diff --git a/bin/gstack-skill-start b/bin/gstack-skill-start
index 663b1e2a8..1b0ca808f 100755
--- a/bin/gstack-skill-start
+++ b/bin/gstack-skill-start
@@ -132,7 +132,7 @@ find "$_GH/sessions" -mmin +120 -type f -exec rm {} + 2>/dev/null || true
 }
 _SESSIONS=$(find "$_GH/sessions" -mmin -120 -type f 2>/dev/null | wc -l | tr -d ' ')
 
-_PROACTIVE=$("$_BIN/gstack-config" get proactive 2>/dev/null || echo "true")
+_PROACTIVE=$("$_BIN/gstack-config" get proactive 2>/dev/null || echo "core")
 _PROACTIVE_PROMPTED=$([ -f "$_GH/.proactive-prompted" ] && echo "yes" || echo "no")
 # Branch name is embedded in concatenated JSON below; strip anything outside
 # the gstack-slug BRANCH alphabet so a hostile ref name can't forge fields.
diff --git a/test/proactive-skill-start-fallback.test.ts b/test/proactive-skill-start-fallback.test.ts
new file mode 100644
index 000000000..fd5fc4017
--- /dev/null
+++ b/test/proactive-skill-start-fallback.test.ts
@@ -0,0 +1,79 @@
+/**
+ * The PROACTIVE STATUS line (GSTA-93 fix 12, proactive core default): every
+ * skill reads `PROACTIVE:` from bin/gstack-skill-start, which asks
+ * bin/gstack-config and falls back to a literal when the config binary is
+ * missing. The default moved from `true` to `core`.
+ *
+ * A fallback that still says `true` means a broken install (no gstack-config
+ * beside the script) auto-invokes every skill while the documented default is
+ * the core four.
+ *
+ * Value: protects=a skill run whose bin has no gstack-config reports the same
+ * proactive mode as the config default; fails_when=lookup_default moves (true
+ * to core) without the skill-start fallback literal following; why_new=no test
+ * ran skill-start without gstack-config beside it; seam=none (the script is
+ * copied next to an empty bin, the OV4 sanitize test's pattern); no_claim=how
+ * the model applies the value (prose, not executed here)
+ *
+ * Regression proof — fails at HEAD: yes · passes at base: yes · passes after fix: yes
+ */
+import { afterAll, beforeAll, describe, expect, test } from 'bun:test';
+import * as fs from 'fs';
+import * as os from 'os';
+import * as path from 'path';
+
+import { runBin } from './helpers/run-bin';
+
+const ROOT = path.resolve(import.meta.dir, '..');
+const BIN = path.join(ROOT, 'bin');
+
+let tmpHome: string;
+
+beforeAll(() => {
+  tmpHome = fs.mkdtempSync(path.join(os.tmpdir(), 'gstack-proactive-ss-home-'));
+});
+
+afterAll(() => {
+  fs.rmSync(tmpHome, { recursive: true, force: true });
+});
+
+function freshState(config: string): string {
+  const state = fs.mkdtempSync(path.join(os.tmpdir(), 'gstack-proactive-ss-state-'));
+  fs.writeFileSync(path.join(state, 'config.yaml'), config);
+  return state;
+}
+
+function runStart(script: string, state: string): string {
+  const r = runBin(script, ['--skill', 'testskill'], {
+    cwd: tmpHome,
+    timeoutMs: 30_000,
+    env: {
+      PATH: process.env.PATH,
+      HOME: tmpHome,
+      GSTACK_HOME: state,
+      GSTACK_STATE_DIR: state,
+      GSTACK_LEASE_DIR: path.join(state, 'leases'),
+    },
+  });
+  expect(r.status, r.stderr).toBe(0);
+  return r.stdout;
+}
+
+const proactiveLine = (out: string) => out.match(/^PROACTIVE: (.*)$/m)?.[1];
+
+describe('gstack-skill-start PROACTIVE fallback', () => {
+  test('regression: with no gstack-config beside it, the fallback reports the config default, not a stale literal', () => {
+    const fakeBin = fs.mkdtempSync(path.join(os.tmpdir(), 'gstack-proactive-ss-bin-'));
+    const state = freshState('update_check: false\n');
+    try {
+      for (const f of ['gstack-skill-start', 'gstack-state-root.sh']) fs.copyFileSync(path.join(BIN, f), path.join(fakeBin, f));
+      fs.chmodSync(path.join(fakeBin, 'gstack-skill-start'), 0o755);
+      const configDefault = runBin(path.join(BIN, 'gstack-config'), ['get', 'proactive'], { gstackHome: state, cwd: ROOT, trim: true }).stdout;
+      expect(configDefault).not.toBe('');
+      expect(proactiveLine(runStart(path.join(fakeBin, 'gstack-skill-start'), state))).toBe(configDefault);
+    } finally {
+      fs.rmSync(fakeBin, { recursive: true, force: true });
+      fs.rmSync(state, { recursive: true, force: true });
+    }
+  });
+});