Concurrent sub-agents fail to spawn | Pressing `Esc` throws “Cannot read properties of undefined (reading 'message')”

Environment

• Platform: Anthropic API
• Claude CLI version: 1.0.51 (Claude Code)
• Operating System: macOS 15.5 (Apple Silicon)
• Terminal: Ghostty

Bug Description

Attempting to launch multiple concurrent sub-agents results in only one agent spawning. No task-list UI appears, and pressing Esc—which should return to a fresh prompt—crashes the CLI.

Steps to Reproduce

1. Start an interactive session:

``bash
claude
``

1. Enter a prompt that should spawn several sub-agents (e.g. Run three parallel searches on …).
2. Observe that only a single agent starts and no task list is shown.
3. Press Esc to return to the prompt.

Expected Behavior

• All requested sub-agents spawn concurrently.
• A task-list UI displays each agent’s status.
• Pressing Esc cleanly returns to an empty prompt without errors.

Actual Behavior

• Only one agent spawns.
• No task list is rendered.
• Pressing Esc crashes the CLI with the stack trace below:

<details>
<summary>Stack trace</summary>

ERROR  Cannot read properties of undefined (reading 'message')

   file:///opt/homebrew/lib/node_modules/@anthropic-ai/claude-code/cli.js:2512:798
   -  (file:///opt/homebrew/lib/node_modules/@anthropic-ai/claude-code/cli.js:2512:798)
   -     at Array.filter (<anonymous>)
   - Object.renderToolUseProgressMessage (file:///opt/homebrew/lib/node_modules/@anthropic-ai/claude-code/cli.js:2512:763)
   - Object.renderToolUseRejectedMessage (file:///opt/homebrew/lib/node_modules/@anthropic-ai/claude-code/cli.js:2512:2307)
   - L_2 (file:///opt/homebrew/lib/node_modules/@anthropic-ai/claude-code/cli.js:1968:3008)
   - $w (file:///opt/homebrew/lib/node_modules/@anthropic-ai/claude-code/cli.js:65:19804)
   - gP (file:///opt/homebrew/lib/node_modules/@anthropic-ai/claude-code/cli.js:67:42569)
   - vP (file:///opt/homebrew/lib/node_modules/@anthropic-ai/claude-code/cli.js:67:38118)
   - VL (file:///opt/homebrew/lib/node_modules/@anthropic-ai/claude-code/cli.js:67:38046)
   - mw (file:///opt/homebrew/lib/node_modules/@anthropic-ai/claude-code/cli.js:67:37900)

</details>

Additional Context

• Reinstalling the CLI (npm install -g @anthropic-ai/claude-code) did not resolve the issue.
• Issue reproduces in both fresh and existing projects.
• Tried the following flags without success:
• --delegate
• --parallel-dirs
• --multi-agent
• --parallel-focus
• --concurrency <n>
• The same prompts and flags worked 100% of the time on earlier CLI versions, so this appears to be a regression.

EDIT: I manually downgraded to 1.0.48 and the issue did not reproduce itself.