diff --git a/bin/gstack-session-kind b/bin/gstack-session-kind
new file mode 100755
index 000000000..8e9bc4e41
--- /dev/null
+++ b/bin/gstack-session-kind
@@ -0,0 +1,53 @@
+#!/usr/bin/env bash
+# gstack-session-kind — classify the current agent session so skills know whether
+# a human can answer an interactive prompt (AskUserQuestion).
+#
+# Usage: gstack-session-kind            → prints one of: spawned | headless | interactive
+#
+# Used by the preamble (generate-preamble-bash.ts) which echoes
+#   SESSION_KIND: <value>
+# so the AskUserQuestion-failure fallback rule can branch without a shell-out at
+# failure time:
+#   spawned     → orchestrator session (OpenClaw). Auto-choose recommended option
+#                 per the skill's SPAWNED_SESSION block. Never prose, never BLOCKED.
+#   headless    → no human present (claude -p evals / CI). BLOCK on AUQ failure.
+#   interactive → a human is present. Prose-fallback on AUQ failure.
+#
+# Detection is best-effort. On ANY ambiguity it prints `interactive` — BLOCK only on
+# a positive headless signal, since a stray prose message in an unmarked one-shot
+# `-p` run just ends the turn (harmless), whereas wrongly BLOCKING a real human is not.
+#
+# Why env vars and not TTY/entrypoint: an interactive Conductor session reports
+# CLAUDE_CODE_ENTRYPOINT=sdk-ts with no TTY — identical to a headless SDK eval. The
+# signals that actually discriminate are the host/orchestrator/CI env markers below.
+set -euo pipefail
+
+# 1. Orchestrator-spawned session (OpenClaw). Authoritative block lives in the skill;
+#    we only surface the classification.
+if [ -n "${OPENCLAW_SESSION:-}" ]; then
+  echo "spawned"
+  exit 0
+fi
+
+# 2. Explicit headless override (set by the eval/E2E harness for determinism).
+if [ -n "${GSTACK_HEADLESS:-}" ]; then
+  echo "headless"
+  exit 0
+fi
+
+# 3. Positive interactive-host signals: a human-driven host is present.
+#    - Conductor app sets CONDUCTOR_* workspace vars.
+#    - Plain interactive `claude` CLI sets CLAUDE_CODE_ENTRYPOINT=cli.
+if [ -n "${CONDUCTOR_WORKSPACE_PATH:-}" ] || [ -n "${CONDUCTOR_PORT:-}" ] || [ "${CLAUDE_CODE_ENTRYPOINT:-}" = "cli" ]; then
+  echo "interactive"
+  exit 0
+fi
+
+# 4. CI / automation markers with no interactive host → headless.
+if [ -n "${CI:-}" ] || [ -n "${GITHUB_ACTIONS:-}" ]; then
+  echo "headless"
+  exit 0
+fi
+
+# 5. No positive headless signal → assume a human is present (degrade-safe default).
+echo "interactive"
diff --git a/scripts/resolvers/preamble/generate-preamble-bash.ts b/scripts/resolvers/preamble/generate-preamble-bash.ts
index d8552421a..ecc30cef7 100644
--- a/scripts/resolvers/preamble/generate-preamble-bash.ts
+++ b/scripts/resolvers/preamble/generate-preamble-bash.ts
@@ -33,6 +33,9 @@ echo "SKILL_PREFIX: $_SKILL_PREFIX"
 source <(${ctx.paths.binDir}/gstack-repo-mode 2>/dev/null) || true
 REPO_MODE=\${REPO_MODE:-unknown}
 echo "REPO_MODE: $REPO_MODE"
+_SESSION_KIND=$(${ctx.paths.binDir}/gstack-session-kind 2>/dev/null || echo "interactive")
+case "$_SESSION_KIND" in spawned|headless|interactive) ;; *) _SESSION_KIND="interactive" ;; esac
+echo "SESSION_KIND: $_SESSION_KIND"
 _LAKE_SEEN=$([ -f ~/.gstack/.completeness-intro-seen ] && echo "yes" || echo "no")
 echo "LAKE_INTRO: $_LAKE_SEEN"
 _TEL=$(${ctx.paths.binDir}/gstack-config get telemetry 2>/dev/null || true)
diff --git a/test/gstack-session-kind.test.ts b/test/gstack-session-kind.test.ts
new file mode 100644
index 000000000..67a047a1b
--- /dev/null
+++ b/test/gstack-session-kind.test.ts
@@ -0,0 +1,70 @@
+/**
+ * gstack-session-kind — classifies the session so skills know whether a human can
+ * answer an AskUserQuestion. Drives the AUQ-failure fallback branch:
+ *   spawned     → auto-choose (orchestrator)
+ *   headless    → BLOCK on AUQ failure
+ *   interactive → prose fallback on AUQ failure
+ *
+ * These permutations are the contract the resolver rule depends on. Run with a
+ * SCRUBBED env (the test process itself runs inside Conductor, so CONDUCTOR_* /
+ * CLAUDE_CODE_* would leak in and contaminate the classification).
+ *
+ * Free, deterministic, gate-tier.
+ */
+import { describe, test, expect } from 'bun:test';
+import { execFileSync } from 'child_process';
+import * as path from 'path';
+
+const BIN = path.resolve(__dirname, '..', 'bin', 'gstack-session-kind');
+
+/** Run the helper with ONLY the supplied env (plus PATH so bash resolves). */
+function kind(env: Record<string, string>): string {
+  return execFileSync(BIN, [], {
+    env: { PATH: process.env.PATH ?? '/usr/bin:/bin', ...env },
+    encoding: 'utf-8',
+  }).trim();
+}
+
+describe('gstack-session-kind', () => {
+  test('OPENCLAW_SESSION → spawned (highest precedence)', () => {
+    expect(kind({ OPENCLAW_SESSION: '1' })).toBe('spawned');
+    // spawned wins even when other markers are also present
+    expect(kind({ OPENCLAW_SESSION: '1', GSTACK_HEADLESS: '1', CONDUCTOR_PORT: '5' })).toBe('spawned');
+  });
+
+  test('GSTACK_HEADLESS → headless', () => {
+    expect(kind({ GSTACK_HEADLESS: '1' })).toBe('headless');
+  });
+
+  test('CONDUCTOR_* → interactive (a human host is present)', () => {
+    expect(kind({ CONDUCTOR_WORKSPACE_PATH: '/tmp/ws' })).toBe('interactive');
+    expect(kind({ CONDUCTOR_PORT: '55010' })).toBe('interactive');
+  });
+
+  test('CLAUDE_CODE_ENTRYPOINT=cli → interactive', () => {
+    expect(kind({ CLAUDE_CODE_ENTRYPOINT: 'cli' })).toBe('interactive');
+  });
+
+  test('interactive host beats CI markers', () => {
+    expect(kind({ CONDUCTOR_PORT: '5', CI: '1' })).toBe('interactive');
+  });
+
+  test('CI / GITHUB_ACTIONS with no host → headless', () => {
+    expect(kind({ CI: '1' })).toBe('headless');
+    expect(kind({ GITHUB_ACTIONS: 'true' })).toBe('headless');
+  });
+
+  test('GSTACK_HEADLESS beats CONDUCTOR (explicit override wins)', () => {
+    expect(kind({ GSTACK_HEADLESS: '1', CONDUCTOR_PORT: '5' })).toBe('headless');
+  });
+
+  test('bare env → interactive (degrade-safe default)', () => {
+    expect(kind({})).toBe('interactive');
+  });
+
+  test('empty GSTACK_HEADLESS is treated as unset (interactive)', () => {
+    // The resolver/helper guard on -n, so an empty string must NOT mean headless —
+    // this is the opt-out path harness suites use to exercise the interactive branch.
+    expect(kind({ GSTACK_HEADLESS: '' })).toBe('interactive');
+  });
+});