths(&original_path).collect();
-    paths.insert(0, ninja_dir.path().to_path_buf());
-    let new_path = std::env::join_paths(paths).expect("join paths");
-    unsafe {
-        std::env::set_var("PATH", &new_path);
-    }
-
-    let temp = tempfile::tempdir().expect("temp dir");
-    let manifest_path = temp.path().join("Netsukefile");
-    std::fs::copy("tests/data/minimal.yml", &manifest_path).expect("copy manifest");
+    let (_ninja_dir, ninja_path, _guard) = ninja_in_path();
+    let (temp, manifest_path) = test_manifest();
     let emit_path = temp.path().join("emitted.ninja");
     let cli = Cli {
         file: manifest_path.clone(),
@@ -126,20 +177,41 @@ fn run_build_with_emit_keeps_file() {
     assert!(emitted.contains("build "));
     assert!(!temp.path().join("build.ninja").exists());
 
-    unsafe {
-        std::env::set_var("PATH", original_path);
-    }
+    // Drop the fake ninja artifacts. PATH is restored by guard drop.
     drop(ninja_path);
 }
 
-#[test]
+#[cfg(unix)]
 #[serial]
-fn run_manifest_subcommand_writes_file() {
-    let original_path = std::env::var_os("PATH").unwrap_or_default();
-    unsafe {
-        std::env::set_var("PATH", "");
-    }
+#[rstest]
+fn run_build_with_emit_creates_parent_dirs() {
+    let (_ninja_dir, ninja_path, _guard) = ninja_with_exit_code(0);
+    let (temp, manifest_path) = test_manifest();
+    let nested_dir = temp.path().join("nested").join("dir");
+    let emit_path = nested_dir.join("emitted.ninja");
+    assert!(!nested_dir.exists());
+    let cli = Cli {
+        file: manifest_path.clone(),
+        directory: Some(temp.path().to_path_buf()),
+        jobs: None,
+        verbose: false,
+        command: Some(Commands::Build(BuildArgs {
+            emit: Some(emit_path.clone()),
+            targets: vec![],
+        })),
+    };
 
+    let result = run(&cli);
+    assert!(result.is_ok());
+    assert!(emit_path.exists());
+    assert!(nested_dir.exists());
+
+    // Drop the fake ninja artifacts. PATH is restored by guard drop.
+    drop(ninja_path);
+}
+
+#[test]
+fn run_manifest_subcommand_writes_file() {
     let temp = tempfile::tempdir().expect("temp dir");
     let manifest_path = temp.path().join("Netsukefile");
     std::fs::copy("tests/data/minimal.yml", &manifest_path).expect("copy manifest");
@@ -158,10 +230,6 @@ fn run_manifest_subcommand_writes_file() {
     assert!(result.is_ok());
     assert!(output_path.exists());
     assert!(!temp.path().join("build.ninja").exists());
-
-    unsafe {
-        std::env::set_var("PATH", original_path);
-    }
 }
 
 #[test]
diff --git a/tests/support/mod.rs b/tests/support/mod.rs
index 443d7221..8a7ed244 100644
--- a/tests/support/mod.rs
+++ b/tests/support/mod.rs
@@ -3,9 +3,10 @@
 //! This module provides helpers for creating fake executables and
 //! generating minimal manifests used in behavioural tests.
 
+use mockable::MockEnv;
 use std::fs::{self, File};
 use std::io::{self, Write};
-use std::path::PathBuf;
+use std::path::{Path, PathBuf};
 use std::sync::{Arc, Mutex};
 use tempfile::TempDir;
 use tracing::Level;
@@ -34,6 +35,48 @@ pub fn fake_ninja(exit_code: i32) -> (TempDir, PathBuf) {
     (dir, path)
 }
 
+/// Set up `env` so `PATH` resolves only to `dir`.
+///
+/// # Examples
+/// ```ignore
+/// let (dir, _) = fake_ninja(0);
+/// let mut env = MockEnv::new();
+/// mock_path_to(&mut env, dir.path());
+/// ```
+#[allow(
+    unfulfilled_lint_expectations,
+    reason = "used only in some test crates"
+)]
+#[expect(dead_code, reason = "used in PATH tests")]
+/// Build a valid `PATH` string that contains exactly one entry pointing to
+/// `dir` and configure the mock to return it. This avoids lossy conversions
+/// and makes the UTF-8 requirement explicit to callers.
+///
+/// Note: `MockEnv::raw` returns a `String`, so callers must accept UTF-8. This
+/// helper returns an error if the constructed `PATH` cannot be represented as
+/// UTF-8.
+pub fn mock_path_to(env: &mut MockEnv, dir: &Path) -> std::io::Result<()> {
+    // Join using the platform-appropriate separator while ensuring exactly one
+    // element is present in the PATH value.
+    let joined = std::env::join_paths([dir.as_os_str()])
+        .map_err(|e| std::io::Error::new(std::io::ErrorKind::InvalidInput, e))?;
+
+    // `MockEnv::raw` expects a `String`. Propagate if the single-entry PATH is
+    // not valid UTF-8 to keep the contract explicit.
+    let path = joined.into_string().map_err(|os| {
+        std::io::Error::new(
+            std::io::ErrorKind::InvalidData,
+            format!("non-UTF-8 PATH entry: {}", os.to_string_lossy()),
+        )
+    })?;
+
+    env.expect_raw()
+        .withf(|key| key == "PATH")
+        .returning(move |_| Ok(path.clone()));
+
+    Ok(())
+}
+
 /// Create a fake Ninja that validates the build file path provided via `-f`.
 ///
 /// The script exits with status `1` if the file is missing or not a regular