Unexpected Model Selection: Claude 3.5 Haiku Forced by Statsig Configuration

Bug Description

Bug Report: Claude Code Using Claude 3.5 Haiku Model

Issue Description

After modifying MCP server configurations, Claude Code CLI is unexpectedly using the Claude 3.5 Haiku model instead of the default Claude 3.7 Sonnet model, as shown in the command output:

Total cost:            $0.0034
Total duration (API):  6.9s
Total duration (wall): 1m 14.1s
Total code changes:    0 lines added, 0 lines removed
Token usage by model:
    claude-3-5-haiku:  4.0k input, 64 output, 0 cache read, 0 cache write

Root Cause

The issue appears to be caused by Statsig configuration data in the Claude Code cache that has enabled the "useHaiku" feature flag. In the ~/.claude/statsig/statsig.cached.evaluations.*.json files, there is a configuration setting:

"useHaiku": true,
"haikuInterval": 5,

This setting is causing Claude Code to use the Haiku model for messages.

Files Affected

• ~/.claude/statsig/statsig.cached.evaluations.3599888669
• ~/.claude/statsig/statsig.cached.evaluations.2932152684

These files contain the following configuration:

"responding": {
  "spinner": "default",
  "messages": "haiku",
  "color": "claude",
  "interval": 100
},
"toolUse": {
  "spinner": "tools",
  "messages": "haiku",
  "color": "claude",
  "interval": 250
},
"useHaiku": true,
"haikuInterval": 5,

How to Reproduce

1. Run claude from terminal
2. Issue any command (e.g., /bug)
3. Observe the token usage output showing Claude 3.5 Haiku being used

Impact

• Uses a smaller, less capable model than expected
• May affect quality of responses
• Different pricing tier than expected

Fix Options

Option 1: Clear Claude Code Configuration Cache

rm -rf ~/.claude/statsig/statsig.cached.evaluations.*

Option 2: Reset Claude Code Configuration

claude config reset

Option 3: Explicitly Set Model

claude config set model claude-3-7-sonnet-20250219

Option 4: Reinstall Claude Code

npm uninstall -g @anthropic-ai/claude-code
npm install -g @anthropic-ai/claude-code

Prevention

To prevent this issue in the future:

1. Avoid modifying MCP server configurations unless necessary
2. Document any changes to configuration files
3. Create backups of configuration files before making changes
4. Add a verification step to check the model being used after configuration changes

Additional Information

The issue appears related to a feature flag configuration in the Statsig system that controls Claude Code's behavior. The specific configuration is enabling "useHaiku" and setting message types to "haiku" for both responding and tool use operations.

Environment Info

• Platform: linux
• Terminal: vscode
• Version: 0.2.114
• Feedback ID: 22c51083-4485-457e-b049-13dfd5488879

Errors

[{"error":"RangeError [ERR_CHILD_PROCESS_STDIO_MAXBUFFER]: stdout maxBuffer length exceeded\n    at Socket.onChildStdout (node:child_process:490:14)\n    at Socket.emit (node:events:518:28)\n    at Socket.emit (node:domain:488:12)\n    at addChunk (node:internal/streams/readable:559:12)\n    at readableAddChunkPushByteMode (node:internal/streams/readable:510:3)\n    at Readable.push (node:internal/streams/readable:390:5)\n    at Pipe.onStreamRead (node:internal/stream_base_commons:190:23)","timestamp":"2025-05-14T19:13:54.102Z"},{"error":"Error: ENOENT: no such file or directory, open '/opt/mExpress/~/.claude'\n    at Module.openSync (node:fs:581:18)\n    at Object.readSync (file:///home/rmbg/.nvm/versions/node/v20.11.1/lib/node_modules/@anthropic-ai/claude-code/cli.js:499:850)\n    at sD (file:///home/rmbg/.nvm/versions/node/v20.11.1/lib/node_modules/@anthropic-ai/claude-code/cli.js:502:260)\n    at mB (file:///home/rmbg/.nvm/versions/node/v20.11.1/lib/node_modules/@anthropic-ai/claude-code/cli.js:504:358)\n    at file:///home/rmbg/.nvm/versions/node/v20.11.1/lib/node_modules/@anthropic-ai/claude-code/cli.js:1357:1059\n    at process.processTicksAndRejections (node:internal/process/task_queues:95:5)","timestamp":"2025-05-14T19:16:43.787Z"},{"error":"Error: ENOENT: no such file or directory, open '/opt/mExpress/~/.claude'\n    at Module.readFileSync (node:fs:453:20)\n    at Object.readFileSync (file:///home/rmbg/.nvm/versions/node/v20.11.1/lib/node_modules/@anthropic-ai/claude-code/cli.js:499:727)\n    at mB (file:///home/rmbg/.nvm/versions/node/v20.11.1/lib/node_modules/@anthropic-ai/claude-code/cli.js:504:376)\n    at file:///home/rmbg/.nvm/versions/node/v20.11.1/lib/node_modules/@anthropic-ai/claude-code/cli.js:1357:1059\n    at process.processTicksAndRejections (node:internal/process/task_queues:95:5)","timestamp":"2025-05-14T19:16:43.787Z"},{"error":"Error: ENOENT: no such file or directory, open '/opt/mExpress/~/.claude/projects'\n    at Module.openSync (node:fs:581:18)\n    at Object.readSync (file:%

Note: Error logs were truncated.