[BUG] Fable 5.1 requires unstable release of Claude Code

### Preflight Checklist

- [x] I have searched [existing issues](https://github.com/anthropics/claude-code/issues?q=is%3Aissue%20state%3Aopen%20label%3Abug) 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?

All attempts to use Fable 5.1 (`claude-fable-5-1`) in the current [stable release of Claude Code](https://code.claude.com/docs/en/setup#configure-release-channel) fail deterministically with the following error message:

```text
⏺ API Error: 400 Claude Code 2.1.236 does not support this model; version 2.1.251 or
newer is required. Run 'claude update', or update the Claude desktop app, then
try again.
```

### What Should Happen?

Fable 5.1 requests should succeed in the current stable release of Claude Code. As of this moment, that is `2.1.236`:

```text
❯ date; npm view @anthropic-ai/claude-code dist-tags.stable
Tue Sep 1 18:07:30 CDT 2026
2.1.236
```

### Error Messages/Logs

```shell
`
2026-09-01T22:55:08.166Z [DEBUG] Acquired PID lock for 2.1.236 (PID 24426)
...
2026-09-01T22:55:10.796Z [DEBUG] [engine] send set_max_thinking_tokens max=null
2026-09-01T22:55:10.796Z [DEBUG] [engine] send set_model model=claude-fable-5-1
...
2026-09-01T22:55:17.794Z [DEBUG] [API:timing] dispatching to firstParty model=claude-haiku-4-5-20251001
2026-09-01T22:55:17.795Z [DEBUG] [API REQUEST] /v1/messages x-client-request-id= source=generate_session_title
...
2026-09-01T22:55:17.797Z [DEBUG] [engine] turn 1 start
...
2026-09-01T22:55:17.803Z [DEBUG] [API:timing] dispatching to firstParty model=claude-fable-5-1
2026-09-01T22:55:17.803Z [DEBUG] [API REQUEST] /v1/messages x-client-request-id= source=repl_main_thread
...
2026-09-01T22:55:18.240Z [ERROR] API error (attempt 1/11): 400 400 {"type":"error","error":{"type":"invalid_request_error","message":"Claude Code 2.1.236 does not support this model; version 2.1.251 or newer is required. Run 'claude update', or update the Claude desktop app, then try again.","details":{"error_code":"claude_code_version_too_old"}},"request_id":""}
2026-09-01T22:55:18.240Z [WARN] [server-fallback] 400 attributed (unattributed) — stripping and retrying
...
2026-09-01T22:55:18.241Z [DEBUG] [API:timing] dispatching to firstParty model=claude-fable-5-1
2026-09-01T22:55:18.241Z [DEBUG] [API REQUEST] /v1/messages x-client-request-id= source=repl_main_thread
2026-09-01T22:55:18.305Z [DEBUG] Stream started - received first chunk
2026-09-01T22:55:18.305Z [DEBUG] [API:timing] first byte after 511ms
...
2026-09-01T22:55:18.537Z [ERROR] API error (attempt 1/11): 400 400 {"type":"error","error":{"type":"invalid_request_error","message":"Claude Code 2.1.236 does not support this model; version 2.1.251 or newer is required. Run 'claude update', or update the Claude desktop app, then try again.","details":{"error_code":"claude_code_version_too_old"}},"request_id":""}
2026-09-01T22:55:18.537Z [WARN] [server-fallback] 400 attributed (unattributed) — stripping and retrying
...
2026-09-01T22:55:18.538Z [DEBUG] [API REQUEST] /v1/messages x-client-request-id= source=repl_main_thread
2026-09-01T22:55:18.795Z [ERROR] API error (attempt 1/11): 400 400 {"type":"error","error":{"type":"invalid_request_error","message":"Claude Code 2.1.236 does not support this model; version 2.1.251 or newer is required. Run 'claude update', or update the Claude desktop app, then try again.","details":{"error_code":"claude_code_version_too_old"}},"request_id":""}
2026-09-01T22:55:18.802Z [ERROR] Error in API request: 400 {"type":"error","error":{"type":"invalid_request_error","message":"Claude Code 2.1.236 does not support this model; version 2.1.251 or newer is required. Run 'claude update', or update the Claude desktop app, then try again.","details":{"error_code":"claude_code_version_too_old"}},"request_id":""}
2026-09-01T22:55:18.802Z [ERROR] API error x-client-request-id= (give this to the API team for server-log lookup)
2026-09-01T22:55:18.804Z [ERROR] Error: Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"Claude Code 2.1.236 does not support this model; version 2.1.251 or newer is required. Run 'claude update', or update the Claude desktop app, then try again.","details":{"error_code":"claude_code_version_too_old"}},"request_id":""}
at generate (/$bunfs/root/cli:39:39337)
at makeRequest (/$bunfs/root/cli:90:8063)
at processTicksAndRejections (native:7:39)
2026-09-01T22:55:18.805Z [DEBUG] [engine] turn 1 end (turns=1 usage in=896 out=11 cost=$0.0010 api=648ms stop=stop_sequence resultLen=0)
2026-09-01T22:55:18.805Z [ERROR] [engine] turn ended in error: API Error: 400 Claude Code 2.1.236 does not support this model; version 2.1.251 or newer is required. Run 'claude update', or update the Claude desktop app, then try again.
...
2026-09-01T23:05:22.843Z [DEBUG] Protecting locked version from cleanup: 2.1.227
`
```

### Steps to Reproduce

1. `claude install stable`
2. `claude --model claude-fable-5-1 --print 'what is your knowledge cutoff?'`
3. observe the following error message and a non-zero exit code (`1`): `API Error: 400 Claude Code 2.1.236 does not support this model; version 2.1.251 or newer is required. Run 'claude update', or update the Claude desktop app, then try again.`

### Claude Model

Other

### Is this a regression?

I don't know

### Last Working Version

_No response_

### Claude Code Version

2.1.236

### Platform

Anthropic API

### Operating System

macOS

### Terminal/Shell

iTerm2

### Additional Information

_No response_

Start by reproducing the failure with `claude install stable` and `claude --model claude-fable-5-1 --print 'what is your knowledge cutoff?'`, then inspect the CLI entry point associated with the bundled `/$bunfs/root/cli` stack trace. The issue names no source file or test; done means the stable release accepts Fable 5.1 without the reported 400 error and the command exits successfully.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.