fix(process): preserve caller overrides during Windows environment normalization

src/process/snapshotEnvironment.test.ts

@@ -25,19 +25,24 @@ it("canonicalizes selected Windows variables without inheriting ambient values",
   expect(Object.isFrozen(snapshot)).toBe(true);
 });
 
-it("uses Node's first lexicographic Windows key even when it is undefined", () => {
-  expect(
-    snapshotEnvironment(
-      {
-        path: "lower",
-        Path: "mixed",
-        PATH: "upper",
-        ComSpec: "mixed-cmd",
-        COMSPEC: "upper-cmd",
-        Temp: "mixed-temp",
-        TEMP: undefined,
-      },
-      "win32",
-    ),
-  ).toEqual({ PATH: "upper", COMSPEC: "upper-cmd", TEMP: undefined });
+it("keeps the last caller value for Windows aliases, including undefined", () => {
+  const snapshot = snapshotEnvironment(
+    {
+      PATH: "upper",
+      path: "lower",
+      COMSPEC: "upper-cmd",
+      ComSpec: "mixed-cmd",
+      TEMP: "ambient-temp",
+      temp: undefined,
+    },
+    "win32",
+  );
+
+  expect(snapshot).toEqual({
+    COMSPEC: "mixed-cmd",
+    PATH: "lower",
+    TEMP: undefined,
+  });
+  expect(Object.keys(snapshot)).toEqual(["COMSPEC", "PATH", "TEMP"]);
+  expect(Object.isFrozen(snapshot)).toBe(true);
 });

src/process/snapshotEnvironment.ts

@@ -1,16 +1,16 @@
-/** Snapshot caller-selected variables using Node child-process key semantics. */
+/** Snapshot selected variables while preserving platform and caller precedence. */
 export const snapshotEnvironment = (
   environment: Readonly<NodeJS.ProcessEnv>,
   platform: NodeJS.Platform = process.platform,
 ): Readonly<NodeJS.ProcessEnv> => {
   if (platform !== "win32") return Object.freeze({ ...environment });
+  const selected = new Map<string, string | undefined>();
+  // Fold aliases in caller order so an explicit later override wins before
+  // this canonical environment crosses a Windows process boundary.
+  for (const [key, value] of Object.entries(environment))
+    selected.set(key.toUpperCase(), value);
   const snapshot: NodeJS.ProcessEnv = {};
-  // Node sorts keys lexicographically and forwards the first case-insensitive
-  // match on Windows. Canonical keys also make subsequent lookups deterministic.
-  for (const key of Object.keys(environment).sort()) {
-    const canonicalKey = key.toUpperCase();
-    if (!Object.hasOwn(snapshot, canonicalKey))
-      snapshot[canonicalKey] = environment[key];
-  }
+  for (const key of [...selected.keys()].sort())
+    snapshot[key] = selected.get(key);
   return Object.freeze(snapshot);
 };

src/setupEnvironment.test.ts

@@ -213,11 +213,30 @@ it.each([
   },
 );
 
-it("prioritizes overridden package runner environment variables over ambient keys with differing case on Windows", () => {
-  const environment = {
-    NPM_COMMAND: "run",
-    npm_command: "exec",
+it("registers the selected Windows package runner after canonicalizing environment aliases", async () => {
+  const environment = snapshotEnvironment(
+    {
+      NPM_COMMAND: "run",
+      npm_command: "exec",
+      NPM_CONFIG_USER_AGENT: "npm/10.9.0 node/v24.11.0 win32 x64",
+    },
+    "win32",
+  );
+  expect(environment).toEqual({
+    NPM_COMMAND: "exec",
     NPM_CONFIG_USER_AGENT: "npm/10.9.0 node/v24.11.0 win32 x64",
-  };
+  });
   expect(invokingPackageRunner(environment, "win32")).toBe("npx");
+
+  const host = systemSetupHost(
+    createDoctorHostFixture({ platform: "win32" }),
+    environment,
+  );
+  try {
+    expect(host.registrationCommand).toEqual(
+      packageRunnerRegistrationCommand("npx", "win32"),
+    );
+  } finally {
+    await host.close?.();
+  }
 });