diff --git a/tests/install-runtime-artifacts.test.cjs b/tests/install-runtime-artifacts.test.cjs
index 310e55e8e..a41ba1d11 100644
--- a/tests/install-runtime-artifacts.test.cjs
+++ b/tests/install-runtime-artifacts.test.cjs
@@ -4646,7 +4646,7 @@ const fs = require('node:fs');
 const path = require('node:path');
 const os = require('node:os');
 
-const { cleanup } = require('./helpers.cjs');
+const { cleanup, TEST_ENV_BASE } = require('./helpers.cjs');
 
 const ROOT = path.join(__dirname, '..');
 const PROFILE_OUTPUT = path.join(ROOT, 'gsd-core', 'bin', 'lib', 'profile-output.cjs');
@@ -4672,7 +4672,12 @@ describe('Bug #2973: dev-preferences default writer path is skills/gsd-dev-prefe
         m.cmdGenerateDevPreferences(${JSON.stringify(tmpHome)}, { analysis: ${JSON.stringify(analysisPath)} }, false);
       `);
       const result = cp.spawnSync(process.execPath, [driver], {
-        env: Object.assign({}, process.env, { HOME: tmpHome, USERPROFILE: tmpHome }),
+        // #2665: TEST_ENV_BASE must be merged in explicitly here. This is a RAW
+        // spawn, not runGsdTools, so nothing scrubs the config-location vars for
+        // it -- and the writer under test resolves them env-FIRST. Sandboxing
+        // HOME alone let an ambient CLAUDE_CONFIG_DIR win, and the SKILL.md
+        // landed in the developer's live config dir instead of tmpHome.
+        env: Object.assign({}, process.env, TEST_ENV_BASE, { HOME: tmpHome, USERPROFILE: tmpHome }),
         encoding: 'utf-8',
         // Bound the subprocess so a regression that hangs the writer
         // (or the dispatcher) cannot deadlock CI (PR #3003 CR feedback).
diff --git a/tests/issue-766-plugin-manifest.test.cjs b/tests/issue-766-plugin-manifest.test.cjs
index 329e29a9f..1a78e243c 100644
--- a/tests/issue-766-plugin-manifest.test.cjs
+++ b/tests/issue-766-plugin-manifest.test.cjs
@@ -24,7 +24,7 @@ const ROOT = path.resolve(__dirname, '..');
 const identity = require(path.join(ROOT, 'gsd-core', 'bin', 'lib', 'package-identity.cjs'));
 const pkg = require(path.join(ROOT, 'package.json'));
 const { MANAGED_HOOKS } = require(path.join(ROOT, 'hooks', 'managed-hooks-registry.cjs'));
-const { cleanup } = require('./helpers.cjs');
+const { cleanup, TEST_ENV_BASE } = require('./helpers.cjs');
 
 const PLUGIN_JSON_PATH = path.join(ROOT, '.claude-plugin', 'plugin.json');
 const HOOKS_JSON_PATH  = path.join(ROOT, 'hooks', 'hooks.json');
@@ -359,9 +359,36 @@ describe('C: plugin.json schema validation', () => {
 
   // ── C2: Opportunistic CLI integration (skipped when claude not on PATH) ──────
 
+  // #2665: the `claude` CLI is a THIRD-PARTY binary that bootstraps its own
+  // config (.claude.json plus a backups/ dir) into whatever CLAUDE_CONFIG_DIR
+  // names. Two things follow, and only the first is obvious:
+  //
+  //   1. Inheriting the developer's ambient CLAUDE_CONFIG_DIR makes even a bare
+  //      `--version` probe write into their live config dir. That alone kept
+  //      `CLAUDE_CONFIG_DIR=<dir> npm run test:unit; find <dir> -type f` from
+  //      returning empty after every GSD-side leak was closed.
+  //   2. BLANKING it (TEST_ENV_BASE's '' convention) is not enough here. GSD's
+  //      own resolvers treat '' as falsy and fall back to the home dir, but this
+  //      binary is not ours and gives no such guarantee -- blanking it produced a
+  //      stray backups/ directory in the REPO ROOT.
+  //
+  // So point it at a real throwaway dir rather than at nothing.
+  const claudeCliHome = fs.mkdtempSync(path.join(os.tmpdir(), 'gsd-2665-claude-cli-'));
+  const claudeCliEnv = () => ({
+    ...process.env,
+    ...TEST_ENV_BASE,
+    HOME: claudeCliHome,
+    USERPROFILE: claudeCliHome,
+    CLAUDE_CONFIG_DIR: path.join(claudeCliHome, '.claude'),
+  });
+
   const claudeAvailable = (() => {
     try {
-      const result = spawnSync('claude', ['--version'], { encoding: 'utf-8', timeout: 5000 });
+      const result = spawnSync('claude', ['--version'], {
+        encoding: 'utf-8',
+        timeout: 5000,
+        env: claudeCliEnv(),
+      });
       return result.status === 0;
     } catch (_) {
       return false;
@@ -384,6 +411,7 @@ describe('C: plugin.json schema validation', () => {
           cwd: ROOT,
           encoding: 'utf-8',
           timeout: 15000,
+          env: claudeCliEnv(),
         });
         assert.equal(
           result.status,