fix(logging): honor NO_COLOR / FORCE_COLOR / tty for non-JSON formatter

The plain (non-JSON) formatter hardcoded ANSI color codes, so redirected
logs and NO_COLOR-respecting environments got raw escape sequences. Add
_should_use_color(): NO_COLOR (no-color.org) wins, FORCE_COLOR overrides,
otherwise fall back to isatty() on the handler stream. Applied to the
CorrelationPlainFormatter selection. Adds tests for the toggle.

litellm/_logging.py

@@ -244,6 +244,17 @@ handler.addFilter(_secret_filter)
 handler.addFilter(_correlation_filter)
 
 
+def _should_use_color(stream: object = None) -> bool:
+    # NO_COLOR (https://no-color.org/) wins. FORCE_COLOR overrides. Otherwise tty-detect.
+    if os.getenv("NO_COLOR"):
+        return False
+    if os.getenv("FORCE_COLOR"):
+        return True
+    if stream is None:
+        stream = sys.stderr
+    return bool(getattr(stream, "isatty", lambda: False)())
+
+
 def _try_parse_json_message(message: str) -> dict[str, Any] | None:
     """
     Try to parse a log message as JSON. Returns parsed dict if valid, else None.
@@ -446,10 +457,11 @@ if json_logs:
     handler.setFormatter(JsonFormatter())
     _setup_json_exception_handlers(JsonFormatter())
 else:
-    formatter: Final = CorrelationPlainFormatter(
-        "\033[92m%(asctime)s - %(name)s:%(levelname)s\033[0m: %(filename)s:%(lineno)s - %(message)s",
-        datefmt="%H:%M:%S",
-    )
+    if _should_use_color(handler.stream):
+        fmt = "\033[92m%(asctime)s - %(name)s:%(levelname)s\033[0m: %(filename)s:%(lineno)s - %(message)s"
+    else:
+        fmt = "%(asctime)s - %(name)s:%(levelname)s: %(filename)s:%(lineno)s - %(message)s"
+    formatter: Final = CorrelationPlainFormatter(fmt, datefmt="%H:%M:%S")
 
     handler.setFormatter(formatter)
 

tests/documentation_tests/test_env_keys.py

@@ -48,6 +48,8 @@ EXCLUDED_TERMINAL_VARS = {
     "WT_SESSION",
     "GNOME_TERMINAL_SCREEN",
     "ALACRITTY_SOCKET",
+    "NO_COLOR",
+    "FORCE_COLOR",
 }
 
 EXCLUDED_KEYS = frozenset(EXCLUDED_TERMINAL_VARS | EXCLUDED_GUARD_ONLY_VARS | EXCLUDED_ROLLOUT_FLAGS)

tests/test_litellm/test_logging.py

@@ -831,3 +831,48 @@ def test_set_session_id_bounds_length():
         assert len(session_id_var.get()) == 256
     finally:
         session_id_var.reset(token)
+
+
+# --- color toggle (NO_COLOR / FORCE_COLOR / tty) -----------------------------
+
+
+class _FakeStream:
+    def __init__(self, is_tty: bool):
+        self._is_tty = is_tty
+
+    def isatty(self) -> bool:
+        return self._is_tty
+
+
+@pytest.fixture
+def clean_color_env(monkeypatch):
+    monkeypatch.delenv("NO_COLOR", raising=False)
+    monkeypatch.delenv("FORCE_COLOR", raising=False)
+
+
+def test_should_use_color_no_color_env_disables(monkeypatch, clean_color_env):
+    from litellm._logging import _should_use_color
+
+    monkeypatch.setenv("NO_COLOR", "1")
+    # Even on a tty + FORCE_COLOR, NO_COLOR must win.
+    monkeypatch.setenv("FORCE_COLOR", "1")
+    assert _should_use_color(_FakeStream(is_tty=True)) is False
+
+
+def test_should_use_color_force_color_enables_on_pipe(monkeypatch, clean_color_env):
+    from litellm._logging import _should_use_color
+
+    monkeypatch.setenv("FORCE_COLOR", "1")
+    assert _should_use_color(_FakeStream(is_tty=False)) is True
+
+
+def test_should_use_color_tty_default_on(clean_color_env):
+    from litellm._logging import _should_use_color
+
+    assert _should_use_color(_FakeStream(is_tty=True)) is True
+
+
+def test_should_use_color_pipe_default_off(clean_color_env):
+    from litellm._logging import _should_use_color
+
+    assert _should_use_color(_FakeStream(is_tty=False)) is False