When I set auto mode as the default at startup in Claude Code, the behavior diverged between supported and unsupported models

I'm emi, a coffee lover.

Last time, I checked model by model whether auto mode could be used with Claude Code via Amazon Bedrock. On Bedrock, the supported models were Sonnet 5 / Opus 4.7 and later / Fable, and the result was that auto mode does not appear with Sonnet 4.6.

https://dev.classmethod.jp/articles/claude-code-auto-mode-bedrock/

This time, I verified how behavior differs between models that support auto mode and those that don't, when the permission mode at startup is set to auto mode.

Conclusion First

• The permission mode at startup can be set to auto via defaultMode in permissions in ~/.claude/settings.json
  • It needs to be written in the user settings (~/.claude/settings.json); settings on the project side (.claude/settings.json or .claude/settings.local.json) will not take effect
• When launched with an auto mode-compatible model (in this case, jp. Opus 4.8), auto mode is on immediately after startup
• With a model that does not support auto mode (in this case, jp. Sonnet 4.6), defaultMode: "auto" is ignored even if written, and it starts with manual mode on
• Model specification via the ANTHROPIC_MODEL environment variable in .bashrc takes priority over model in settings.json

Environment

• Windows + WSL2 (Ubuntu)
• Claude Code v2.1.248
• Via Amazon Bedrock (launched by injecting temporary credentials with aws-vault)

As before, I'm using the Bedrock startup function claude-bedrock. This function launches claude inside aws-vault exec with CLAUDE_CODE_USE_BEDROCK=1, the region, and the model specified with the jp. prefix. For details, please refer to the following article.

https://dev.classmethod.jp/articles/claude-code-bedrock-jp-region/

Setting defaultMode to auto

To make auto mode the mode at startup, add defaultMode to the permissions block in the user settings ~/.claude/settings.json.

"permissions": {
  "allow": [
    "Read(//tmp/**)"
  ]
}

↓ Add defaultMode here.

"permissions": {
  "defaultMode": "auto",
  "allow": [
    "Read(//tmp/**)"
  ]
}

Even if defaultMode: "auto" is written in the project-side settings (.claude/settings.json or .claude/settings.local.json), it will not take effect. You need to write it in the user settings ~/.claude/settings.json, or pass --permission-mode at each startup.

I also specify which model to use at startup via model in settings.json.

Initially, I was passing the environment variable ANTHROPIC_MODEL='jp.anthropic.claude-sonnet-4-6' inside the claude-bedrock function in .bashrc. Since this environment variable takes priority over model in settings.json, no matter what I wrote in settings.json, it would always be Sonnet 4.6 when launched via claude-bedrock.

Since I want to consolidate model specification into settings.json this time, I removed the ANTHROPIC_MODEL line from the function in .bashrc. I kept ANTHROPIC_SMALL_FAST_MODEL for the lightweight model.

claude-bedrock() {
  aws-vault exec cm-kitani.emi -- env \
    CLAUDE_CODE_USE_BEDROCK=1 \
    AWS_REGION=ap-northeast-1 \
    ANTHROPIC_SMALL_FAST_MODEL='jp.anthropic.claude-haiku-4-5-20251001-v1:0' \
    claude "$@"
}

Since I changed .bashrc, I run the source ~/.bashrc command to apply the changes.

Execution result▼

emiki@<hostname>:~/work/xx$ source ~/.bashrc
emiki@<hostname>:~/work/xx$ 

Changes to settings.json are applied just by restarting Claude Code.

When I wrote the alias "opus" for model in settings.json, it resolved to the global model without jp. (Opus 5 in this case). Since I want to fix it to the domestic profile with the jp. prefix, I write the profile ID directly.

"model": "jp.anthropic.claude-opus-4-8",

With the compatible model jp. Opus 4.8, it launched in auto mode

I rewrite the settings and launch claude-bedrock.

 ▐▛███▛█   Claude Code v2.1.248
▝▜██████▀  Opus 4.8 · Amazon Bedrock
  ▝▝ ▝▝    ~/work/xx

                                                                                                                       ● high · /effort
─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
❯
─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
  ⏵⏵ auto mode on (shift+tab to cycle) · ← for agents · shift+click to native select

With jp. Opus 4.8, auto mode was on immediately after startup.

I'll exit the TUI once with /exit.

With the incompatible model jp. Sonnet 4.6, it was manual mode

Next, I change model to jp. Sonnet 4.6. defaultMode: "auto" remains as is.

"model": "jp.anthropic.claude-sonnet-4-6",

I restarted claude-bedrock.

 ▐▛███▛█   Claude Code v2.1.248
▝▜██████▀  Sonnet 4.6 · Amazon Bedrock
  ▝▝ ▝▝    ~/work/xx

                                                                                                   auto mode unavailable for this model
─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
❯
─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
  ⏸ manual mode on · ? for shortcuts · ← for agents · shift+click to native select

Even though defaultMode: "auto" is written, it shows manual mode on. The message auto mode unavailable for this model is displayed. I was able to confirm the behavior where auto is ignored for incompatible models and falls back to the built-in default of manual.

Closing

With the auto mode-compatible Opus 4.8, auto mode was active immediately after startup, while with the incompatible Sonnet 4.6, it reverted to manual even with the same settings.

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