Dry Run Skill

Executes any command or skill in simulation mode, preventing all file modifications except session state.

Usage

/dry-run <any command or prompt>

Examples:

• /dry-run /mux-ospec path/to/spec.md - Preview full spec workflow
• /dry-run /spec IMPLEMENT path/to/spec.md - Test implementation without changes
• /dry-run why did you implement X this way? - Normal chat (no files affected)

Workflow

Step 1: Initialize Session Status

Resolve the Claude Code PID using process-tree tracing (same logic as the dry-run hook), then create the session directory:

CLAUDE_PID=$(python3 - <<'PY'
import os
import subprocess

pid = os.getpid()
for _ in range(10):
    result = subprocess.run(
        ["ps", "-o", "pid=,ppid=,comm=", "-p", str(pid)],
        capture_output=True,
        text=True,
    )
    line = result.stdout.strip()
    if not line:
        break

    parts = line.split()
    if len(parts) >= 3:
        current_pid, ppid, comm = int(parts[0]), int(parts[1]), parts[2]
        if "claude" in comm.lower():
            print(current_pid)
            break
        pid = ppid
    else:
        break
else:
    print("shared")
PY
)

[ -n "$CLAUDE_PID" ] || CLAUDE_PID="shared"
mkdir -p "outputs/session/${CLAUDE_PID}"

Check if outputs/session/<CLAUDE_PID>/status.yml exists. If not, create with initial schema:

dry_run: false

Step 2: Set Dry-Run Mode

Update outputs/session/<CLAUDE_PID>/status.yml:

dry_run: true

This signals to THIS session's file operations that writes are prohibited. Other Claude sessions (different PIDs) are not affected.

Step 3: Execute Delegated Command

Execute the provided command/prompt EXACTLY as given. All behavior remains normal EXCEPT:

CRITICAL CONSTRAINTS:

• NO file modifications allowed (Read, Grep, Glob, LSP, Bash read-only commands OK)
• ONLY exception: outputs/session/<CLAUDE_PID>/status.yml can be modified
• If command requires file writes, DESCRIBE what WOULD be changed instead
• For chat-only prompts (no file operations needed), respond normally

Step 4: Reset Dry-Run Mode

After execution completes (success or failure), reset state in outputs/session/<CLAUDE_PID>/status.yml:

dry_run: false

Step 5: Report Results

Provide summary:

• What was executed
• What file changes WOULD have occurred (if any)
• Verification that dry_run mode is reset

Implementation Notes

The skill acts as a wrapper:

1. It does NOT interpret or execute the delegated work itself
2. It sets the flag, then lets normal AI behavior handle the prompt
3. The dry_run flag is checked by pretooluse hook (hard enforcement at tool level)
4. After completion, it ensures cleanup

State Schema

outputs/session/<claude_pid>/status.yml:

dry_run: bool  # true = prevent file writes, false = normal mode

Session isolation by Claude PID ensures parallel agents don't interfere with each other. Future extensions may add additional session state fields.