[BUG] haiku subagent delegates to 3.5

Found 3 possible duplicate issues:

1. https://github.com/anthropics/claude-code/issues/7251
2. https://github.com/anthropics/claude-code/issues/8932
3. https://github.com/anthropics/claude-code/issues/9573

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🤖 Generated with Claude Code

hi @sorryhyun, how are you checking that it's delegating to haiku 3.5? We upgrade all first party users to haiku 4.5 (whereas we haven't upgraded third party users -- AWS & GCP -- yet as they need to provision separate quota).

@km-anthropic oh, I've set model to haiku in github project directly by modifying .claude/agents/*.md and I met the following issue, so I thought this would be caused by delegation.

{
  "type": "assistant",
  "message": {
    "id": "b8e0d850-5658-40cc-acd5-aa4877f51e14",
    "container": null,
    "model": "<synthetic>",
    "role": "assistant",
    "stop_reason": "stop_sequence",
    "stop_sequence": "",
    "type": "message",
    "usage": {
      "input_tokens": 0,
      "output_tokens": 0,
      "cache_creation_input_tokens": 0,
      "cache_read_input_tokens": 0,
      "server_tool_use": {
        "web_search_requests": 0
      },
      "service_tier": null,
      "cache_creation": {
        "ephemeral_1h_input_tokens": 0,
        "ephemeral_5m_input_tokens": 0
      }
    },
    "content": [
      {
        "type": "text",
        "text": "API Error: 500 {\"type\":\"error\",\"error\":{\"type\":\"api_error\",\"message\":\"Internal server error\"},\"request_id\":\"req_011CU9zpfvzPGo7CF18WNa1H\"}"
      }
    ]
  },
  "parent_tool_use_id": null,
  "session_id": "685b9c4e-d238-41c7-85a5-45d5a78aad89",
  "uuid": "2ca7c1b1-2d96-40e3-ab4c-a8e310e6599a"
}
{
  "type": "result",
  "subtype": "success",
  "is_error": true,
  "duration_ms": 60590,
  "duration_api_ms": 5[1063](https://github.com/sorryhyun/DiPeO/actions/runs/18547266700/job/52867616821#step:3:1069),
  "num_turns": 27,
  "result": "API Error: 500 {\"type\":\"error\",\"error\":{\"type\":\"api_error\",\"message\":\"Internal server error\"},\"request_id\":\"req_011CU9zpfvzPGo7CF18WNa1H\"}",
  "session_id": "685b9c4e-d238-41c7-85a5-45d5a78aad89",
  "total_cost_usd": 0.16578330000000002,
  "usage": {
    "input_tokens": 411,
    "cache_creation_input_tokens": 27948,
    "cache_read_input_tokens": 96611,
    "output_tokens": 1622,
    "server_tool_use": {
      "web_search_requests": 0
    },
    "service_tier": "standard",
    "cache_creation": {
      "ephemeral_1h_input_tokens": 0,
      "ephemeral_5m_input_tokens": 27948
    }
  },
  "modelUsage": {
    "claude-3-5-haiku-20241022": {
      "inputTokens": 6245,
      "outputTokens": 359,
      "cacheReadInputTokens": 0,
      "cacheCreationInputTokens": 0,
      "webSearchRequests": 0,
      "costUSD": 0.006432,
      "contextWindow": 200000
    },
    "claude-sonnet-4-5-20250929": {
      "inputTokens": 411,
      "outputTokens": 1622,
      "cacheReadInputTokens": 96611,
      "cacheCreationInputTokens": 27948,
      "webSearchRequests": 0,
      "costUSD": 0.15935129999999997,
      "contextWindow": 200000
    }
  },
  "permission_denials": [],
  "uuid": "eb548aab-40c7-40e4-aae5-b1349e03a6ae"
}

Could you please see if you've set ANTHROPIC_DEFAULT_HAIKU_MODEL anywhere? possibly to claude-3-5-haiku-20241022? or ANTHROPIC_SMALL_FAST_MODEL? And lastly if you can run claude --version

I couldnt find any env variables you mentioned and I always use the recent version.
Also, claude code sdk has the same problem, when ClaudeAgentOption(model="haiku")
@km-anthropic

Gotcha, could you check claude --version? I think if you update it to your latest version you should be set.

Confirmation: --model haiku still defaults to Haiku 3.5

I can confirm this behavior. Testing with Claude Code 2.0.27 on Bedrock backend:

Current Behavior

Using --model haiku with the recommended Bedrock configuration:

{
  "env": {
    "CLAUDE_CODE_MAX_OUTPUT_TOKENS": "4096",
    "MAX_THINKING_TOKENS": "1024"
  },
  "alwaysThinkingEnabled": true
}

Results in:

claude --model haiku -p "what model are you using?"

Error:

API Error: 400 'claude-3-5-haiku-20241022' does not support thinking.

The haiku alias resolves to Haiku 3.5 (October 2024), which doesn't support thinking - causing errors with the recommended configuration from the documentation.

Workaround

As documented at https://docs.claude.com/en/docs/claude-code/amazon-bedrock#4-model-configuration, set the environment variable:

{
  "env": {
    "ANTHROPIC_DEFAULT_HAIKU_MODEL": "us.anthropic.claude-haiku-4-5-20251001-v1:0"
  }
}

This successfully resolves to Haiku 4.5.

Question

While the documentation correctly states this variable must be set manually, why does the haiku alias still default to a year-old model (3.5) that's incompatible with the recommended configuration?

Given that:

• Haiku 4.5 has been available since October 2025
• Haiku 3.5 doesn't support thinking (breaking the recommended MAX_THINKING_TOKENS setting)
• Other aliases like sonnet point to the latest version (4.5)

Shouldn't the haiku alias be updated to point to 4.5 by default now, with users explicitly opting into 3.5 if needed?

I also confirmed out-of-date haiku alias with Claude Code (v2.0.27) on Bedrock. Regarding quota provisioning, my understanding is that Bedrock's Haiku 4.5 quota is adjusted to their past Haiku 3.5 usage, so this shouldn't be a blocking factor for updating the default mapping. I hope the default can be changed to Haiku 4.5 for Bedrock users soon.

Yeah the current default is set to 3.5 for 3p (Bedrock + GCP) because we don't want to cause errors due to quota provisioning. The simplest fix right now would be to set ANTHROPIC_DEFAULT_HAIKU_MODEL=anthropic.claude-haiku-4-5-20251001-v1:0 in your environment variables if you're confident you have quota.

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