diff --git a/CHANGELOG.md b/CHANGELOG.md
index 983a520f..bdbe7a1d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,9 @@
 
 ## Unreleased
 
+### Changed
+- Core string assertions (`assert_contains`/`assert_not_contains`, `assert_matches`/`assert_not_matches`, `assert_string_starts_with`/`assert_string_ends_with` and their negations) no longer fork a subshell per call to join their arguments; a fork-free join with identical behaviour replaces it (#844)
+
 ## [0.42.0](https://github.com/TypedDevs/bashunit/compare/0.41.0...0.42.0) - 2026-07-20
 
 ### Fixed
diff --git a/src/assert.sh b/src/assert.sh
index a7d309cf..6436fb5b 100755
--- a/src/assert.sh
+++ b/src/assert.sh
@@ -28,6 +28,21 @@ function bashunit::assert::label_to_slot() {
   _BASHUNIT_ASSERT_LABEL_OUT=$_BASHUNIT_HELPER_NORMALIZED_OUT
 }
 
+_BASHUNIT_ASSERT_JOINED_OUT=""
+
+# Join positional args into _BASHUNIT_ASSERT_JOINED_OUT with no fork.
+# Mirrors $(printf '%s\n' "$@"): joins with newlines and strips trailing
+# newlines, so callers that match on the result behave exactly as the previous
+# command-substitution did.
+function bashunit::assert::join_to_slot() {
+  local IFS=$'\n'
+  local joined="$*"
+  while [ "$joined" != "${joined%$'\n'}" ]; do
+    joined="${joined%$'\n'}"
+  done
+  _BASHUNIT_ASSERT_JOINED_OUT=$joined
+}
+
 # Resolve assertion label: use custom label if provided, otherwise derive from test function name
 function bashunit::assert::label() {
   bashunit::assert::label_to_slot "${1:-}"
@@ -273,8 +288,8 @@ function assert_contains() {
   local -a actual_arr
   actual_arr=("${@:2}")
   local label_override=""
-  local actual
-  actual=$(printf '%s\n' "${actual_arr[@]}")
+  bashunit::assert::join_to_slot "${actual_arr[@]}"
+  local actual=$_BASHUNIT_ASSERT_JOINED_OUT
 
   case "$actual" in
   *"$expected"*) ;;
@@ -326,8 +341,8 @@ function assert_not_contains() {
   local expected="$1"
   local -a actual_arr
   actual_arr=("${@:2}")
-  local actual
-  actual=$(printf '%s\n' "${actual_arr[@]}")
+  bashunit::assert::join_to_slot "${actual_arr[@]}"
+  local actual=$_BASHUNIT_ASSERT_JOINED_OUT
 
   case "$actual" in
   *"$expected"*)
@@ -349,8 +364,8 @@ function assert_matches() {
   local expected="$1"
   local -a actual_arr
   actual_arr=("${@:2}")
-  local actual
-  actual=$(printf '%s\n' "${actual_arr[@]}")
+  bashunit::assert::join_to_slot "${actual_arr[@]}"
+  local actual=$_BASHUNIT_ASSERT_JOINED_OUT
 
   if [ "$(printf '%s' "$actual" | "$GREP" -cE "$expected" || true)" -eq 0 ]; then
     # Retry with newlines collapsed for cross-line patterns
@@ -375,8 +390,8 @@ function assert_not_matches() {
   local expected="$1"
   local -a actual_arr
   actual_arr=("${@:2}")
-  local actual
-  actual=$(printf '%s\n' "${actual_arr[@]}")
+  bashunit::assert::join_to_slot "${actual_arr[@]}"
+  local actual=$_BASHUNIT_ASSERT_JOINED_OUT
 
   # Check both line-by-line and with newlines collapsed for cross-line patterns
   if [ "$(printf '%s' "$actual" | "$GREP" -cE "$expected" || true)" -gt 0 ] ||
@@ -652,8 +667,8 @@ function assert_string_starts_with() {
   local expected="$1"
   local -a actual_arr
   actual_arr=("${@:2}")
-  local actual
-  actual=$(printf '%s\n' "${actual_arr[@]}")
+  bashunit::assert::join_to_slot "${actual_arr[@]}"
+  local actual=$_BASHUNIT_ASSERT_JOINED_OUT
 
   case "$actual" in
   "$expected"*) ;;
@@ -697,8 +712,8 @@ function assert_string_ends_with() {
   local expected="$1"
   local -a actual_arr
   actual_arr=("${@:2}")
-  local actual
-  actual=$(printf '%s\n' "${actual_arr[@]}")
+  bashunit::assert::join_to_slot "${actual_arr[@]}"
+  local actual=$_BASHUNIT_ASSERT_JOINED_OUT
 
   case "$actual" in
   *"$expected") ;;
@@ -722,8 +737,8 @@ function assert_string_not_ends_with() {
   local expected="$1"
   local -a actual_arr
   actual_arr=("${@:2}")
-  local actual
-  actual=$(printf '%s\n' "${actual_arr[@]}")
+  bashunit::assert::join_to_slot "${actual_arr[@]}"
+  local actual=$_BASHUNIT_ASSERT_JOINED_OUT
 
   case "$actual" in
   *"$expected")
diff --git a/tests/unit/assert_string_test.sh b/tests/unit/assert_string_test.sh
index 3bd2720f..6b110cf1 100644
--- a/tests/unit/assert_string_test.sh
+++ b/tests/unit/assert_string_test.sh
@@ -142,6 +142,28 @@ function test_unsuccessful_assert_string_not_ends_with() {
     "$(assert_string_not_ends_with "bar" "foobar")"
 }
 
+# Characterization: the haystack join must strip trailing newlines (the behavior
+# $(printf '%s\n' ...) gave), preserve internal newlines, and join multiple args
+# with newlines. These pin the contract the fork-free join must keep.
+function test_assert_contains_ignores_trailing_newlines_in_haystack() {
+  assert_empty "$(assert_contains "bar" "$(printf 'foobar')"$'\n')"
+  assert_empty "$(assert_contains "bar" "foobar"$'\n\n\n')"
+}
+
+function test_assert_string_ends_with_ignores_trailing_newlines_in_haystack() {
+  assert_empty "$(assert_string_ends_with "bar" "foobar"$'\n')"
+  assert_empty "$(assert_string_ends_with "bar" "foobar"$'\n\n\n')"
+}
+
+function test_assert_contains_preserves_internal_newlines_in_haystack() {
+  assert_empty "$(assert_contains "b" "a"$'\n'"b"$'\n'"c")"
+}
+
+function test_assert_contains_joins_multiple_haystack_args_with_newlines() {
+  assert_empty "$(assert_contains "b" "a" "b" "c")"
+  assert_empty "$(assert_string_ends_with "c" "a" "b" "c")"
+}
+
 function test_assert_string_start_end_with_special_chars() {
   assert_empty "$(assert_string_starts_with "foo." "foo.bar")"
   assert_empty "$(assert_string_ends_with ".bar" "foo.bar")"