diff --git a/src/mcp/client/auth/oauth2.py b/src/mcp/client/auth/oauth2.py
index c513a99b46..594760d6fe 100644
--- a/src/mcp/client/auth/oauth2.py
+++ b/src/mcp/client/auth/oauth2.py
@@ -188,7 +188,10 @@ def is_token_valid(self) -> bool:
         return bool(
             self.current_tokens
             and self.current_tokens.access_token
-            and (not self.token_expiry_time or time.time() <= self.token_expiry_time)
+            # None means the authorization server declared no expiry. Compare against that
+            # explicitly: a falsy test would also treat an expiry of 0.0 (the epoch, long
+            # expired) as "no expiry" and keep presenting a dead token.
+            and (self.token_expiry_time is None or time.time() <= self.token_expiry_time)
         )
 
     def can_refresh_token(self) -> bool:
diff --git a/src/mcp/shared/auth_utils.py b/src/mcp/shared/auth_utils.py
index 3ba880f40d..8665b4fa60 100644
--- a/src/mcp/shared/auth_utils.py
+++ b/src/mcp/shared/auth_utils.py
@@ -65,16 +65,18 @@ def check_resource_allowed(requested_resource: str, configured_resource: str) ->
     return requested_path.startswith(configured_path)
 
 
-def calculate_token_expiry(expires_in: int | str | None) -> float | None:
+def calculate_token_expiry(expires_in: int | float | str | None) -> float | None:
     """Calculate token expiry timestamp from expires_in seconds.
 
     Args:
-        expires_in: Seconds until token expiration (may be string from some servers)
+        expires_in: Seconds until token expiration (may be a string or a float from some servers)
 
     Returns:
         Unix timestamp when token expires, or None if no expiry specified
     """
     if expires_in is None:
         return None  # pragma: no cover
-    # Defensive: handle servers that return expires_in as string
-    return time.time() + int(expires_in)
+    # Defensive: handle servers that return expires_in as a string, including ones that
+    # send a fractional value such as "3599.5". int() would raise ValueError on those and
+    # silently truncate the float case, expiring the token up to a second early.
+    return time.time() + float(expires_in)
diff --git a/tests/client/test_auth.py b/tests/client/test_auth.py
index 18a1566705..909936c98e 100644
--- a/tests/client/test_auth.py
+++ b/tests/client/test_auth.py
@@ -253,6 +253,19 @@ async def test_token_validity_checking(self, oauth_provider: OAuthClientProvider
         context.client_info = None
         assert not context.can_refresh_token()
 
+    @pytest.mark.anyio
+    async def test_token_expiry_of_zero_is_expired(self, oauth_provider: OAuthClientProvider, valid_tokens: OAuthToken):
+        """An expiry timestamp of 0 means expired, not 'no expiry'.
+
+        `token_expiry_time` is a float|None where None means the server declared no
+        expiry. Testing it for truthiness conflates 0.0 (the epoch, long expired) with
+        absence, so a token whose expiry resolves to 0 is treated as never expiring.
+        """
+        context = oauth_provider.context
+        context.current_tokens = valid_tokens
+        context.token_expiry_time = 0.0
+        assert not context.is_token_valid()
+
     def test_clear_tokens(self, oauth_provider: OAuthClientProvider, valid_tokens: OAuthToken):
         """Test clear_tokens() removes token data."""
         context = oauth_provider.context
diff --git a/tests/shared/test_auth_utils.py b/tests/shared/test_auth_utils.py
index 5ae0e22b0c..bcbd3703d0 100644
--- a/tests/shared/test_auth_utils.py
+++ b/tests/shared/test_auth_utils.py
@@ -1,8 +1,10 @@
 """Tests for OAuth 2.0 Resource Indicators utilities."""
 
+import time
+
 from pydantic import HttpUrl
 
-from mcp.shared.auth_utils import check_resource_allowed, resource_url_from_server_url
+from mcp.shared.auth_utils import calculate_token_expiry, check_resource_allowed, resource_url_from_server_url
 
 # Tests for resource_url_from_server_url function
 
@@ -121,3 +123,52 @@ def test_check_resource_allowed_empty_paths():
     assert check_resource_allowed("https://example.com", "https://example.com") is True
     assert check_resource_allowed("https://example.com/", "https://example.com") is True
     assert check_resource_allowed("https://example.com/api", "https://example.com") is True
+
+
+# Tests for calculate_token_expiry function
+
+
+def test_calculate_token_expiry_none_returns_none():
+    """A server that declares no expiry yields None, not a timestamp."""
+    assert calculate_token_expiry(None) is None
+
+
+def test_calculate_token_expiry_integer_seconds():
+    """The common case: expires_in arrives as an integer."""
+    before = time.time()
+    expiry = calculate_token_expiry(3600)
+    after = time.time()
+    assert expiry is not None
+    assert before + 3600 <= expiry <= after + 3600
+
+
+def test_calculate_token_expiry_numeric_string():
+    """Some servers emit expires_in as a JSON string."""
+    before = time.time()
+    expiry = calculate_token_expiry("3600")
+    after = time.time()
+    assert expiry is not None
+    assert before + 3600 <= expiry <= after + 3600
+
+
+def test_calculate_token_expiry_fractional_string():
+    """A fractional string must round, not raise.
+
+    The declared signature accepts str, and the docstring promises the string case is
+    handled defensively, but int("3599.5") raises ValueError. Servers that send a
+    fractional expires_in as a string fail token setup instead of expiring 0.5s later.
+    """
+    before = time.time()
+    expiry = calculate_token_expiry("3599.5")
+    after = time.time()
+    assert expiry is not None
+    assert before + 3599.5 <= expiry <= after + 3599.5
+
+
+def test_calculate_token_expiry_float():
+    """A float expires_in keeps its fraction instead of being truncated."""
+    before = time.time()
+    expiry = calculate_token_expiry(3599.5)
+    after = time.time()
+    assert expiry is not None
+    assert before + 3599.5 <= expiry <= after + 3599.5