[BUG] Claude Agent SDK Fails with HTTP 404 `not_found_error` for `model="default"`

Preflight Checklist

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

What's Wrong?

Starting today (08 December 2025) at 12:43 PM IST, Claude Agent SDK (Python) on top of Claude Code CLI suddenly began failing with a 404 not_found_error related to the model name.

We are using model="default" inside ClaudeAgentOptions. This configuration had been working reliably earlier, but without any code changes on our side, requests now fail with:

API Error: 404 {"type":"error","error":{"type":"not_found_error","message":"model: default"}}

It appears that the default model identifier is no longer being resolved by Claude Code CLI or the underlying API.

What Should Happen?

model="default" should continue to work as it has been working previously.

Error Messages/Logs

API Error: 404 {"type":"error","error":{"type":"not_found_error","message":"model: default"}}

Steps to Reproduce

1. Install Claude Code CLI 2.0.59
2. Install Claude Agent SDK Python 0.1.4
3. Use the following minimal agent script:

from claude_agent_sdk import ClaudeSDKClient, ClaudeAgentOptions

    options=ClaudeAgentOptions(
        model="default",
        permission_mode="bypassPermissions",
    )

client = ClaudeSDKClient(options=opts)

await client.query(prompt)

Execute the script.

Observe the 404 response with "model: default".

Claude Model

Other

Is this a regression?

Yes, this worked in a previous version

Last Working Version

2.0.59

Claude Code Version

2.0.59

Platform

Anthropic API

Operating System

Other Linux

Terminal/Shell

Other

Additional Information

_No response_