diff --git a/src/hai_agents_common/credentials.py b/src/hai_agents_common/credentials.py
index 97c8913..2a89bb0 100644
--- a/src/hai_agents_common/credentials.py
+++ b/src/hai_agents_common/credentials.py
@@ -78,12 +78,12 @@ def absolute_share_url(client: Client | AsyncClient, share_path: str) -> str:
 
 
 def save_api_key(key: str) -> Path:
-    """Persist the API key to the global `.env` (chmod 600) and the process env."""
-    GLOBAL_ENV_PATH.parent.mkdir(parents=True, exist_ok=True)
-    if not GLOBAL_ENV_PATH.exists():
-        GLOBAL_ENV_PATH.write_text("", encoding="utf-8")
-    with contextlib.suppress(OSError):
-        GLOBAL_ENV_PATH.chmod(0o600)
+    """Persist the API key to the global `.env` (file 600, directory 700) and the process env."""
+    GLOBAL_ENV_PATH.parent.mkdir(mode=0o700, parents=True, exist_ok=True)
+    GLOBAL_ENV_PATH.parent.chmod(0o700)  # mkdir leaves an existing directory's mode untouched, so we set it explicitly
+    # Create owner-only from the start
+    os.close(os.open(GLOBAL_ENV_PATH, os.O_WRONLY | os.O_CREAT, 0o600))
+    GLOBAL_ENV_PATH.chmod(0o600)  # tighten a file that already existed
     set_key(str(GLOBAL_ENV_PATH), API_KEY_VAR, key)
     os.environ[API_KEY_VAR] = key
     return GLOBAL_ENV_PATH
diff --git a/tests/test_credentials.py b/tests/test_credentials.py
index 8632ae4..dff6c05 100644
--- a/tests/test_credentials.py
+++ b/tests/test_credentials.py
@@ -1,5 +1,8 @@
 from __future__ import annotations
 
+import stat
+import sys
+
 import httpx
 import pytest
 from typer.testing import CliRunner
@@ -57,6 +60,27 @@ def test_save_then_clear_roundtrip(monkeypatch):
         credentials.resolve_api_key()
 
 
+@pytest.mark.skipif(sys.platform == "win32", reason="POSIX permissions")
+def test_saved_key_is_owner_only(tmp_path, monkeypatch):
+    directory = tmp_path / "hai"
+    monkeypatch.setattr(credentials, "GLOBAL_ENV_PATH", directory / ".env")
+    directory.mkdir(mode=0o755)
+    credentials.GLOBAL_ENV_PATH.write_text("OTHER=1\n")
+    credentials.GLOBAL_ENV_PATH.chmod(0o644)
+
+    credentials.save_api_key("hk-minted")
+
+    assert stat.S_IMODE(directory.stat().st_mode) == 0o700
+    assert stat.S_IMODE(credentials.GLOBAL_ENV_PATH.stat().st_mode) == 0o600
+
+    credentials.GLOBAL_ENV_PATH.unlink()
+    directory.rmdir()
+    credentials.save_api_key("hk-fresh")
+
+    assert stat.S_IMODE(directory.stat().st_mode) == 0o700
+    assert stat.S_IMODE(credentials.GLOBAL_ENV_PATH.stat().st_mode) == 0o600
+
+
 def test_absolute_share_url_prepends_base():
     class _Wrapper:
         def get_base_url(self) -> str:
diff --git a/tests/test_polling_helpers.py b/tests/test_polling_helpers.py
index 97e468c..025ca33 100644
--- a/tests/test_polling_helpers.py
+++ b/tests/test_polling_helpers.py
@@ -13,7 +13,6 @@
 import pytest
 
 from hai_agents.polling import (
-    MAX_REQUEST_BYTES,
     assert_request_under_limit,
     is_settled_session_status,
     is_terminal_session_status,
@@ -80,10 +79,6 @@ class Shot(pydantic.BaseModel):
         assert_request_under_limit(Shot(png="x" * 200), max_bytes=50)
 
 
-def test_default_limit_is_five_megabytes() -> None:
-    assert MAX_REQUEST_BYTES == 5 * 1024 * 1024
-
-
 def test_wait_times_out_on_wall_clock() -> None:
     client = SimpleNamespace(sessions=_AlwaysRunning())
     with pytest.raises(TimeoutError, match="did not settle within 0"):