diff --git a/bin/gstack-config b/bin/gstack-config
index 01176c44c..0eca9ed49 100755
--- a/bin/gstack-config
+++ b/bin/gstack-config
@@ -264,7 +264,7 @@ case "${1:-}" in
     # endpoint-namespaced keys introduced by the brain-aware planning layer).
     # Endpoint ids are sha8/sha16 hex for remote MCP URLs, or the literal
     # "local" for stdio/PGLite engines (see endpoint_hash).
-    if ! printf '%s' "$KEY" | grep -qE '^[a-zA-Z0-9_]+(@[a-zA-Z0-9]+)?$'; then
+    if ! printf '%s' "$KEY" | LC_ALL=C grep -qE '^[a-zA-Z0-9_]+(@[a-zA-Z0-9]+)?$'; then
       echo "Error: key must contain only alphanumeric characters, underscores, and an optional @<endpoint-id> suffix" >&2
       exit 1
     fi
@@ -280,7 +280,7 @@ case "${1:-}" in
     VALUE="${3:?Usage: gstack-config set <key> <value>}"
     # Validate key (alphanumeric + underscore + optional @<endpoint-id> suffix).
     # Accepts hex hashes and the literal "local" from endpoint_hash.
-    if ! printf '%s' "$KEY" | grep -qE '^[a-zA-Z0-9_]+(@[a-zA-Z0-9]+)?$'; then
+    if ! printf '%s' "$KEY" | LC_ALL=C grep -qE '^[a-zA-Z0-9_]+(@[a-zA-Z0-9]+)?$'; then
       echo "Error: key must contain only alphanumeric characters, underscores, and an optional @<endpoint-id> suffix" >&2
       exit 1
     fi
diff --git a/test/gstack-config-key-locale.test.ts b/test/gstack-config-key-locale.test.ts
new file mode 100644
index 000000000..6fdbd5db1
--- /dev/null
+++ b/test/gstack-config-key-locale.test.ts
@@ -0,0 +1,76 @@
+/**
+ * Locale-independent key validation tests for bin/gstack-config.
+ *
+ * POSIX bracket ranges such as a-z follow the active collation order. Under
+ * GNU grep with tr_TR.UTF-8, that excludes the ASCII letter i and silently
+ * breaks most stored preferences. macOS BSD grep does not reproduce the bug,
+ * so the source-level tripwire pins the C-locale boundary on every platform.
+ */
+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 ROOT = path.resolve(import.meta.dir, "..");
+const CONFIG = path.join(ROOT, "bin", "gstack-config");
+
+let stateRoot: string;
+
+function run(args: string[]) {
+  const result = spawnSync(CONFIG, args, {
+    encoding: "utf8",
+    env: { ...process.env, GSTACK_STATE_ROOT: stateRoot },
+  });
+
+  return {
+    status: result.status ?? -1,
+    stdout: result.stdout ?? "",
+    stderr: result.stderr ?? "",
+  };
+}
+
+beforeEach(() => {
+  stateRoot = fs.mkdtempSync(path.join(os.tmpdir(), "gstack-config-locale-"));
+});
+
+afterEach(() => {
+  fs.rmSync(stateRoot, { recursive: true, force: true });
+});
+
+describe("gstack-config key validation is locale-independent", () => {
+  test("both get and set validate ASCII ranges under the C locale", () => {
+    const source = fs.readFileSync(CONFIG, "utf8");
+    const guardedValidators = source.match(
+      /LC_ALL=C grep -qE '\^\[a-zA-Z0-9_\]\+\(@\[a-zA-Z0-9\]\+\)\?\$'/g,
+    );
+
+    expect(guardedValidators).toHaveLength(2);
+  });
+
+  test("round-trips existing keys that contain i", () => {
+    expect(run(["set", "skill_prefix", "true"]).status).toBe(0);
+
+    const get = run(["get", "skill_prefix"]);
+    expect(get.status).toBe(0);
+    expect(get.stdout).toBe("true");
+  });
+
+  test("accepts an existing endpoint-scoped ASCII key", () => {
+    expect(run(["set", "brain_trust_policy@local", "personal"]).status).toBe(0);
+
+    const get = run(["get", "brain_trust_policy@local"]);
+    expect(get.status).toBe(0);
+    expect(get.stdout).toBe("personal");
+  });
+
+  test("continues to reject non-ASCII keys", () => {
+    const set = run(["set", "skïll_prefix", "true"]);
+    expect(set.status).toBe(1);
+    expect(set.stderr).toContain("key must contain only alphanumeric characters");
+
+    const get = run(["get", "skïll_prefix"]);
+    expect(get.status).toBe(1);
+    expect(get.stderr).toContain("key must contain only alphanumeric characters");
+  });
+});