diff --git a/src/_pytest/assertion/truncate.py b/src/_pytest/assertion/truncate.py
index fb2377db3d3..7a03ab74136 100644
--- a/src/_pytest/assertion/truncate.py
+++ b/src/_pytest/assertion/truncate.py
@@ -4,7 +4,8 @@
 Current default behaviour is to truncate assertion explanations at
 ~8 terminal lines, unless running in "-vv" mode or running on CI.
 """
-import os
+from _pytest.assertion.util import _running_on_ci
+
 
 DEFAULT_MAX_LINES = 8
 DEFAULT_MAX_CHARS = 8 * 80
@@ -31,12 +32,6 @@ def _should_truncate_item(item):
     return int(level) > verbose
 
 
-def _running_on_ci():
-    """Check if we're currently running on a CI system."""
-    env_vars = ["CI", "BUILD_NUMBER"]
-    return any(var in os.environ for var in env_vars)
-
-
 def _truncate_explanation(input_lines, max_lines=None, max_chars=None):
     """
     Truncate given list of strings that makes up the assertion explanation.
diff --git a/src/_pytest/assertion/util.py b/src/_pytest/assertion/util.py
index 7521c08e4e4..27e3cb20ffa 100644
--- a/src/_pytest/assertion/util.py
+++ b/src/_pytest/assertion/util.py
@@ -1,5 +1,6 @@
 """Utilities for assertion debugging"""
 import collections.abc
+import os
 import pprint
 from typing import AbstractSet
 from typing import Any
@@ -461,3 +462,7 @@ def _notin_text(term: str, text: str, verbose: int = 0) -> List[str]:
         else:
             newdiff.append(line)
     return newdiff
+
+
+def _running_on_ci():
+    return os.environ.get("CI") == "true" or "BUILD_NUMBER" in os.environ
diff --git a/src/_pytest/terminal.py b/src/_pytest/terminal.py
index a4e4ea1ba05..f1819d904a3 100644
--- a/src/_pytest/terminal.py
+++ b/src/_pytest/terminal.py
@@ -26,6 +26,7 @@
 
 import pytest
 from _pytest import nodes
+from _pytest.assertion.util import _running_on_ci
 from _pytest.compat import shell_quote
 from _pytest.main import ExitCode
 
@@ -1048,8 +1049,7 @@ def short_test_summary(self) -> None:
         if not self.reportchars:
             return
 
-        # NOTE: there's also _pytest.assertion._running_on_ci.
-        if os.environ.get("CI") == "true" or not self.isatty:
+        if not self.isatty or _running_on_ci():
             termwidth = None
         else:
             termwidth = self._tw.fullwidth
diff --git a/testing/test_assertion.py b/testing/test_assertion.py
index 47d1ddafd9c..52284159e82 100644
--- a/testing/test_assertion.py
+++ b/testing/test_assertion.py
@@ -999,7 +999,13 @@ def test_many_lines():
         result = testdir.runpytest("-vv")
         result.stdout.fnmatch_lines(["* 6*"])
 
-        monkeypatch.setenv("CI", "1")
+        monkeypatch.setenv("CI", "")
+        result = testdir.runpytest()
+        result.stdout.fnmatch_lines(
+            ["E         ...Full output truncated (2 lines hidden), use '-vv' to show"]
+        )
+
+        monkeypatch.setenv("CI", "true")
         result = testdir.runpytest()
         result.stdout.fnmatch_lines(["* 6*"])