diff --git a/CHANGELOG.md b/CHANGELOG.md
index 70728ba8..b3fec0b6 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -10,6 +10,7 @@
 - Performance: a test in a file that defines `set_up` or `tear_down` is about 2.6x faster, which brings it level with a hookless test (14.1ms to 5.4ms per test on macOS arm64, bash 3.2). Each hook minted its output file with `mktemp` and removed it with `rm`, and the ownership marker that left behind made the runner `rm -rf` the test's temp files at exit — five forks per test, even for a test that created no temp file of its own (#1345)
 
 ### Fixed
+- `assert_contains_ignore_case` documents what it actually guarantees: ASCII case folding. Whether `ñ` matches `Ñ` depends on the shell version, the locale and which `tr` the system ships — BSD `tr` folds it, GNU and busybox `tr` do not, and `nocasematch` only folds it in a UTF-8 locale. Behaviour is unchanged (#1351)
 - A test function whose name another selected name starts with is no longer rejected as a duplicate. `test_a` alongside `test_ab` read as one name twice, which made the whole file select nothing. Only definition order reached it, so nothing hit it before (#1347)
 - Coverage counts every line of a multiline statement: array literals, quoted strings, heredocs, backslash continuations. An array written one element per line cost one uncovered line per element. Bash 3.x records the assignment on its closing `)`, so there the whole array read as uncovered. A parent statement's hits no longer mark commands inside command or process substitutions as covered, even within quotes or arrays (#1338)
 - `--exclude-filter` keeps a comma as part of the test function name. The value was split at the comma, so `--exclude-filter 'test_a,{b}'` also excluded `test_a`. Repeat the flag to exclude several names. `BASHUNIT_EXCLUDE_FILTER` stays comma-separated, so one of its filters cannot hold a literal comma (#1340)
diff --git a/docs/assertions.md b/docs/assertions.md
index b9ff3228..e93e4fec 100644
--- a/docs/assertions.md
+++ b/docs/assertions.md
@@ -200,6 +200,8 @@ function test_failure() {
 
 Reports an error if `needle` is not a substring of `haystack`.
 Differences in casing are ignored when needle is searched for in haystack.
+Case folding is guaranteed for ASCII only: whether `ñ` matches `Ñ` depends on
+the shell version, the locale and which `tr` the system ships.
 
 ::: code-group
 ```bash [Example]
diff --git a/src/assert/core.sh b/src/assert/core.sh
index d175a72a..3f2c3951 100755
--- a/src/assert/core.sh
+++ b/src/assert/core.sh
@@ -462,10 +462,22 @@ function assert_contains_ignore_case() {
 
   # nocasematch (Bash 3.1+) folds case inside the `case` itself, which costs no
   # fork at all; the two `tr` pipelines below cost two. Measured on Bash 3.2:
-  # 0.087ms per call versus 12.8ms. Both fold non-ASCII identically in a UTF-8
-  # locale -- `ñü` matches `ÑÜ` either way -- which rules out the tempting
-  # pure-bash A-Z loop, since that is ASCII-only and would silently stop
-  # matching accented text that matches today.
+  # 0.087ms per call versus 12.8ms.
+  #
+  # Only ASCII folding is promised, because only ASCII folding is portable.
+  # Measured on `ÑÜ`:
+  #
+  #   BSD tr (macOS)        folds      -> ñü
+  #   GNU tr (Debian)       does not   -> ÑÜ
+  #   busybox tr            does not   -> ÑÜ
+  #   nocasematch, UTF-8    folds      -> matches
+  #   nocasematch, LC_ALL=C does not   -> no match
+  #
+  # So the two branches disagree on non-ASCII wherever `tr` is not BSD, and the
+  # fast path alone disagrees with itself across locales. The Bash 3.0 job runs
+  # on Debian, so that is CI, not a hypothetical. This comment used to claim the
+  # two agreed on non-ASCII in a UTF-8 locale, and used that to rule out a
+  # pure-bash A-Z fold for the fallback; the claim was wrong (#1351).
   #
   # Prior state is saved and restored rather than blindly unset: nocasematch is
   # a global shell option and a user's test file may already have set it. `shopt
@@ -492,7 +504,8 @@ function assert_contains_ignore_case() {
     return
   fi
 
-  # Bash 3.0 only: nocasematch does not exist, so fold with tr.
+  # Bash 3.0 only: nocasematch does not exist, so fold with tr. ASCII-only in
+  # practice on GNU and busybox tr; see the note above.
   local expected_lower
   local actual_lower
   expected_lower=$(printf '%s' "$expected" | tr '[:upper:]' '[:lower:]')
diff --git a/tests/acceptance/snapshots/bashunit_test_sh.test_bashunit_should_display_all_assert_docs.snapshot b/tests/acceptance/snapshots/bashunit_test_sh.test_bashunit_should_display_all_assert_docs.snapshot
index adaacac3..6504b02f 100644
--- a/tests/acceptance/snapshots/bashunit_test_sh.test_bashunit_should_display_all_assert_docs.snapshot
+++ b/tests/acceptance/snapshots/bashunit_test_sh.test_bashunit_should_display_all_assert_docs.snapshot
@@ -53,6 +53,8 @@ Reports an error if `needle` is not a substring of `haystack`.
 
 Reports an error if `needle` is not a substring of `haystack`.
 Differences in casing are ignored when needle is searched for in haystack.
+Case folding is guaranteed for ASCII only: whether `ñ` matches `Ñ` depends on
+the shell version, the locale and which `tr` the system ships.
 
 
 ## assert_empty
diff --git a/tests/unit/assert/string_test.sh b/tests/unit/assert/string_test.sh
index d2b94298..03f816af 100644
--- a/tests/unit/assert/string_test.sh
+++ b/tests/unit/assert/string_test.sh
@@ -53,6 +53,19 @@ function test_successful_assert_contains_ignore_case() {
   assert_empty "$(assert_contains_ignore_case "Linux" "GNU/LINUX")"
 }
 
+# ASCII case folding is the whole of what this assertion promises, and it has
+# two implementations -- nocasematch from Bash 3.1, two `tr` pipelines below
+# that. This runs on both: the Bash 3.0 job takes the `tr` branch, every other
+# job the fast one. Non-ASCII is deliberately not asserted, because BSD `tr`
+# folds it, GNU and busybox `tr` do not, and nocasematch itself only folds it
+# in a UTF-8 locale (#1351).
+function test_assert_contains_ignore_case_folds_ascii_on_either_implementation() {
+  assert_empty "$(assert_contains_ignore_case "linux" "GNU/LINUX")"
+  assert_empty "$(assert_contains_ignore_case "LINUX" "gnu/linux")"
+  assert_empty "$(assert_contains_ignore_case "LiNuX" "GnU/lInUx")"
+  assert_empty "$(assert_contains_ignore_case "0-9_." "0-9_.")"
+}
+
 function test_unsuccessful_assert_contains_ignore_case() {
   local expected
   expected="$(bashunit::console_results::print_failed_test \