Rubber Duck

[tanselmi-appliedsurety]

Describe the bug

rubber-duck reviews sometimes use the same model family as the primary session instead of selecting an independent reviewer, reducing the value of adversarial review.

i have observed this on a variety of models. most recently using 5.6 Terra - Max

Affected version

v1.0.78

Steps to reproduce the behavior

Have code/plan/contract whilst working in a gpt-5.6-terra session and request a rubber-duck review...

you then see this: Rubber-duck(gpt-5.6-terra) Assess audit design quality
└ Agent started in background with agent_id: audit-design-duck. You'll be notif...

Expected behavior

I would expect a different model family selected from the current model being used in the session.

Additional context

No response

[nick-thinks]

Hi @tanselmi-appliedsurety I agree, typically you should have received a complementary model.

Are you able to provide a session process log of when this happened to you? That will help to make sure we have identified the exact problem 😄 You can find the session's log file by using /session info and the log file will look like /Users/nick/.copilot/logs/process-1786017957609-61781.log

Just to double check, can you also confirm if you have overridden the rubber duck subagent model in your subagent settings? If you go to /subagents you are able to change the model mapping behavior for rubber duck from the default of "complementary (different family)". If it is set to "inherit" it will use your session model, and if you have set a specific model it will use that.

It is also possible that the session agent itself decided to override the rubber duck model choice, which we can identify from the process log. We discourage this option in instructions but leave it open for some flexibility in-session.

[tanselmi-appliedsurety]

Thanks @nick-thinks — checked both.

1. Rubber-duck model mapping

•  /subagents :  rubber-duck  →  complementary — unavailable, disabled 
• Overridden: No
• Status: On
I did not set this.  settings.json  has no  rubber-duck  model override (only explore/task/research/etc.). So complementary is the built-in default, but the UI treats it as unavailable for the current session model/catalog.

2. Process log
   Attached a redacted extract from yesterday’s session ( v1.0.78 ) with multiple  rubber-duck  execute/complete events.
   Note: INFO logs show the duck runs, but do not emit complementary selection debug ( parent_model  /  available_models  /  selected_model ). Happy to capture a fresher log if you want a specific debug flag/level.

That matches the original report: complementary path fails open to same-family/session model (e.g. terra-on-terra) instead of a different family.

copilot-cli-4380-rubber-duck-evidence.REDACTED.txt