diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index 818ced2..59a70f2 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -207,6 +207,12 @@ add_executable(srgb_format_test srgb_format_test.cpp)
 target_include_directories(srgb_format_test PRIVATE ${CMAKE_SOURCE_DIR}/host/src)
 add_test(NAME srgb_format_test COMMAND srgb_format_test)
 
+# Unit test for the host's shared text helpers: contains_ci (the panels' case-insensitive filter)
+# and narrow/widen (UTF-8 <-> wide). Pure logic; header-only, no window / device.
+add_executable(text_util_test text_util_test.cpp)
+target_include_directories(text_util_test PRIVATE ${CMAKE_SOURCE_DIR}/host/src)
+add_test(NAME text_util_test COMMAND text_util_test)
+
 # Unit test for the keyed-mutex acquire decision (capture/keyed_mutex.hpp): WAIT_ABANDONED must be
 # treated as "acquired" so a mirror recovers after a host crash instead of freezing. Header-only.
 add_executable(keyed_mutex_test keyed_mutex_test.cpp)
@@ -405,6 +411,7 @@ coop_output_subdir(tests
 	audio_loopback_test
 	audio_hook_test
 	srgb_format_test
+	text_util_test
 	present_hook_test
 	dx12_present_hook_test
 	d3d9_hook_test
diff --git a/tests/text_util_test.cpp b/tests/text_util_test.cpp
new file mode 100644
index 0000000..2fde92d
--- /dev/null
+++ b/tests/text_util_test.cpp
@@ -0,0 +1,67 @@
+// Unit test for the host's shared text helpers: the case-insensitive filter predicate the Log and
+// Injection panels use (ui/text_match.hpp) and the UTF-8 <-> wide conversions (util/utf8.hpp) every
+// panel bridges Win32 strings through. Pure logic; no window / device.
+#include <cstdio>
+#include <string>
+
+#include "ui/text_match.hpp"
+#include "util/utf8.hpp"
+
+using namespace coop;
+
+namespace
+{
+int g_failures = 0;
+void check(bool ok, const char* what)
+{
+	std::printf("%s %s\n", ok ? "  ok:" : "FAIL:", what);
+	if (!ok)
+	{
+		++g_failures;
+	}
+}
+} // namespace
+
+int main()
+{
+	// --- contains_ci: the filter contract the panels rely on -------------------------------------
+	// An empty (or null) needle matches everything, so an empty filter box shows every row.
+	check(contains_ci("anything", ""), "empty needle matches");
+	check(contains_ci("anything", nullptr), "null needle matches");
+	check(contains_ci("", ""), "empty needle matches an empty haystack");
+
+	// Case-insensitive both ways (so typing 'error' finds 'ERROR', and 'ERR' finds 'stderr').
+	check(contains_ci("ERROR: disk full", "error"), "lowercase needle finds uppercase text");
+	check(contains_ci("stderr stream", "ERR"), "uppercase needle finds lowercase text");
+	check(contains_ci("MixedCase", "edca"), "case-folded substring across a case boundary");
+
+	// Substring position: start, middle, end, and whole-string.
+	check(contains_ci("game.exe", "game"), "match at the start");
+	check(contains_ci("coop_host.exe", "host"), "match in the middle");
+	check(contains_ci("coop_host.exe", ".exe"), "match at the end");
+	check(contains_ci("exact", "exact"), "whole-string match");
+
+	// Non-matches (so a filter actually hides non-matching rows).
+	check(!contains_ci("game.exe", "xyz"), "no false match");
+	check(!contains_ci("short", "longer than the haystack"), "needle longer than haystack");
+	check(!contains_ci("", "x"), "non-empty needle never matches an empty haystack");
+
+	// ascii_lower folds A-Z only and leaves other bytes untouched.
+	check(ascii_lower("AbC123!") == "abc123!", "ascii_lower folds letters, leaves digits/punct");
+
+	// --- narrow / widen: round-trip + boundary cases ---------------------------------------------
+	check(narrow(L"") == "", "narrow of an empty string is empty");
+	check(widen("") == L"", "widen of an empty string is empty");
+	check(narrow(L"game.exe") == "game.exe", "narrow of ASCII is byte-identical");
+	check(widen("game.exe") == L"game.exe", "widen of ASCII is byte-identical");
+
+	// Round-trip preserves the string (ASCII and a non-ASCII code point that must survive
+	// persist/reload of an override key -- the reason narrow/widen exist instead of a byte mask).
+	for (const wchar_t* s : {L"CoopAllTheThings", L"path\\to\\Game (2).exe", L"café.exe", L"游戏.exe"})
+	{
+		check(widen(narrow(std::wstring(s))) == std::wstring(s), "narrow->widen round-trips");
+	}
+
+	std::printf(g_failures == 0 ? "PASS text_util_test\n" : "FAILED text_util_test (%d)\n", g_failures);
+	return g_failures == 0 ? 0 : 1;
+}