diff --git a/server.py b/server.py
index 6aee04a..4d0fa98 100644
--- a/server.py
+++ b/server.py
@@ -92,6 +92,8 @@ async def middleware(self, request: web.Request, handler) -> web.Response:
         return await handler(request)
 
     def _is_valid_key(self, candidate_key: str) -> bool:
+        if not candidate_key.isascii():
+            return False
         valid = False
         for stored_key in self._keys:
             valid |= secrets.compare_digest(candidate_key, stored_key)
@@ -101,6 +103,8 @@ def _is_valid_key(self, candidate_key: str) -> bool:
     def _validated_key(key: str) -> str:
         if not key.strip():
             raise ValueError("API key must not be blank")
+        if not key.isascii():
+            raise ValueError("API key must contain only ASCII characters")
         return key
 
     def add_key(self, key: str) -> None:
diff --git a/tests/test_api_auth.py b/tests/test_api_auth.py
index 4bbe94f..3038663 100644
--- a/tests/test_api_auth.py
+++ b/tests/test_api_auth.py
@@ -79,6 +79,15 @@ async def test_malformed_header_returns_401(self, aiohttp_client, auth):
         resp = await client.get("/test", headers={"Authorization": "Basic abc123"})
         assert resp.status == 401
 
+    async def test_non_ascii_bearer_token_fails_closed_without_rate_log_entry(self, aiohttp_client, auth):
+        client = await aiohttp_client(_make_app(auth))
+
+        resp = await client.get("/test", headers={"Authorization": "Bearer é"})
+
+        assert resp.status == 403
+        assert await resp.json() == {"error": "Invalid API key."}
+        assert "é" not in auth._request_log
+
     def test_blank_configured_key_is_rejected(self):
         with pytest.raises(ValueError, match="API key must not be blank"):
             APIKeyAuth(api_keys=[""])
@@ -87,6 +96,10 @@ def test_whitespace_configured_key_is_rejected(self):
         with pytest.raises(ValueError, match="API key must not be blank"):
             APIKeyAuth(api_keys=["   \t"])
 
+    def test_non_ascii_configured_key_is_rejected(self):
+        with pytest.raises(ValueError, match="API key must contain only ASCII characters"):
+            APIKeyAuth(api_keys=["é"])
+
 
 class TestAPIKeyConstantTimeComparison:
     def test_helper_compares_candidate_against_each_stored_key_without_self_compare(self, monkeypatch):
@@ -126,6 +139,19 @@ def fake_compare_digest(left, right):
             ("matching-key", "other-key"),
         ]
 
+    def test_non_ascii_candidate_key_returns_false_without_comparing(self, monkeypatch):
+        auth = APIKeyAuth(api_keys=["test-key-123", "another-key"])
+        calls = []
+
+        def fake_compare_digest(left, right):
+            calls.append((left, right))
+            return left == right
+
+        monkeypatch.setattr(server.secrets, "compare_digest", fake_compare_digest)
+
+        assert auth._is_valid_key("é") is False
+        assert calls == []
+
 
 class TestAPIKeyManagement:
     def test_add_key(self):
@@ -147,6 +173,12 @@ def test_add_key_rejects_whitespace_key(self):
             auth.add_key("  \n")
         assert auth.enabled is False
 
+    def test_add_key_rejects_non_ascii_key(self):
+        auth = APIKeyAuth()
+        with pytest.raises(ValueError, match="API key must contain only ASCII characters"):
+            auth.add_key("é")
+        assert auth.enabled is False
+
     def test_revoke_key(self):
         auth = APIKeyAuth(api_keys=["only-key"])
         assert auth.enabled is True