Mixed Models spec mode auto-switching does not work with custom BYOK models

[Rubyj]

Description

After approving a spec in Specification Mode, the session does not automatically switch from the specModeModel to the default model. It remains on the spec mode model for execution.

Environment

• OS: macOS (darwin 25.4.0)
• Date observed: 2026-04-09

Custom Model Configuration (~/.factory/settings.json)

• Default model: custom:Sonnet-4.6-1 (claude-sonnet-4-6 via Anthropic provider)
• Spec mode model: custom:Opus-4.6-0 (claude-opus-4-6 via Anthropic provider)
• Both models use the same Anthropic provider and base URL

Session Default Settings

{
  "model": "custom:Sonnet-4.6-1",
  "specModeModel": "custom:Opus-4.6-0",
  "autonomyMode": "spec",
  "reasoningEffort": "none",
  "interactionMode": "spec",
  "autonomyLevel": "medium"
}

Steps to Reproduce

1. Configure two custom Anthropic BYOK models (Opus and Sonnet)
2. Set Sonnet as default model and Opus as specModeModel in sessionDefaultSettings
3. Start a new session (enters spec mode, uses Opus as expected)
4. Provide a prompt, Opus generates a spec
5. Approve the spec
6. Observe: session remains on Opus instead of switching to Sonnet for execution

What I Tried

• Verified sessionDefaultSettings has correct model IDs in both new and existing sessions
• Configured spec mode model via /model → "Configure Spec Mode Model" UI
• Removed/modified thinking and reasoningEffort settings on Sonnet
• Started multiple fresh sessions after each config change
• None of the above resolved the issue

Expected Behavior

After spec approval, the session should automatically switch to the default model (custom Sonnet) for execution, as documented in the Mixed Models feature.

Actual Behavior

The session stays on the spec mode model (custom Opus) during execution. The model indicator in the session header continues to show Opus after spec approval.

[Rubyj]

Additional Debugging Performed

We exhaustively tested the following config variations -- none resolved the issue:

1. Sonnet thinking config: Tried disabled, enabled (8k budget), and removed entirely -- no effect on switching
2. reasoningEffort: Tried none, low, and off -- no effect
3. Removed redundant interactionMode field from sessionDefaultSettings -- no effect
4. Cleared legacy config.json to eliminate duplicate model definitions -- no effect (Droid appears to regenerate it)
5. Used /model UI → 'Configure Spec Mode Model' to set the pairing interactively -- no effect

All tests were done in fresh sessions after each change. The sessionDefaultSettings correctly shows model: custom:Sonnet-4.6-1 and specModeModel: custom:Opus-4.6-0, and new sessions reflect these defaults, but the auto-switch after spec approval never occurs.

Conclusion: The mixed models auto-switching feature does not appear to work with custom BYOK models. The session stays on the specModeModel (Opus) after spec approval instead of switching to the default model (Sonnet).