diff --git a/test/setup-runtime-lib-command.test.ts b/test/setup-runtime-lib-command.test.ts
new file mode 100644
index 000000000..d116bdfb3
--- /dev/null
+++ b/test/setup-runtime-lib-command.test.ts
@@ -0,0 +1,190 @@
+import { describe, test, expect } from 'bun:test';
+import { spawnSync } from 'child_process';
+import * as path from 'path';
+import * as fs from 'fs';
+import * as os from 'os';
+
+const ROOT = path.resolve(import.meta.dir, '..');
+const SETUP_SRC = fs.readFileSync(path.join(ROOT, 'setup'), 'utf-8');
+
+// gstack-learnings-log is the command from the original bug report: bin scripts
+// import shared modules via `$SCRIPT_DIR/../lib`, so a runtime root that
+// exposes bin/ without lib/ fails with "Cannot find module .../lib/jsonl-store.ts".
+// Running it end-to-end from each installed root proves bin and lib travel together.
+const PAYLOAD = JSON.stringify({
+  skill: 'review',
+  type: 'pattern',
+  key: 'runtime-lib-e2e',
+  insight: 'bin commands resolve sibling lib modules after setup',
+  confidence: 8,
+  source: 'observed',
+});
+
+// Slice a named shell function out of setup by its anchors so the tests are
+// resilient to line-number drift (same idiom as setup-windows-fallback.test.ts).
+function extractFunction(name: string): string {
+  const start = SETUP_SRC.indexOf(`${name}() {`);
+  const end = SETUP_SRC.indexOf('\n}\n', start);
+  if (start < 0 || end < 0) throw new Error(`Could not locate ${name}() in setup`);
+  return SETUP_SRC.slice(start, end + 2);
+}
+
+// The Kiro install is an inline block, not a function. Slice from the runtime
+// root assignment through the last runtime-asset link so the extracted code is
+// a complete statement list.
+function extractKiroBlock(): string {
+  const startAnchor = 'KIRO_GSTACK="$KIRO_SKILLS/gstack"';
+  const endAnchor = '_link_or_copy "$SOURCE_GSTACK_DIR/browse/bin" "$KIRO_GSTACK/browse/bin"';
+  const start = SETUP_SRC.indexOf(startAnchor);
+  const end = SETUP_SRC.indexOf(endAnchor, start);
+  if (start < 0 || end < 0) throw new Error('Could not locate the Kiro install block in setup');
+  return SETUP_SRC.slice(start, end + endAnchor.length);
+}
+
+interface CommandResult {
+  buildStatus: number | null;
+  buildStderr: string;
+  runStatus: number | null;
+  runStderr: string;
+  learningsWritten: boolean;
+  libIsSymlink: boolean | null;
+}
+
+// Build one host runtime root inside a sandbox using the real setup shell code
+// (IS_WINDOWS toggles _link_or_copy between symlink and copy), then execute
+// gstack-learnings-log from the installed root and check the learning landed.
+function buildRootAndRunCommand(
+  isWindows: '0' | '1',
+  buildScript: (sandbox: string) => { script: string; rootDir: string },
+): CommandResult {
+  const sandbox = fs.mkdtempSync(path.join(os.tmpdir(), 'gstack-runtime-lib-'));
+  try {
+    const home = path.join(sandbox, 'home');
+    const project = path.join(sandbox, 'project');
+    fs.mkdirSync(home, { recursive: true });
+    fs.mkdirSync(project, { recursive: true });
+
+    const { script, rootDir } = buildScript(sandbox);
+    const build = spawnSync(
+      'bash',
+      ['-c', `IS_WINDOWS=${isWindows}\n${extractFunction('_link_or_copy')}\n${script}`],
+      { encoding: 'utf-8', timeout: 30000 },
+    );
+
+    const libLst = fs.lstatSync(path.join(rootDir, 'lib'), { throwIfNoEntry: false });
+    const run = spawnSync('bash', [path.join(rootDir, 'bin', 'gstack-learnings-log'), PAYLOAD], {
+      cwd: project,
+      encoding: 'utf-8',
+      timeout: 30000,
+      env: { ...process.env, HOME: home, GSTACK_HOME: path.join(home, '.gstack') },
+    });
+
+    const projectsDir = path.join(home, '.gstack', 'projects');
+    const learningsWritten = fs.existsSync(projectsDir)
+      && fs.readdirSync(projectsDir).some((slug) => {
+        const file = path.join(projectsDir, slug, 'learnings.jsonl');
+        return fs.existsSync(file) && fs.readFileSync(file, 'utf-8').includes('runtime-lib-e2e');
+      });
+
+    return {
+      buildStatus: build.status,
+      buildStderr: build.stderr,
+      runStatus: run.status,
+      runStderr: run.stderr,
+      learningsWritten,
+      libIsSymlink: libLst ? libLst.isSymbolicLink() : null,
+    };
+  } finally {
+    fs.rmSync(sandbox, { recursive: true, force: true });
+  }
+}
+
+// One builder per host root the lib fix touches. Each returns the shell that
+// setup itself runs plus where the installed runtime root lands.
+const HOST_ROOTS: Record<string, (sandbox: string) => { script: string; rootDir: string }> = {
+  'agents sidecar': (sandbox) => ({
+    script: [
+      `SOURCE_GSTACK_DIR="${ROOT}"`,
+      extractFunction('create_agents_sidecar'),
+      `mkdir -p "${sandbox}/repo"`,
+      `create_agents_sidecar "${sandbox}/repo"`,
+    ].join('\n'),
+    rootDir: path.join(sandbox, 'repo', '.agents', 'skills', 'gstack'),
+  }),
+  codex: (sandbox) => ({
+    script: [
+      extractFunction('create_codex_runtime_root'),
+      `create_codex_runtime_root "${ROOT}" "${sandbox}/home/.codex/skills/gstack"`,
+    ].join('\n'),
+    rootDir: path.join(sandbox, 'home', '.codex', 'skills', 'gstack'),
+  }),
+  factory: (sandbox) => ({
+    script: [
+      extractFunction('create_factory_runtime_root'),
+      `create_factory_runtime_root "${ROOT}" "${sandbox}/home/.factory/skills/gstack"`,
+    ].join('\n'),
+    rootDir: path.join(sandbox, 'home', '.factory', 'skills', 'gstack'),
+  }),
+  opencode: (sandbox) => ({
+    script: [
+      extractFunction('create_opencode_runtime_root'),
+      `create_opencode_runtime_root "${ROOT}" "${sandbox}/home/.opencode/skills/gstack"`,
+    ].join('\n'),
+    rootDir: path.join(sandbox, 'home', '.opencode', 'skills', 'gstack'),
+  }),
+  kiro: (sandbox) => ({
+    script: [
+      `HOME="${sandbox}/home"`,
+      `SOURCE_GSTACK_DIR="${ROOT}"`,
+      `KIRO_SKILLS="$HOME/.kiro/skills"`,
+      `mkdir -p "$KIRO_SKILLS"`,
+      extractKiroBlock(),
+    ].join('\n'),
+    rootDir: path.join(sandbox, 'home', '.kiro', 'skills', 'gstack'),
+  }),
+};
+
+// The IS_WINDOWS=0 cells rely on Unix `ln -snf` semantics; on a real Windows
+// runner without Developer Mode that silently degrades to a copy — the exact
+// bug _link_or_copy works around — so skip there, matching the behavior-matrix
+// precedent in setup-windows-fallback.test.ts. The IS_WINDOWS=1 cells exercise
+// the Windows copy branch itself, which is plain `cp -R` and portable.
+describe.skipIf(process.platform === 'win32')('setup: bin commands resolve sibling lib from every host root', () => {
+  for (const [host, buildScript] of Object.entries(HOST_ROOTS)) {
+    test(`${host} root (symlink install): gstack-learnings-log imports ../lib and writes the learning`, () => {
+      const r = buildRootAndRunCommand('0', buildScript);
+      expect(r.buildStatus).toBe(0);
+      expect(r.libIsSymlink).toBe(true);
+      expect(r.runStderr).not.toContain('lib/jsonl-store.ts');
+      expect(r.runStatus).toBe(0);
+      expect(r.learningsWritten).toBe(true);
+    });
+
+    test(`${host} root (Windows copy install): gstack-learnings-log imports ../lib and writes the learning`, () => {
+      const r = buildRootAndRunCommand('1', buildScript);
+      expect(r.buildStatus).toBe(0);
+      // Windows branch copies: lib must be a real directory, not a symlink.
+      expect(r.libIsSymlink).toBe(false);
+      expect(r.runStderr).not.toContain('lib/jsonl-store.ts');
+      expect(r.runStatus).toBe(0);
+      expect(r.learningsWritten).toBe(true);
+    });
+  }
+
+  // Negative control: a root with bin/ but no lib/ (the pre-fix layout) must
+  // fail on the ../lib import. Proves the positive cells actually detect the
+  // regression rather than passing vacuously.
+  test('a root missing lib/ beside bin/ fails the ../lib import (pre-fix layout)', () => {
+    const r = buildRootAndRunCommand('0', (sandbox) => ({
+      script: [
+        `mkdir -p "${sandbox}/broken"`,
+        `_link_or_copy "${ROOT}/bin" "${sandbox}/broken/bin"`,
+      ].join('\n'),
+      rootDir: path.join(sandbox, 'broken'),
+    }));
+    expect(r.buildStatus).toBe(0);
+    expect(r.runStatus).not.toBe(0);
+    expect(r.runStderr).toContain('lib/jsonl-store.ts');
+    expect(r.learningsWritten).toBe(false);
+  });
+});