diff --git a/tests/copilot-install.test.cjs b/tests/copilot-install.test.cjs
index c66d1e56b..c60dc671d 100644
--- a/tests/copilot-install.test.cjs
+++ b/tests/copilot-install.test.cjs
@@ -1362,7 +1362,10 @@ const EXPECTED_AGENTS = listAgentFiles().length;
 const { PROBE_TIMEOUT_MS, INSTALL_TIMEOUT_MS } = require('./helpers/timeouts.cjs');
 
 function runCopilotInstall(cwd) {
-  const env = { ...process.env };
+  // #3156: sandbox HOME — the installer writes <home>/.gsd/defaults.json via
+  // os.homedir() directly, which no env scrub can reach. See installSpawnEnv.
+  const { installSpawnEnv } = require('./helpers.cjs');
+  const env = installSpawnEnv();
   delete env.GSD_TEST_MODE;
   const r = runNode([INSTALL_PATH, '--copilot', '--local', '--no-sdk'], {
     cwd,
@@ -1374,7 +1377,10 @@ function runCopilotInstall(cwd) {
 }
 
 function runCopilotUninstall(cwd) {
-  const env = { ...process.env };
+  // #3156: sandbox HOME — the installer writes <home>/.gsd/defaults.json via
+  // os.homedir() directly, which no env scrub can reach. See installSpawnEnv.
+  const { installSpawnEnv } = require('./helpers.cjs');
+  const env = installSpawnEnv();
   delete env.GSD_TEST_MODE;
   const r = runNode([INSTALL_PATH, '--copilot', '--local', '--uninstall', '--no-sdk'], {
     cwd,
@@ -1673,7 +1679,10 @@ describe('E2E: Copilot uninstall verification', () => {
 // ─── E2E: Copilot global scope (#786) ──────────────────────────────────────────
 
 function runCopilotInstallGlobal(cwd, configDir) {
-  const env = { ...process.env };
+  // #3156: sandbox HOME — the installer writes <home>/.gsd/defaults.json via
+  // os.homedir() directly, which no env scrub can reach. See installSpawnEnv.
+  const { installSpawnEnv } = require('./helpers.cjs');
+  const env = installSpawnEnv();
   delete env.GSD_TEST_MODE;
   const r = runNode(
     [INSTALL_PATH, '--copilot', '--global', '--config-dir', configDir, '--no-sdk'],
@@ -1684,7 +1693,10 @@ function runCopilotInstallGlobal(cwd, configDir) {
 }
 
 function runCopilotUninstallGlobal(cwd, configDir) {
-  const env = { ...process.env };
+  // #3156: sandbox HOME — the installer writes <home>/.gsd/defaults.json via
+  // os.homedir() directly, which no env scrub can reach. See installSpawnEnv.
+  const { installSpawnEnv } = require('./helpers.cjs');
+  const env = installSpawnEnv();
   delete env.GSD_TEST_MODE;
   const r = runNode(
     [INSTALL_PATH, '--copilot', '--global', '--config-dir', configDir, '--uninstall', '--no-sdk'],
@@ -1734,7 +1746,10 @@ describe('E2E: Copilot global install (#786)', () => {
 // ─── Claude uninstall: user file preservation (#1423) ─────────────────────────
 
 function runClaudeInstall(cwd) {
-  const env = { ...process.env };
+  // #3156: sandbox HOME — the installer writes <home>/.gsd/defaults.json via
+  // os.homedir() directly, which no env scrub can reach. See installSpawnEnv.
+  const { installSpawnEnv } = require('./helpers.cjs');
+  const env = installSpawnEnv();
   delete env.GSD_TEST_MODE;
   const r = runNode([INSTALL_PATH, '--claude', '--local', '--no-sdk'], {
     cwd,
@@ -1746,7 +1761,10 @@ function runClaudeInstall(cwd) {
 }
 
 function runClaudeUninstall(cwd) {
-  const env = { ...process.env };
+  // #3156: sandbox HOME — the installer writes <home>/.gsd/defaults.json via
+  // os.homedir() directly, which no env scrub can reach. See installSpawnEnv.
+  const { installSpawnEnv } = require('./helpers.cjs');
+  const env = installSpawnEnv();
   delete env.GSD_TEST_MODE;
   const r = runNode([INSTALL_PATH, '--claude', '--local', '--uninstall', '--no-sdk'], {
     cwd,
diff --git a/tests/helpers.cjs b/tests/helpers.cjs
index e17c57b59..7e224d74a 100644
--- a/tests/helpers.cjs
+++ b/tests/helpers.cjs
@@ -911,7 +911,43 @@ function clearSessionEnv() {
   for (const k of SESSION_ENV_KEYS) delete process.env[k];
 }
 
-module.exports = { runGsdTools, createTempDir, createTempProject, createTempGitProject, cleanup, tmpRootCandidates, readFileNormalized, readWorkflowCombined, parseFrontmatter, isUsageOutput, captureConsole, toPosixPath, absPlanningPath, runNpm, isolatedNpmEnv, withIsolatedProcessState, delay, waitFor, resetRuntimeWarningCaches, SESSION_ENV_KEYS, saveSessionEnv, restoreSessionEnv, clearSessionEnv, TOOLS_PATH, SESSION_IDENTITY_ENV_KEYS, scrubConfigLocationEnv };
+/**
+ * #3156: env for a RAW installer spawn — one that bypasses runGsdTools and so
+ * never receives TEST_ENV_BASE on its own.
+ *
+ * Blanking config-LOCATION vars is necessary but NOT sufficient here.
+ * bin/install.js writes GSD's own user-owned store through os.homedir()
+ * DIRECTLY (writeNonClaudeDefaults -> <home>/.gsd/defaults.json, #2834), and
+ * os.homedir() consults no GSD variable at all — so nothing in
+ * CONFIG_LOCATION_ENV_KEYS can reach it, and blanking GSD_HOME does not reach
+ * it either, because a blank GSD_HOME falls back to exactly that homedir().
+ * Only a sandboxed HOME/USERPROFILE contains it.
+ *
+ * HOME stays deliberately OUT of TEST_ENV_BASE — blanking it would break far
+ * more than it fixed — so it is sandboxed per spawn instead, which is the
+ * discipline the suite already applies by hand elsewhere. USERPROFILE is set
+ * with it because os.homedir() reads that one on Windows.
+ *
+ * The sandbox home is per-process and removed on exit, so a caller gets
+ * containment without having to own a lifecycle.
+ */
+let installSpawnHomeDir = null;
+function installSpawnHome() {
+  if (installSpawnHomeDir === null) {
+    installSpawnHomeDir = fs.mkdtempSync(path.join(os.tmpdir(), 'gsd-install-home-'));
+    process.on('exit', () => {
+      try { fs.rmSync(installSpawnHomeDir, { recursive: true, force: true }); } catch { /* best effort */ }
+    });
+  }
+  return installSpawnHomeDir;
+}
+
+function installSpawnEnv(overrides = {}) {
+  const home = installSpawnHome();
+  return { ...process.env, ...testEnvBase(), HOME: home, USERPROFILE: home, ...overrides };
+}
+
+module.exports = { runGsdTools, createTempDir, createTempProject, createTempGitProject, cleanup, tmpRootCandidates, readFileNormalized, readWorkflowCombined, parseFrontmatter, isUsageOutput, captureConsole, toPosixPath, absPlanningPath, runNpm, isolatedNpmEnv, withIsolatedProcessState, delay, waitFor, resetRuntimeWarningCaches, SESSION_ENV_KEYS, saveSessionEnv, restoreSessionEnv, clearSessionEnv, TOOLS_PATH, SESSION_IDENTITY_ENV_KEYS, scrubConfigLocationEnv, installSpawnEnv, installSpawnHome };
 
 // Lazy, for the reason builtLib() is lazy: reading either of these is what
 // forces the built-lib require, so a test file that needs neither can still
diff --git a/tests/helpers/install-shared.cjs b/tests/helpers/install-shared.cjs
index b4c5ce580..153a449cb 100644
--- a/tests/helpers/install-shared.cjs
+++ b/tests/helpers/install-shared.cjs
@@ -520,7 +520,15 @@ function simulateHookCopy(hooksSrc, hooksDest) {
 /** Build a clean env for spawned installer processes.
  *  Must strip GSD_TEST_MODE so the child runs the real install, not the no-op guard. */
 function installerEnv(overrides = {}) {
-  const env = { ...process.env, ...overrides };
+  // #3156: delegate to the ONE canonical raw-installer-spawn env rather than
+  // carrying a second shape of it. The installer writes GSD's own user store to
+  // <home>/.gsd/defaults.json through os.homedir() DIRECTLY
+  // (bin/install.js writeNonClaudeDefaults, #2834), which reads no GSD variable,
+  // so no config-location scrub can reach it — only a sandboxed HOME can. Every
+  // caller that already passes an explicit { HOME, USERPROFILE } still wins:
+  // overrides spread last.
+  const { installSpawnEnv } = require('../helpers.cjs');
+  const env = installSpawnEnv(overrides);
   delete env.GSD_TEST_MODE;
   return env;
 }
diff --git a/tests/install.test.cjs b/tests/install.test.cjs
index 670ab886e..ae8e62133 100644
--- a/tests/install.test.cjs
+++ b/tests/install.test.cjs
@@ -5535,7 +5535,10 @@ function ensureHooksDist() {
  * GSD_TEST_MODE is cleared so the install() main block executes.
  */
 function runInstall(cwd, args) {
-  const env = { ...process.env };
+  // #3156: sandbox HOME — the installer writes <home>/.gsd/defaults.json via
+  // os.homedir() directly, which no env scrub can reach. See installSpawnEnv.
+  const { installSpawnEnv } = require('./helpers.cjs');
+  const env = installSpawnEnv();
   delete env.GSD_TEST_MODE;
   // 120s, not 60s. A full install copies and converts the whole shipped
   // payload (117 workflows, 100 references, 34 agents, ~71 skills) and
@@ -9597,7 +9600,10 @@ function ensureHooksDist() {
  * GSD_TEST_MODE is cleared so the install() main block executes.
  */
 function runInstall(cwd, args) {
-  const env = { ...process.env };
+  // #3156: sandbox HOME — the installer writes <home>/.gsd/defaults.json via
+  // os.homedir() directly, which no env scrub can reach. See installSpawnEnv.
+  const { installSpawnEnv } = require('./helpers.cjs');
+  const env = installSpawnEnv();
   delete env.GSD_TEST_MODE;
   // 120s, not 60s. A full install copies and converts the whole shipped
   // payload (117 workflows, 100 references, 34 agents, ~71 skills) and
@@ -10022,7 +10028,10 @@ const {
  * GSD_TEST_MODE must be cleared so the install() main block executes.
  */
 function runClaudeLocalInstall(cwd) {
-  const env = { ...process.env };
+  // #3156: sandbox HOME — the installer writes <home>/.gsd/defaults.json via
+  // os.homedir() directly, which no env scrub can reach. See installSpawnEnv.
+  const { installSpawnEnv } = require('./helpers.cjs');
+  const env = installSpawnEnv();
   delete env.GSD_TEST_MODE;
   const r = runNode([INSTALL_PATH, '--claude', '--local', '--no-sdk'], {
     cwd,
diff --git a/tests/opencode-plugin-adapter.test.cjs b/tests/opencode-plugin-adapter.test.cjs
index 6d4c11ecc..49b3ff23a 100644
--- a/tests/opencode-plugin-adapter.test.cjs
+++ b/tests/opencode-plugin-adapter.test.cjs
@@ -389,6 +389,9 @@ test('installer copies plugin as .js, records it in the manifest, and removes it
   const run = (args) => {
     const result = runNode([installer, '--opencode', '--global', '--config-dir', cfg, ...args], {
       timeoutMs: 120000,
+      // #3156: sandbox HOME — the installer writes <home>/.gsd/defaults.json via
+      // os.homedir() directly, which no env scrub can reach. See installSpawnEnv.
+      env: require('./helpers.cjs').installSpawnEnv(),
     });
     result.status = result.exitCode;
     return result;