diff --git a/scripts/run-tests.cjs b/scripts/run-tests.cjs
index 810438534..133d9c9db 100644
--- a/scripts/run-tests.cjs
+++ b/scripts/run-tests.cjs
@@ -236,6 +236,14 @@ function main() {
   // Build the gitignored bin/lib artifact if absent, before any test requires it.
   ensureBuiltArtifacts();
 
+  // Hermeticity: in-process tests resolve `.planning` via planningDir(cwd), which
+  // honours GSD_PROJECT/GSD_WORKSTREAM. A developer shell inside a GSD workstream
+  // exports GSD_WORKSTREAM, which would redirect fixture STATE.md reads away from
+  // each <tmp>/.planning and silently diverge from the clean CI/Docker env. Strip
+  // them so the local runner matches CI; tests that need them set them explicitly.
+  delete process.env.GSD_PROJECT;
+  delete process.env.GSD_WORKSTREAM;
+
   // Log selected files to stderr for CI / harness-test visibility.
   // node:test default reporter doesn't echo filenames, so this gives
   // operators a single stable line they can grep.
diff --git a/tests/roadmap-phase-fallback.test.cjs b/tests/roadmap-phase-fallback.test.cjs
index 4587b5dd3..beac818ec 100644
--- a/tests/roadmap-phase-fallback.test.cjs
+++ b/tests/roadmap-phase-fallback.test.cjs
@@ -11,6 +11,26 @@ const fs = require('fs');
 const path = require('path');
 const { runGsdTools, createTempProject, cleanup } = require('./helpers.cjs');
 
+// The planning-dir resolver (planningDir) is workstream-aware and honours
+// GSD_PROJECT / GSD_WORKSTREAM. These suites write STATE.md to <tmp>/.planning
+// and assume that is where it is read from, so a developer shell inside a GSD
+// workstream would otherwise redirect the read and break extractCurrentMilestone.
+// Isolate the vars so the file is hermetic when run directly via `node --test`.
+let savedGsdProject;
+let savedGsdWorkstream;
+beforeEach(() => {
+  savedGsdProject = process.env.GSD_PROJECT;
+  savedGsdWorkstream = process.env.GSD_WORKSTREAM;
+  delete process.env.GSD_PROJECT;
+  delete process.env.GSD_WORKSTREAM;
+});
+afterEach(() => {
+  if (savedGsdProject !== undefined) process.env.GSD_PROJECT = savedGsdProject;
+  else delete process.env.GSD_PROJECT;
+  if (savedGsdWorkstream !== undefined) process.env.GSD_WORKSTREAM = savedGsdWorkstream;
+  else delete process.env.GSD_WORKSTREAM;
+});
+
 /**
  * Helper: write STATE.md with a milestone version so extractCurrentMilestone
  * will slice the roadmap to only that milestone's section.
@@ -365,6 +385,51 @@ This is the active milestone body.
     );
   });
 
+  test('(7) workstream-aware: STATE.md under GSD_WORKSTREAM is read from the workstream subdir', () => {
+    // Regression guard for the env-leak that made these suites pass in clean CI but
+    // fail in a developer's GSD_WORKSTREAM shell. planningDir() is workstream-aware,
+    // so STATE.md lives at <cwd>/.planning/workstreams/<ws>/STATE.md. Setting the env
+    // here makes clean CI exercise the polluted-env resolution path.
+    process.env.GSD_WORKSTREAM = 'guard-ws';
+    try {
+      const wsPlanning = path.join(tmpDir, '.planning', 'workstreams', 'guard-ws');
+      fs.mkdirSync(wsPlanning, { recursive: true });
+      fs.writeFileSync(path.join(wsPlanning, 'STATE.md'), '---\nmilestone: v8.0\n---\n');
+      const roadmap = `# Project Roadmap
+
+## v8.0 Overview — v8.0-F (CLOSED FAIL 2026-05-18)
+
+This is the closed milestone body with some text.
+
+### Phase 24: ARCHIVED
+**Goal:** This phase is done and archived.
+
+## v8.0-B Overview (STARTED 2026-05-18)
+
+This is the active milestone body.
+
+### Phase 31: EVAL
+**Goal:** Evaluate the new system.
+
+## v9.0 Future Milestone
+
+### Phase 40: FUTURE
+**Goal:** Future work.
+`;
+      const slice = core.extractCurrentMilestone(roadmap, tmpDir);
+      assert.ok(
+        slice.includes('Phase 31: EVAL'),
+        'workstream-scoped STATE.md must select the active v8.0-B section',
+      );
+      assert.ok(
+        !slice.includes('Phase 24: ARCHIVED'),
+        'closed section must still be excluded under a workstream env',
+      );
+    } finally {
+      delete process.env.GSD_WORKSTREAM;
+    }
+  });
+
   test('(2) double-closed-skip: third sibling (active) selected when first two are closed', () => {
     writeState(tmpDir, 'v9.0');
     const roadmap = `# Project Roadmap
diff --git a/tests/run-tests-harness.test.cjs b/tests/run-tests-harness.test.cjs
index c819000de..343ca5fb8 100644
--- a/tests/run-tests-harness.test.cjs
+++ b/tests/run-tests-harness.test.cjs
@@ -260,6 +260,32 @@ test('boom', () => { throw new Error('intentional'); });
     });
   });
 
+  describe('env hermeticity', () => {
+    // Regression guard for the two `delete process.env.GSD_PROJECT/GSD_WORKSTREAM`
+    // lines added in scripts/run-tests.cjs main() right after ensureBuiltArtifacts().
+    // If those deletions are removed, the fixture's assertions fail inside the child
+    // node:test process → non-zero harness exit → this test fails → CI catches it.
+    test('harness strips GSD_PROJECT and GSD_WORKSTREAM before running child tests', () => {
+      // Write a fixture that asserts both vars are absent in the child process env.
+      const FIXTURE = `'use strict';
+const { test } = require('node:test');
+const assert = require('node:assert/strict');
+test('ambient GSD workstream vars are stripped by the runner', () => {
+  assert.strictEqual(process.env.GSD_PROJECT, undefined);
+  assert.strictEqual(process.env.GSD_WORKSTREAM, undefined);
+});
+`;
+      fs.writeFileSync(path.join(tmpDir, 'env-hermeticity.test.cjs'), FIXTURE, 'utf8');
+      // Pass both vars in the ambient env given to the harness process.
+      // The harness must delete them before spawning the child node:test process.
+      const r = runHarness(tmpDir, [], {
+        GSD_PROJECT: 'ambient-proj',
+        GSD_WORKSTREAM: 'ambient-ws',
+      });
+      assert.strictEqual(r.status, 0, r.stderr);
+    });
+  });
+
   describe('Windows argv-overflow chunking (issue #3597)', () => {
     // Windows CreateProcess caps lpCommandLine at 32,767 chars. With ~550
     // tests the unchunked spawn fails instantly on Windows with no test