diff --git a/src/harperLifecycle.ts b/src/harperLifecycle.ts
index e88c911..23952bd 100644
--- a/src/harperLifecycle.ts
+++ b/src/harperLifecycle.ts
@@ -143,6 +143,28 @@ export interface StartHarperOptions {
 	harperBinPath?: string;
 }
 
+/**
+ * Build the environment for the spawned Harper child process. Exported for testing.
+ *
+ * `HOME`/`USERPROFILE` are applied LAST so the dataRootDir isolation always takes precedence over
+ * caller-supplied `env` (and over a spread of `process.env`, which contains HOME) — otherwise a caller could
+ * clobber it and re-expose the developer's real home. The isolation keeps Harper's global boot pointer
+ * (`$HOME/.harperdb/hdb_boot_properties.file`) and generated license keys inside the throwaway dataRootDir:
+ * cleaned up with it, never touching the real home, and isolated across concurrent suites. On first start
+ * Harper records its rootPath in that global boot file and — with an explicit `--ROOTPATH` — never overwrites
+ * it again; teardown removes only dataRootDir, so without this isolation the developer's real `~/.harperdb`
+ * would be left pointing at a since-deleted temp install, silently breaking the next `harper dev`/`harper run`
+ * anywhere on the machine.
+ */
+export function buildHarperChildEnv(dataRootDir: string, config: any, env?: any): Record<string, string | undefined> {
+	return {
+		HARPER_SET_CONFIG: JSON.stringify(config),
+		...env,
+		HOME: dataRootDir,
+		USERPROFILE: dataRootDir,
+	};
+}
+
 export interface HarperContext {
 	/** Absolute path to the Harper installation directory */
 	dataRootDir: string;
@@ -552,12 +574,10 @@ export async function startHarper(ctx: HarperTestContext, options?: StartHarperO
 		args.push(`--HTTP_SECUREPORT=${loopbackAddress}:${HTTPS_PORT}`);
 	}
 
-	// HARPER_SET_CONFIG must be passed as an environment variable, not a CLI arg,
-	// because applyRuntimeEnvVarConfig reads from process.env.HARPER_SET_CONFIG
-	const harperEnv = {
-		HARPER_SET_CONFIG: JSON.stringify(config),
-		...options?.env,
-	};
+	// HARPER_SET_CONFIG must be passed as an environment variable, not a CLI arg, because
+	// applyRuntimeEnvVarConfig reads from process.env.HARPER_SET_CONFIG. buildHarperChildEnv also isolates the
+	// child's HOME into dataRootDir so Harper's global boot pointer never lands in the developer's real home.
+	const harperEnv = buildHarperChildEnv(dataRootDir, config, options?.env);
 
 	const result = await runHarperCommand({
 		args,
diff --git a/test/harperLifecycle.test.ts b/test/harperLifecycle.test.ts
index 3dec57f..a710dcd 100644
--- a/test/harperLifecycle.test.ts
+++ b/test/harperLifecycle.test.ts
@@ -6,7 +6,13 @@ import { mkdtempSync, writeFileSync, rmSync } from 'node:fs';
 import { tmpdir } from 'node:os';
 import { join } from 'node:path';
 import { setTimeout as sleep } from 'node:timers/promises';
-import { killHarper, runHarperCommand, HarperStartupError, type StartedHarperTestContext } from '../src/harperLifecycle.ts';
+import {
+	killHarper,
+	runHarperCommand,
+	HarperStartupError,
+	buildHarperChildEnv,
+	type StartedHarperTestContext,
+} from '../src/harperLifecycle.ts';
 
 // Standalone scripts used as a fake "Harper binary" (passed via harperBinPath) to drive
 // runHarperCommand's startup watchdog through specific timing scenarios without a real Harper.
@@ -243,3 +249,24 @@ test('killHarper returns immediately for an already-exited process', async () =>
 	// than waiting out the grace, while leaving plenty of headroom for a contended-CI stall.
 	ok(Date.now() - start < 1000, 'should not wait the grace period for an already-dead process');
 });
+
+// Regression guard: the dataRootDir HOME isolation must take precedence over any caller-supplied env (or a
+// spread of process.env, which contains HOME). If a caller could clobber HOME/USERPROFILE, Harper's global
+// boot pointer would land in the developer's real ~/.harperdb and outlive the throwaway instance.
+test('buildHarperChildEnv: HOME/USERPROFILE isolation wins over caller-supplied env', () => {
+	const env = buildHarperChildEnv('/tmp/data-root', { logging: { level: 'debug' } }, {
+		HOME: '/should/not/win',
+		USERPROFILE: '/should/not/win',
+		CUSTOM_VAR: 'kept',
+	});
+	strictEqual(env.HOME, '/tmp/data-root', 'isolated HOME must override caller env');
+	strictEqual(env.USERPROFILE, '/tmp/data-root', 'isolated USERPROFILE must override caller env');
+	strictEqual(env.CUSTOM_VAR, 'kept', 'non-isolation caller env is still passed through');
+	strictEqual(env.HARPER_SET_CONFIG, JSON.stringify({ logging: { level: 'debug' } }));
+});
+
+test('buildHarperChildEnv: isolates HOME/USERPROFILE to dataRootDir by default', () => {
+	const env = buildHarperChildEnv('/tmp/data-root', {});
+	strictEqual(env.HOME, '/tmp/data-root');
+	strictEqual(env.USERPROFILE, '/tmp/data-root');
+});