diff --git a/bin/gstack-config b/bin/gstack-config
index 3521825f4..46f82bfd0 100755
--- a/bin/gstack-config
+++ b/bin/gstack-config
@@ -403,6 +403,14 @@ case "${1:-}" in
       echo "Error: codex_reviews '$VALUE' not recognized. Valid values: enabled, disabled. Existing value left unchanged." >&2
       exit 1
     fi
+    # cross_project_learnings: empty get is the first-run prompt sentinel.
+    # Skills enable only on the literal "true". A typo must not persist — that
+    # keeps the feature off and suppresses the prompt. Reject, like
+    # codex_reviews; do not coerce (a stored default still kills the sentinel).
+    if [ "$KEY" = "cross_project_learnings" ] && [ "$VALUE" != "true" ] && [ "$VALUE" != "false" ]; then
+      echo "Error: cross_project_learnings '$VALUE' not recognized. Valid values: true, false. Existing value left unchanged." >&2
+      exit 1
+    fi
     mkdir -p "$STATE_DIR"
     # Write annotated header on first creation
     if [ ! -f "$CONFIG_FILE" ]; then
diff --git a/test/gstack-config-cross-project.test.ts b/test/gstack-config-cross-project.test.ts
new file mode 100644
index 000000000..93c323948
--- /dev/null
+++ b/test/gstack-config-cross-project.test.ts
@@ -0,0 +1,67 @@
+/**
+ * #2673: gstack-config set must reject malformed cross_project_learnings.
+ *
+ * Empty get is the first-run prompt sentinel (pinned in
+ * gstack-config-defaults.test.ts). Skills only enable on the literal "true".
+ * A typo used to store verbatim and exit 0, so the feature stayed off and
+ * the prompt never returned. Unlike pair_agent / redact_prepush_hook, do
+ * not coerce to a default — that would still persist a value and still
+ * kill the sentinel. Follow codex_reviews: reject, leave existing.
+ */
+import { afterEach, beforeEach, describe, expect, test } from "bun:test";
+import { spawnSync } from "child_process";
+import * as fs from "fs";
+import * as os from "os";
+import * as path from "path";
+
+const CONFIG = path.resolve(import.meta.dir, "..", "bin", "gstack-config");
+let stateRoot: string;
+
+function cfg(args: string[]): { code: number; out: string; err: string } {
+  const r = spawnSync(CONFIG, args, {
+    encoding: "utf8",
+    env: { ...process.env, GSTACK_STATE_ROOT: stateRoot },
+  });
+  return { code: r.status ?? 0, out: r.stdout ?? "", err: r.stderr ?? "" };
+}
+
+beforeEach(() => {
+  stateRoot = fs.mkdtempSync(path.join(os.tmpdir(), "gstack-config-xproj-"));
+});
+
+afterEach(() => {
+  fs.rmSync(stateRoot, { recursive: true, force: true });
+});
+
+describe("cross_project_learnings set domain (#2673)", () => {
+  test("empty get is still the first-run sentinel", () => {
+    const r = cfg(["get", "cross_project_learnings"]);
+    expect(r.code).toBe(0);
+    expect(r.out).toBe("");
+  });
+
+  test("true and false round-trip", () => {
+    expect(cfg(["set", "cross_project_learnings", "true"]).code).toBe(0);
+    expect(cfg(["get", "cross_project_learnings"]).out).toBe("true");
+    expect(cfg(["set", "cross_project_learnings", "false"]).code).toBe(0);
+    expect(cfg(["get", "cross_project_learnings"]).out).toBe("false");
+  });
+
+  test("typo is rejected and does not write", () => {
+    const r = cfg(["set", "cross_project_learnings", "ture"]);
+    expect(r.code).toBe(1);
+    expect(r.err).toContain("not recognized");
+    expect(r.err).toContain("cross_project_learnings");
+    const got = cfg(["get", "cross_project_learnings"]);
+    expect(got.code).toBe(0);
+    expect(got.out).toBe("");
+  });
+
+  test("typo leaves an existing valid value unchanged", () => {
+    expect(cfg(["set", "cross_project_learnings", "true"]).code).toBe(0);
+    const r = cfg(["set", "cross_project_learnings", "yes"]);
+    expect(r.code).toBe(1);
+    expect(r.err).toContain("Existing value left unchanged");
+    expect(cfg(["get", "cross_project_learnings"]).out).toBe("true");
+  });
+});