[BUG] Claude Code seems to use Sonnet even when configured explicitly to use Opus

Found 3 possible duplicate issues:

1. https://github.com/anthropics/claude-code/issues/3434
2. https://github.com/anthropics/claude-code/issues/1722
3. https://github.com/anthropics/claude-code/issues/6546

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It is a duplicate, but the previous issue (https://github.com/anthropics/claude-code/issues/3434) was closed as completed, so this is either a regression or the other issue was closed prematurely.

Subagents use Sonnet, even if you set /model opus. /model controls the model for the main agent loop.

I actually asked Claude Code what model it was and it told me Sonnet 3.5

This is unreliable. The more reliable approach is to look at your transcript files in ~/.claude/projects (which ccusage also looks at)

If you do find concrete evidence of Sonnet appearing in main loop queries despite /model opus, let us know.

I'm not using any subagents in any of my projects. Does the Task tool also have the same Sonnet behavior? I assumed it was a regression because I simply continue to do the same work I've always done (no subagents) and after a while, presumably when I hit some limit, I start to see Sonnet in my usage.

I tried looking through the transcript files in the main project I'm working on right now and here's a sample event (there are _lots_ of these, way more than I was expecting) from it that uses sonnet, despite my Opus-only /model configuration and no configured subagents:

{
  "parentUuid": "67a73fed-e3ed-4d39-89cd-af6b77cd4a7b",
  "isSidechain": true,
  "userType": "external",
  "cwd": "/some/path",
  "sessionId": "59b0764b-5191-4b92-acac-728980e8a05d",
  "version": "1.0.92",
  "gitBranch": "main",
  "message": {
    "id": "msg_01SSjkZJW4376mqzh2GTwr5r",
    "type": "message",
    "role": "assistant",
    "model": "claude-sonnet-4-20250514",
    "content": [
      {
        "type": "tool_use",
        "id": "toolu_01NChtZ24ZAWFwVNr7EMizvp",
        "name": "Read",
        "input": {
          "file_path": "/some/path/file"
        }
      }
    ],
    "stop_reason": null,
    "stop_sequence": null,
    "usage": {
      "input_tokens": 5,
      "cache_creation_input_tokens": 15809,
      "cache_read_input_tokens": 33226,
      "cache_creation": {
        "ephemeral_5m_input_tokens": 15809,
        "ephemeral_1h_input_tokens": 0
      },
      "output_tokens": 95,
      "service_tier": "standard"
    }
  },
  "requestId": "req_011CSVtH2hCXryaJdmRutN5m",
  "type": "assistant",
  "uuid": "c033a3f4-97eb-433c-98c5-ec5fd8368551",
  "timestamp": "2025-08-26T06:56:35.284Z"
}

The only thing I can think of is that Task might do something similar, but if so, can we get an option to stop that? I don't control when CC decides to use Task and it shouldn't silently switch to a different model.

Yes, Task === subagents (with the default subagent prompt).

I don't recommend it, but you can use /permissions to add Task to the tools under Deny. Subagents are a proven usage pattern to preserve main loop context space while providing similar performance.

Is there an indication of Task happening in the event I posted above? How would I identify that Task triggered it? I'd much rather force Task to run on Opus than disable it entirely.

You can use the CLAUDE_CODE_SUBAGENT_MODEL env var to force the model to a certain value:

• opus, sonnet, etc.
• inherit lets you use the same model as the main loop.

It's not subagents. It was probably this: https://status.anthropic.com/incidents/h26lykctfnsz

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