diff --git a/tests/test_tag_value_invariants.py b/tests/test_tag_value_invariants.py
new file mode 100644
index 0000000000..41cc3c3d35
--- /dev/null
+++ b/tests/test_tag_value_invariants.py
@@ -0,0 +1,23 @@
+import pytest
+
+def sanitize_sentry_tag(key: str, value: str, max_len: int = 200) -> tuple[str, str] | None:
+    if not key or not isinstance(key, str):
+        return None
+    k = key.strip()
+    if not k:
+        return None
+    v = str(value) if value is not None else ""
+    if len(v) > max_len:
+        v = v[:max_len]
+    return k, v
+
+def test_valid_tag():
+    assert sanitize_sentry_tag("environment", "production") == ("environment", "production")
+
+def test_truncated_tag():
+    long_str = "x" * 250
+    k, v = sanitize_sentry_tag("payload_id", long_str)
+    assert len(v) == 200
+
+def test_invalid_key():
+    assert sanitize_sentry_tag("", "production") is None