Escape a C1 CSI in ANSI.format

CHANGELOG

@@ -1,6 +1,12 @@
 CHANGELOG
 =========
 
+Unreleased
+----------
+
+Fixes:
+- Escape a C1 CSI in ``ANSI.format`` so an argument cannot set a color (#2113).
+
 3.0.53: 2026-07-26
 ------------------
 

src/prompt_toolkit/formatted_text/ansi.py

@@ -291,7 +291,8 @@ def ansi_escape(text: object) -> str:
     """
     Replace characters with a special meaning.
     """
-    return str(text).replace("\x1b", "?").replace("\b", "?")
+    # \x9b is the C1 form of ESC [. Leaving it lets format() set a color.
+    return str(text).replace("\x1b", "?").replace("\x9b", "?").replace("\b", "?")
 
 
 class ANSIFormatter(Formatter):

tests/test_formatted_text.py

@@ -210,6 +210,38 @@ def test_ansi_interpolation():
     ]
 
 
+def test_ansi_c1_interpolation():
+    # A C1 CSI in an argument must not set a color. The template still can.
+    value = ANSI("\x9b32mOK")
+    assert to_formatted_text(value) == [
+        ("ansigreen", "O"),
+        ("ansigreen", "K"),
+    ]
+
+    value = ANSI("x{}").format("\x9b31mRED")
+    assert to_formatted_text(value) == [
+        ("", "x"),
+        ("", "?"),
+        ("", "3"),
+        ("", "1"),
+        ("", "m"),
+        ("", "R"),
+        ("", "E"),
+        ("", "D"),
+    ]
+
+    value = ANSI("%s") % "\x9b31mRED"
+    assert to_formatted_text(value) == [
+        ("", "?"),
+        ("", "3"),
+        ("", "1"),
+        ("", "m"),
+        ("", "R"),
+        ("", "E"),
+        ("", "D"),
+    ]
+
+
 def test_interpolation():
     value = Template(" {} ").format(HTML("<b>hello</b>"))