Task tool subagents fail with 404 'model: haiku' regardless of model parameter

Bug Description

All Task tool subagent invocations fail with a 404 error for model "haiku", even when explicitly specifying a different model.

Environment

• Claude Code version: 2.0.76
• Platform: macOS Darwin 24.6.0
• Main session model: claude-opus-4-5-20251101 (works fine)

Reproduction

Any Task tool invocation fails:

# Attempt 1: deep agent (has custom definition with model: opus)
Task(subagent_type="deep", prompt="test")
→ API Error: 404 {"type":"error","error":{"type":"not_found_error","message":"model: haiku"}}

# Attempt 2: deep agent with explicit model parameter
Task(subagent_type="deep", model="opus", prompt="test")  
→ API Error: 404 {"type":"error","error":{"type":"not_found_error","message":"model: haiku"}}

# Attempt 3: Explore agent
Task(subagent_type="Explore", prompt="test")
→ API Error: 404 {"type":"error","error":{"type":"not_found_error","message":"model: haiku"}}

# Attempt 4: general-purpose agent
Task(subagent_type="general-purpose", prompt="test")
→ API Error: 404 {"type":"error","error":{"type":"not_found_error","message":"model: haiku"}}

Custom Agent Definition

User has ~/.claude/agents/deep.md with:

---
name: deep
model: opus
tools: Read, Glob, Grep, Bash, WebSearch, WebFetch
---

Expected Behavior

• Subagents should use the model specified in the agent definition or model parameter
• If "haiku" isn't available, should fall back gracefully or use an available model

Actual Behavior

• All subagents attempt to use "haiku" regardless of configuration
• The model parameter passed to Task tool is ignored
• Custom agent definitions with explicit model settings are ignored
• Results in 404 error, completely breaking subagent functionality

Suspected Cause

Claude Code appears to have hardcoded "haiku" somewhere in the subagent spawning logic, bypassing both:

1. The model parameter on the Task tool
2. The model field in custom agent definitions

The "haiku" shorthand may also not be resolving to a valid full model ID.

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