diff --git a/src/openai/auth/_workload.py b/src/openai/auth/_workload.py
index 4f9797f092..dbae7f6137 100644
--- a/src/openai/auth/_workload.py
+++ b/src/openai/auth/_workload.py
@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import math
 import time
 import threading
 from typing import Any, Generic, TypeVar, Callable, TypedDict, cast
@@ -305,9 +306,20 @@ def _handle_token_response(self, response: httpx2.Response) -> dict[str, Any]:
         )
 
     def _validate_expires_in(self, expires_in: object) -> float:
-        if not isinstance(expires_in, (int, float)):
+        # `bool` is a subclass of `int`, so guard against it explicitly to avoid
+        # treating `true`/`false` as a `1`/`0` second lifetime.
+        if isinstance(expires_in, bool) or not isinstance(expires_in, (int, float)):
             raise OpenAIError("Token exchange response did not include a valid expires_in")
-        return float(expires_in)
+        try:
+            expires_in_value = float(expires_in)
+        except OverflowError:
+            raise OpenAIError("Token exchange response did not include a valid expires_in") from None
+        # A non-positive lifetime yields an already-expired token, while a
+        # non-finite one (e.g. `inf`) would never trigger a refresh, leaving the
+        # client to reuse a token indefinitely after it has expired server-side.
+        if not math.isfinite(expires_in_value) or expires_in_value <= 0:
+            raise OpenAIError("Token exchange response did not include a valid expires_in")
+        return expires_in_value
 
     def _token_unusable(self) -> bool:
         return self._cached_token is None or self._token_expired()
diff --git a/tests/test_auth.py b/tests/test_auth.py
index fc717973e6..0d352c7961 100644
--- a/tests/test_auth.py
+++ b/tests/test_auth.py
@@ -7,7 +7,7 @@
 from inline_snapshot import snapshot
 
 from tests import respx2
-from openai import OpenAI, OAuthError
+from openai import OpenAI, OAuthError, OpenAIError
 from openai.auth import WorkloadIdentity, WorkloadIdentityAuth, SubjectTokenWorkloadIdentity
 from tests.respx2.models import Call
 from openai.auth._workload import (
@@ -166,6 +166,43 @@ def test_workload_identity_exchange_error() -> None:
     assert api_route.call_count == 0
 
 
+@respx2.mock
+@pytest.mark.parametrize("expires_in", [0, -1, True, "3600", None, 10**400])
+def test_workload_identity_rejects_nonpositive_or_nonnumeric_expiration(expires_in: object) -> None:
+    exchange_route = respx2.post("https://auth.openai.com/oauth/token").mock(
+        return_value=httpx2.Response(
+            200,
+            json={
+                "access_token": "fake_access_token",
+                "issued_token_type": "urn:ietf:params:oauth:token-type:access_token",
+                "token_type": "Bearer",
+                "expires_in": expires_in,
+            },
+        )
+    )
+    api_route = respx2.get("https://api.openai.com/v1/models").mock(
+        return_value=httpx2.Response(200, json={"data": [], "object": "list"})
+    )
+
+    client = OpenAI(
+        max_retries=0,
+        workload_identity={
+            "identity_provider_id": "idp_123",
+            "service_account_id": "sa_123",
+            "provider": {
+                "get_token": lambda: "fake_subject_token",
+                "token_type": "jwt",
+            },
+        },
+    )
+
+    with pytest.raises(OpenAIError, match="expires_in"):
+        client.models.list()
+
+    assert exchange_route.call_count == 1
+    assert api_route.call_count == 0
+
+
 def test_k8s_service_account_token_provider(tmp_path: Path) -> None:
     token_file = tmp_path / "token"
     token_file.write_text("my-k8s-token")