diff --git a/get-shit-done/bin/lib/core.cjs b/get-shit-done/bin/lib/core.cjs
index 7810e5932..b38719259 100644
--- a/get-shit-done/bin/lib/core.cjs
+++ b/get-shit-done/bin/lib/core.cjs
@@ -561,6 +561,15 @@ function planningDir(cwd, ws, project) {
   if (project === undefined) project = process.env.GSD_PROJECT || null;
   if (ws === undefined) ws = process.env.GSD_WORKSTREAM || null;
 
+  // Reject path separators and traversal components in project/workstream names
+  const BAD_SEGMENT = /[/\\]|\.\./;
+  if (project && BAD_SEGMENT.test(project)) {
+    throw new Error(`GSD_PROJECT contains invalid path characters: ${project}`);
+  }
+  if (ws && BAD_SEGMENT.test(ws)) {
+    throw new Error(`GSD_WORKSTREAM contains invalid path characters: ${ws}`);
+  }
+
   let base = path.join(cwd, '.planning');
   if (project) base = path.join(base, project);
   if (ws) base = path.join(base, 'workstreams', ws);
diff --git a/tests/core.test.cjs b/tests/core.test.cjs
index 4211b3f8e..f99a31718 100644
--- a/tests/core.test.cjs
+++ b/tests/core.test.cjs
@@ -30,6 +30,7 @@ const {
   findPhaseInternal,
   findProjectRoot,
   detectSubRepos,
+  planningDir,
 } = require('../get-shit-done/bin/lib/core.cjs');
 
 // ─── loadConfig ────────────────────────────────────────────────────────────────
@@ -1565,3 +1566,78 @@ describe('reapStaleTempFiles', () => {
     });
   });
 });
+
+// ─── planningDir ──────────────────────────────────────────────────────────────
+
+describe('planningDir', () => {
+  const cwd = '/fake/repo';
+  let savedProject, savedWorkstream;
+
+  beforeEach(() => {
+    savedProject = process.env.GSD_PROJECT;
+    savedWorkstream = process.env.GSD_WORKSTREAM;
+    delete process.env.GSD_PROJECT;
+    delete process.env.GSD_WORKSTREAM;
+  });
+
+  afterEach(() => {
+    if (savedProject !== undefined) process.env.GSD_PROJECT = savedProject;
+    else delete process.env.GSD_PROJECT;
+    if (savedWorkstream !== undefined) process.env.GSD_WORKSTREAM = savedWorkstream;
+    else delete process.env.GSD_WORKSTREAM;
+  });
+
+  test('returns .planning/ when neither project nor workstream is set', () => {
+    const result = planningDir(cwd, null, null);
+    assert.strictEqual(result, path.join(cwd, '.planning'));
+  });
+
+  test('returns .planning/{project}/ when project is set', () => {
+    const result = planningDir(cwd, null, 'my-app');
+    assert.strictEqual(result, path.join(cwd, '.planning', 'my-app'));
+  });
+
+  test('returns .planning/workstreams/{ws}/ when workstream is set', () => {
+    const result = planningDir(cwd, 'feature-x', null);
+    assert.strictEqual(result, path.join(cwd, '.planning', 'workstreams', 'feature-x'));
+  });
+
+  test('returns .planning/{project}/workstreams/{ws}/ when both are set', () => {
+    const result = planningDir(cwd, 'feature-x', 'my-app');
+    assert.strictEqual(result, path.join(cwd, '.planning', 'my-app', 'workstreams', 'feature-x'));
+  });
+
+  test('reads GSD_PROJECT from env when project param is undefined', () => {
+    process.env.GSD_PROJECT = 'env-project';
+    const result = planningDir(cwd);
+    assert.strictEqual(result, path.join(cwd, '.planning', 'env-project'));
+  });
+
+  test('rejects path traversal in project name', () => {
+    assert.throws(
+      () => planningDir(cwd, null, '../../etc'),
+      /invalid path characters/
+    );
+  });
+
+  test('rejects forward slash in project name', () => {
+    assert.throws(
+      () => planningDir(cwd, null, 'foo/bar'),
+      /invalid path characters/
+    );
+  });
+
+  test('rejects backslash in project name', () => {
+    assert.throws(
+      () => planningDir(cwd, null, 'foo\\bar'),
+      /invalid path characters/
+    );
+  });
+
+  test('rejects path traversal in workstream name', () => {
+    assert.throws(
+      () => planningDir(cwd, '../../../tmp', null),
+      /invalid path characters/
+    );
+  });
+});