diff --git a/agent/ssl_guard.py b/agent/ssl_guard.py
index ac1b7841b2..35c3e47621 100644
--- a/agent/ssl_guard.py
+++ b/agent/ssl_guard.py
@@ -55,7 +55,13 @@ def _validate_bundle_path(label: str, value: str, *, require_substantial: bool =
         ctx = ssl.create_default_context(cafile=str(path))
     except Exception as exc:
         raise _ssl_err(f"{label} CA bundle at {value} cannot be loaded: {exc}") from exc
-    if not ctx.get_ca_certs():
+    try:
+        loaded_certs = ctx.get_ca_certs()
+    except NotImplementedError:
+        # truststore-backed SSLContext (Windows OS trust store) doesn't
+        # implement get_ca_certs(); bundle was already validated above.
+        return
+    if not loaded_certs:
         raise _ssl_err(f"{label} CA bundle at {value} did not load any certificates")
 
 
diff --git a/tests/agent/test_ssl_ca_guard.py b/tests/agent/test_ssl_ca_guard.py
index 4b9da7d32f..a381352a45 100644
--- a/tests/agent/test_ssl_ca_guard.py
+++ b/tests/agent/test_ssl_ca_guard.py
@@ -19,8 +19,6 @@ def test_healthy_bundle_passes(monkeypatch):
     verify_ca_bundle()
 
 
-
-
 def test_empty_certifi_bundle_raises_ssl_error(monkeypatch, tmp_path):
     """Empty file is treated as a corrupted bundle."""
     fake = tmp_path / "empty.pem"
@@ -44,7 +42,33 @@ def test_missing_explicit_ca_bundle_env_raises_before_httpx(monkeypatch, tmp_pat
     assert "force-reinstall" in message
 
 
+def test_truststore_get_ca_certs_not_implemented_is_accepted(monkeypatch, tmp_path):
+    """A truststore-backed SSLContext (Windows OS trust store) raises
+    NotImplementedError from get_ca_certs(). The guard must accept the
+    already-loaded bundle rather than fail.
 
+    Regression for the empty-message ``Failed to initialize OpenAI client:``
+    seen on every fresh agent init on Windows (str(NotImplementedError()) == "").
+    """
+    from agent import ssl_guard
 
+    bundle = tmp_path / "bundle.pem"
+    bundle.write_text(
+        "-----BEGIN CERTIFICATE-----\nfake\n-----END CERTIFICATE-----\n",
+        encoding="utf-8",
+    )
 
+    class _TruststoreLikeContext:
+        def get_ca_certs(self, binary_form=False):  # noqa: ARG002 - mirror ssl API
+            raise NotImplementedError()
 
+    # create_default_context(cafile=...) loads the bundle fine; only the
+    # post-load introspection is unsupported under truststore.
+    monkeypatch.setattr(
+        ssl_guard.ssl, "create_default_context", lambda *a, **k: _TruststoreLikeContext()
+    )
+    monkeypatch.setenv("SSL_CERT_FILE", str(bundle))
+
+    # Must not raise on the explicit env bundle nor the certifi check.
+    verify_ca_bundle()
+    verify_ca_bundle_with_fallback()