diff --git a/api/libs/helper.py b/api/libs/helper.py
index 752342bfad6..0992553442a 100644
--- a/api/libs/helper.py
+++ b/api/libs/helper.py
@@ -289,7 +289,11 @@ UUIDStr = Annotated[str, AfterValidator(_strict_uuid)]
 
 def alphanumeric(value: str):
     # check if the value is alphanumeric and underlined
-    if re.match(r"^[a-zA-Z0-9_]+$", value):
+    # Use re.fullmatch instead of re.match to reject trailing newlines.
+    # In Python, '$' matches at end-of-string OR just before a trailing newline,
+    # so re.match accepts "tool_name\n". re.fullmatch requires the entire
+    # string to match. Regression for #39666 (sibling of #39234 / #39548).
+    if re.fullmatch(r"^[a-zA-Z0-9_]+$", value):
         return value
 
     raise ValueError(f"{value} is not a valid alphanumeric value")
diff --git a/api/tests/unit_tests/libs/test_helper.py b/api/tests/unit_tests/libs/test_helper.py
index dbc2cd6cba8..d5f78e67abc 100644
--- a/api/tests/unit_tests/libs/test_helper.py
+++ b/api/tests/unit_tests/libs/test_helper.py
@@ -2,7 +2,7 @@ from datetime import datetime
 
 import pytest
 
-from libs.helper import OptionalTimestampField, email, escape_like_pattern, extract_tenant_id
+from libs.helper import OptionalTimestampField, alphanumeric, email, escape_like_pattern, extract_tenant_id
 from models.account import Account
 from models.model import EndUser
 
@@ -153,3 +153,47 @@ class TestEmailValidator:
     def test_invalid_email_rejected(self):
         with pytest.raises(ValueError, match="not a valid email"):
             email("not-an-email")
+
+
+class TestAlphanumericValidator:
+    """Tests for the alphanumeric() validator — regression for #39666."""
+
+    def test_valid_alphanumeric_accepted(self):
+        assert alphanumeric("tool_name") == "tool_name"
+        assert alphanumeric("Tool123") == "Tool123"
+        assert alphanumeric("_underscore_start") == "_underscore_start"
+        assert alphanumeric("a") == "a"
+
+    def test_trailing_newline_rejected(self):
+        # re.match with $ accepts a trailing \n in Python; re.fullmatch does not.
+        # This was the pre-fix behaviour: alphanumeric("tool\n") returned "tool\n".
+        with pytest.raises(ValueError, match="not a valid alphanumeric value"):
+            alphanumeric("tool_name\n")
+
+    def test_trailing_carriage_return_rejected(self):
+        with pytest.raises(ValueError, match="not a valid alphanumeric value"):
+            alphanumeric("tool_name\r")
+
+    def test_trailing_crlf_rejected(self):
+        with pytest.raises(ValueError, match="not a valid alphanumeric value"):
+            alphanumeric("tool_name\r\n")
+
+    def test_leading_newline_rejected(self):
+        with pytest.raises(ValueError, match="not a valid alphanumeric value"):
+            alphanumeric("\ntool_name")
+
+    def test_embedded_whitespace_rejected(self):
+        with pytest.raises(ValueError, match="not a valid alphanumeric value"):
+            alphanumeric("tool name")
+
+    def test_empty_string_rejected(self):
+        with pytest.raises(ValueError, match="not a valid alphanumeric value"):
+            alphanumeric("")
+
+    def test_special_characters_rejected(self):
+        with pytest.raises(ValueError, match="not a valid alphanumeric value"):
+            alphanumeric("tool-name")
+        with pytest.raises(ValueError, match="not a valid alphanumeric value"):
+            alphanumeric("tool.name")
+        with pytest.raises(ValueError, match="not a valid alphanumeric value"):
+            alphanumeric("tool/name")