Subagent fails with 'Minified React error #185' during non-UI operations

Description

Background subagent (Task tool) fails with a React rendering error even though the agent is performing non-UI operations (bash scripts, file edits).

Environment

• Claude Code Version: 2.1.31
• Platform: macOS (darwin)
• Date: 2026-02-04

Error Message

Agent "Implement RuVector CoherenceEngine" failed: Minified React error #185; 
visit https://react.dev/errors/185 for the full message

React error #185 = "Objects are not valid as a React child"

Steps to Reproduce

1. Spawn a background subagent using Task tool with run_in_background: true
2. Agent performs file operations (Glob, Read, Bash, Edit)
3. Agent fails mid-execution with React error

Expected Behavior

Subagent should complete or fail with a relevant error, not a UI rendering error.

Additional Context

• The agent was creating bash scripts, not doing any UI work
• Error appears to be in Claude Code's progress rendering, not the agent's actual work
• Retrying the same task with a new agent worked fine
• Agent output file shows PreToolUse:Bash hook error repeated before crash

Workaround

Retry the task - subsequent agents complete successfully.