ralph-loop: setup script crashes with unbound variable when no prompt provided

[Pran-Ker]

Bug

Running /ralph-loop:ralph-loop without a prompt argument crashes with:

setup-ralph-loop.sh: line 113: PROMPT_PARTS[*]: unbound variable

Root Cause

The script uses set -euo pipefail (line 6), which includes -u (treat unbound variables as errors). When no positional arguments are passed, PROMPT_PARTS remains an empty array initialized on line 9:

PROMPT_PARTS=()

On line 113, the expansion ${PROMPT_PARTS[*]} triggers the unbound variable error because bash's set -u treats empty arrays as unbound:

PROMPT="${PROMPT_PARTS[*]}"

This crashes before the helpful usage message on lines 116–127 can be displayed.

Expected Behavior

Running /ralph-loop:ralph-loop without arguments should display the usage instructions:

❌ Error: No prompt provided

   Ralph needs a task description to work on.

   Examples:
     /ralph-loop Build a REST API for todos
     /ralph-loop Fix the auth bug --max-iterations 20
     ...

Suggested Fix

Replace line 113 with a safe expansion that handles empty arrays:

PROMPT="${PROMPT_PARTS[*]:-}"

The :- default value syntax prevents the unbound variable error while still allowing the empty check on line 116 to work correctly.

Environment

• macOS (Darwin 25.3.0)
• bash with set -euo pipefail
• Plugin version: d5c15b861cd2 (commit d5c15b861cd23e3102215c26020368ad5134dc47)