[MODEL] Sub Agent delegatation does not work

Preflight Checklist

• [x] I have searched existing issues for similar behavior reports
• [x] This report does NOT contain sensitive information (API keys, passwords, etc.)

Type of Behavior Issue

Claude ignored my instructions or configuration

What You Asked Claude to Do

Since you guys release Opus 4.5
Sonnet 4.5 refuse to delegate work to sub agent (Haiku 4.5)

I have 2 pc with claude code, one for my work the other one for my personal
So i tested this on 2 pc

Usually, to save token, I always ask Haiku 4.5 to do the work, Sonnet 4.5 for thinking and planning (like what you teached us)

What Claude Actually Did

Before

• Sonnet create a plan and let Haiku to implement

After

• Sonnet create a plan and continue to work in implementation
• Even if asking Sonnet to delegate the work, it keeps doing it by himself but still never delegate the work
• Even during planning asking Sonnet to let Haiku to implement, it makes Haiku also doing planning and then Sonnet working on implementation

Expected Behavior

It should let sub agent to do the implementation
This is working before

Files Affected

Permission Mode

Accept Edits was ON (auto-accepting changes)

Can You Reproduce This?

Yes, every time with the same prompt

Steps to Reproduce

1. During planning mode, ask Sonnet to make a plan and let sub agent (Haiku) to implement

Claude Model

Sonnet

Relevant Conversation

Impact

High - Significant unwanted changes

Claude Code Version

2.0.55 (Claude Code)

Platform

Anthropic API

Additional Context

Usually I have long sessions of Claude Code due saving with Haiku, now Sonnet keep doing his own think then the session is very short.

While I understand the urge of keep updating stuff but stuff start to broken.