diff --git a/CHANGELOG.md b/CHANGELOG.md
index 68f2507..7f1e710 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -25,6 +25,13 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   - `.github/dependabot.yml` — monthly GitHub Actions updates
   - README badges, attribution line, and Contributing/Security/License/Citation sections
 
+### Fixed
+- `ralph-loop.sh` no longer crashes on macOS (bash 3.2) with
+  `mem_limit_prefix[@]: unbound variable`. Empty-array expansions now use the
+  `${arr[@]:+"${arr[@]}"}` idiom, which is safe under `set -u` on bash < 4.4
+  (macOS default) and expands identically to `"${arr[@]}"` when the RAM-limit
+  prefix is populated on Linux.
+
 ### Changed
 - `NOTICE`, source headers and `CITATION.cff` declare **César Gallego Rodríguez**
   as the original author and copyright holder, with a link to the original
diff --git a/ralph-loop.sh b/ralph-loop.sh
index f588c20..596d602 100755
--- a/ralph-loop.sh
+++ b/ralph-loop.sh
@@ -305,7 +305,7 @@ run_tool() {
       if [ "$ralph_thinking" = false ]; then
         extra_flags=(-c 'model_reasoning_summary="none"' -c hide_agent_reasoning=true)
       fi
-      "${mem_limit_prefix[@]}" "$tool_command" exec "${tool_flags[@]}" \
+      ${mem_limit_prefix[@]:+"${mem_limit_prefix[@]}"} "$tool_command" exec "${tool_flags[@]}" \
         -m "$tool_model" \
         -o "$current_console_output" \
         -c "model_reasoning_effort=\"$tool_reasoning_effort\"" \
@@ -319,7 +319,7 @@ run_tool() {
       else
         env_prefix=(env -u CLAUDE_CODE_DISABLE_THINKING "CLAUDE_CODE_EFFORT_LEVEL=$tool_reasoning_effort")
       fi
-      "${mem_limit_prefix[@]}" "${env_prefix[@]}" "$tool_command" "${tool_flags[@]}" --model "$tool_model" --effort "$tool_reasoning_effort" -p <"$prompt_path"
+      ${mem_limit_prefix[@]:+"${mem_limit_prefix[@]}"} "${env_prefix[@]}" "$tool_command" "${tool_flags[@]}" --model "$tool_model" --effort "$tool_reasoning_effort" -p <"$prompt_path"
       ;;
     gemini)
       gemini_settings_dir=$(mktemp -d "$RALPH_LOCAL_DIR/gemini-settings.XXXXXX") || return 1
@@ -343,7 +343,7 @@ run_tool() {
         printf '}\n'
       } >"$gemini_settings_file"
 
-      "${mem_limit_prefix[@]}" \
+      ${mem_limit_prefix[@]:+"${mem_limit_prefix[@]}"} \
         env GEMINI_CLI_SYSTEM_SETTINGS_PATH="$gemini_settings_file" \
         "$tool_command" "${tool_flags[@]}" --model ralph-selected <"$prompt_path"
       run_exit_code=$?
@@ -354,7 +354,7 @@ run_tool() {
       # Devin reads the prompt from --prompt-file, not stdin (piping the prompt
       # makes it drop into REPL mode and panic). It has no reasoning/thinking
       # knob, so the capability tier only selects the model. -p is print mode.
-      "${mem_limit_prefix[@]}" "$tool_command" "${tool_flags[@]}" \
+      ${mem_limit_prefix[@]:+"${mem_limit_prefix[@]}"} "$tool_command" "${tool_flags[@]}" \
         --model "$tool_model" --prompt-file "$prompt_path" -p
       ;;
   esac
diff --git a/tests/ralph-loop.bats b/tests/ralph-loop.bats
index 1343125..a306b7e 100644
--- a/tests/ralph-loop.bats
+++ b/tests/ralph-loop.bats
@@ -52,6 +52,16 @@ teardown() {
   [[ "$output" == *"PROMPT_FILE must exist"* ]]
 }
 
+@test "mem_limit_prefix expansions are safe under set -u (bash 3.2)" {
+  # Regression for the macOS bash 3.2 crash: expanding an empty array with
+  # "${arr[@]}" under set -u raises "unbound variable" on bash < 4.4. Every
+  # mem_limit_prefix expansion must use the ${arr[@]:+"${arr[@]}"} idiom.
+  # An unsafe expansion is a bare "${mem_limit_prefix[@]}" (quote preceded by
+  # whitespace); the safe idiom has the quote preceded by ':+'.
+  run grep -nE '[[:space:]]"\$\{mem_limit_prefix\[@\]\}"' "$SCRIPT"
+  [ "$status" -ne 0 ]
+}
+
 @test "stop.md present at startup: exits 0 without running an agent" {
   echo "do something" > prompt.md
   echo "stop" > stop.md