[BUG] Builtins Subagents use hardcoded model names

[0wn2pwn]

Preflight Checklist

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

What's Wrong?

Built-in agents (like Explore, claude-code-guide, etc.) ignore the ANTHROPIC_SMALL_FAST_MODEL environment variable configured in ~/.claude/settings.json and instead use hardcoded model names like "haiku" which get mapped to claude-haiku-4-5-20251001 instead of the configured custom model.

This causes authentication errors when using custom model endpoints (e.g., Vertex AI, Bedrock, or proxy services) that require specific model identifiers.

What Should Happen?

Built-in agents should:

• Respect ANTHROPIC_SMALL_FAST_MODEL when using model: "haiku"
• OR allow custom agents to override built-in agents
• OR provide a way to configure model mapping for short names like "haiku", "sonnet", "opus"

Error Messages/Logs

Built-in agents have hardcoded model strings like:
  LL={
      agentType:"Explore",
      // ... 
      model:"haiku",  // <- Hardcoded
      // ...
  }

  This "haiku" gets mapped to claude-haiku-4-5-20251001 instead of mine.

Steps to Reproduce

1. Configure custom model in ~/.claude/settings.json:
   {
   "env": {
   "ANTHROPIC_MODEL": "custom/claude-sonnet-4-5",
   "ANTHROPIC_SMALL_FAST_MODEL": "custom/claude-haiku-4-5",
   "ANTHROPIC_BASE_URL": "https://custom.proxy.url/"
   }
   }

2. Use the Task tool with a built-in agent:
   Task(
   subagent_type="Explore",
   prompt="Find all .sh files in the lxc/ directory"
   )

3. Observe the error:
   API Error: 401 {"error":{"message":"team not allowed to access model.
   Tried to access claude-haiku-4-5-20251001",...}}

Claude Model

Other

Is this a regression?

No, this never worked

Last Working Version

No response

Claude Code Version

2.0.76

Platform

Other

Operating System

Ubuntu/Debian Linux

Terminal/Shell

VS Code integrated terminal

Additional Information

In /node_modules/@anthropic-ai/claude-code/cli.js line 2067:

LL={
agentType:"Explore",
whenToUse:'Fast agent specialized for exploring codebases...',
disallowedTools:[n3,mJ1,j3,FI,lM],
source:"built-in",
baseDir:"built-in",
model:"haiku", // <- Problem: hardcoded
getSystemPrompt:()=>Jg5,
// ...
}

Similarly for claude-code-guide agent (line 623):
QCB={
agentType:ln1,
whenToUse:'Use this agent when the user asks questions...',
tools:[qV,OX,T3,VI,iM],
source:"built-in",
baseDir:"built-in",
model:"haiku", // ← Same issue
// ...
}

Workaround

Manual patch of cli.js:

Backup

cp cli.js cli.js.backup

Patch Explore agent

sed -i 's/model:"haiku"/model:"custom/claude-haiku-4-5"/' cli.js

Note: This workaround is fragile and breaks on each update.

Proposed Solutions

1. Option A: Make built-in agents respect ANTHROPIC_SMALL_FAST_MODEL:
   model: process.env.ANTHROPIC_SMALL_FAST_MODEL || "haiku"
2. Option B: Allow custom agents in ~/.claude/agents/ to override built-in agents with the same name
3. Option C: Add a configuration option for model mapping:
   {
   "modelMapping": {
   "haiku": "custom/claude-haiku-4-5",
   "sonnet": "custom/claude-sonnet-4-5",
   "opus": "custom/claude-opus-4-5"
   }
   }
4. Option D: Make agents use model: "inherit" by default instead of hardcoded values

Impact

This bug prevents users from:

• Using Claude Code with Vertex AI or Bedrock
• Using custom LLM proxy services
• Deploying Claude Code in enterprise environments with custom model routing

Additional Context

The Task tool does have a model parameter, but it only accepts short names ("haiku", "sonnet", "opus"), not full model identifiers like "custom/claude-haiku-4-5".

[github-actions]

Found 3 possible duplicate issues:

1. [BUG] Excessive Retry Delays When Fast Model Returns 401 - No Configurable Fallback #10932
2. [BUG] Intermittent Task/subagents fail with API Error: 400 The provided model identifier is invalid while main agent succeeds (Bedrock) #15907
3. [BUG] Explore mode not working with vertex #14646

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