From dafc86fdff45d48a456c246e022c1d493ac4a930 Mon Sep 17 00:00:00 2001
From: WatchTree-19 <119982314+WatchTree-19@users.noreply.github.com>
Date: Sun, 15 Nov 2026 02:22:06 +0000
Subject: [PATCH] Reject NaN increments on Counter

Counter.inc rejected negative amounts but accepted NaN, which left the
counter's total at NaN from then on. OpenMetrics requires a counter's
total to be non-NaN and monotonically non-decreasing, so a NaN increment
now raises ValueError and the total is left unchanged.

Signed-off-by: WatchTree-19 <119982314+WatchTree-19@users.noreply.github.com>
---
 prometheus_client/metrics.py | 2 ++
 tests/test_core.py           | 5 +++++
 2 files changed, 7 insertions(+)

diff --git a/prometheus_client/metrics.py b/prometheus_client/metrics.py
index a0ad9b92..8e0d74d4 100644
--- a/prometheus_client/metrics.py
+++ b/prometheus_client/metrics.py
@@ -347,6 +347,8 @@ def inc(self, amount: float = 1, exemplar: Optional[Dict[str, str]] = None) -> N
         self._raise_if_not_observable()
         if amount < 0:
             raise ValueError('Counters can only be incremented by non-negative amounts.')
+        if math.isnan(amount):
+            raise ValueError('Counters cannot be incremented by NaN.')
         self._value.inc(amount)
         if exemplar:
             _validate_exemplar(exemplar)
diff --git a/tests/test_core.py b/tests/test_core.py
index 2ed2da90..e0ec830a 100644
--- a/tests/test_core.py
+++ b/tests/test_core.py
@@ -71,6 +71,11 @@ def test_clear_without_labels_is_noop(self):
     def test_negative_increment_raises(self):
         self.assertRaises(ValueError, self.counter.inc, -1)
 
+    def test_nan_increment_raises(self):
+        self.counter.inc(2)
+        self.assertRaises(ValueError, self.counter.inc, float('nan'))
+        self.assertEqual(2, self.registry.get_sample_value('c_total'))
+
     def test_function_decorator(self):
         @self.counter.count_exceptions(ValueError)
         def f(r):