Accidentally created my first fork bomb with Claude Code

TL;DR Highlight

A real incident where Claude Code's SessionStart hook recursively spawned infinite Claude instances, creating a fork bomb that crashed a computer overnight and nearly resulted in a shocking API bill.

Who Should Read

Developers actively using AI coding agents like Claude Code or Cursor — especially anyone who has configured hooks or automation scripts — should read this.

Core Mechanics

A developer created a SessionStart hook in Claude Code (a script that runs automatically when a session starts), configured to spawn 2 background CC (Claude Code) instances via the `claude -p ...` command. The problem was that each time a new instance started, the hook fired again, causing processes to explode exponentially: 1→2→4→8→...→2^N, creating a classic fork bomb.
A fork bomb is a classic attack/mistake pattern where a process continuously clones itself until all system resources are exhausted. In this case, the developer left their desk at 2 AM without noticing, and the computer spent the entire night spinning up hundreds of Claude Code instances on its own before becoming completely unresponsive.
When the developer came back at 11 AM the next morning, the mouse, keyboard, and trackpad were all unresponsive, and the machine was burning hot. Opening Activity Monitor revealed hundreds of CC instances running, with memory pressure maxed out in the red.
After a forced restart, the first thing checked was the API bill — fortunately, only about $600 had been added. The damage was less than expected because Claude Code itself consumes enormous memory per instance through its Bun → React → TUI chain, causing memory to run out early and crashing the computer before the API charges could grow further. In other words, the software being heavyweight actually stopped the billing from spiraling further.
The fix was simple: open `~/.claude/settings.json` and remove the offending SessionStart hook. Afterward, the developer verified no remaining instances were running in Activity Monitor, kept one hand on the power button, and cautiously ran `claude` again.
This developer had been a heavy user of AI coding tools since early 2025, having used Cursor for over 310 million tokens. Initially skeptical about adopting agentic workflows, they decided that if it was inevitable, they might as well master it — signed up for Claude Code, dove deep, and that's when this incident occurred.

Evidence

"Other developers shared similar experiences. One recounted unintentionally creating a fork bomb with Python multiprocessing code on Windows by failing to wrap the entry point in an `if __name__ == '__main__'` block — with context that Windows lacks Unix's `fork()`, so child processes re-execute the module. Another comment described experiencing a fork bomb from running official Microsoft OLE/COM sample code from the 90s, leading to the lesson of never blindly trusting even official documentation — a parallel to this incident where AI-generated code was placed directly into a hook. A humorous comment noted that calling it a \"first fork bomb\" implies a growth mindset — suggesting there will be more — and the community generally treated the incident as a rite of passage rather than a disaster. Questions were raised about why Claude Code intended to spawn 2 more CC instances in the SessionStart hook and what the purpose was, as the original post didn't explain this sufficiently — speculation pointed to an experiment with parallel task processing or a specific skill like `/adhd`. Many commenters were curious about the `/adhd` skill mentioned in the post (presumed to be a custom Claude Code feature for developers with ADHD), with some noting it sounded genuinely useful."

How to Apply

"When configuring hooks like SessionStart or PostToolUse in Claude Code, running the `claude` or `cc` command inside a hook will spawn a new instance that triggers the same hook again, potentially creating a fork bomb. Never spawn Claude Code itself from within a hook script, and always review for recursion risks before deploying. Hook configurations for Claude Code are managed in `~/.claude/settings.json`. After creating a new hook, open Activity Monitor (Mac) or Task Manager (Windows) and visually verify that processes are not multiplying explosively while testing. If you spot anything suspicious, immediately remove the offending hook entry from `settings.json`. If you're using the Anthropic Claude API on a company account, set up daily cost alerts on the Usage page. As this incident shows, a single automation script mistake can rack up hundreds or thousands of dollars in a short time — early alerts can minimize the damage. Similar risks exist when writing parallel processing code using Python `multiprocessing` or `subprocess`. On Windows, always wrap your entry point in an `if __name__ == '__main__':` block to prevent child processes from re-executing the parent code."

Code Example

snippet

# Dangerous example - spawning a claude instance from a SessionStart hook causes a fork bomb
# ~/.claude/settings.json
{
  "hooks": {
    "SessionStart": [
      {
        "hooks": [
          {
            "type": "command",
            # This command starts a new CC instance, which also runs the same hook → infinite spawning
            "command": "claude -p 'some background task' &"
          }
        ]
      }
    ]
  }
}

# Safely removing the hook
# 1. Open settings.json from the terminal
nano ~/.claude/settings.json
# 2. Delete the SessionStart hook entry and save

# If a fork bomb has already occurred (Mac)
pkill -f claude  # Kill all claude processes
# If that doesn't work, force restart with the power button

Terminology

fork bombA pattern where a process continuously clones (forks) itself until all system resources are exhausted. One becomes two, two become four, four become eight — growing exponentially until the computer crashes.

SessionStart hookA script in Claude Code that can be configured to run automatically whenever a new session starts. Configured in `~/.claude/settings.json`, if written incorrectly it can execute infinitely every time a session opens.

CC (Claude Code)An AI coding agent CLI tool built by Anthropic. Run via the `claude` command in the terminal, it allows collaborative AI-assisted code writing, editing, and execution.

agentic workflowA mode of operation where AI autonomously makes decisions and uses multiple tools in sequence to complete tasks without step-by-step human instruction — going beyond simple Q&A to independently execute code, modify files, and more.

Activity MonitorThe macOS task manager. Displays a real-time list of running processes along with CPU and memory usage. In this incident, it was the tool used to discover hundreds of running `claude` processes.

BunA high-performance JavaScript runtime designed as a Node.js replacement. Claude Code uses it internally via a Bun → React → TUI chain, which causes each instance to consume a significant amount of memory.