[BUG] UI Freezes When Spawning Multiple Parallel Task Agents

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

Environment:

• Platform: Windows (win32)
• Claude Code CLI
• Model: Claude Sonnet 4.5

Description:

The Claude Code UI becomes completely unresponsive for 10-60+ seconds when Claude spawns multiple Task agents in parallel in a
single response.

Reproduction Steps:

1. Ask Claude to perform multiple independent tasks in parallel (e.g., "Fix these 7 linting errors in parallel, one agent per

file")

1. Claude responds with a single message containing 7+ Task tool calls
2. UI freezes - no input accepted, no visual updates, appears hung
3. After 10-60 seconds, UI becomes responsive and shows "Running N Task agents..."
4. Agents run fine in background, UI remains responsive during execution
5. Brief freeze again during post-processing ("Hashing..." phase)

Expected Behavior:

• UI should remain responsive during agent spawning
• Show progress indicator: "Spawning agent 1/7... 2/7..."
• Tool calls should be processed asynchronously
• User should be able to cancel, type, or interact with UI during spawn phase

Actual Behavior:

• UI completely freezes during the tool call invocation phase
• No visual feedback that anything is happening
• Appears hung/crashed (though it eventually recovers)
• Duration scales with number of parallel agents (7 agents = ~30s freeze)

Workarounds:

• Spawn agents in smaller batches (2-3 at a time)
• Use sequential spawning across multiple messages
• Accept the freeze and wait it out

Additional Context:

This makes the "run tasks in parallel" workflow nearly unusable for large batches. Users expect parallel execution to be
fast/responsive, but the spawning overhead negates the benefits.

Similar Issues:

• Post-processing phase ("Hashing...") also blocks UI when many files are edited
• Suggests synchronous file hashing after agent completion

Suggested Fix:

• Make tool call processing async in UI
• Stream agent spawn events to UI
• Show progress indicator during initialization
• Process file hashing in background with progress updates

What Should Happen?

Expected Behavior:

• UI should remain responsive during agent spawning
• Show progress indicator: "Spawning agent 1/7... 2/7..."
• Tool calls should be processed asynchronously
• User should be able to cancel, type, or interact with UI during spawn phase

Error Messages/Logs

No error messages - UX hung.

Steps to Reproduce

Fix these linting errors (100's) in parallel, one agent per file (10)

Claude Model

Sonnet (default)

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.37 (Claude Code)

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

PowerShell

Additional Information

_No response_