[Feature]: Choose different model / harnesses during planning and implementation.

[thehorse2000]

Before submitting

• I searched existing issues and did not find a duplicate.
• I am describing a concrete problem or use case, not just a vague idea.

Area

apps/web

Problem or use case

"Implement in a new thread" locks the user into whatever model and harness were active on the plan thread — there is no opportunity to change them before the implementation run started.
Plan generation and implementation often benefit from different models / harnesses (e.g. plan with claude code but implement with codex).

Proposed solution

Clicking "Implement in a new thread" opens a new thread with the implementation prompt pre-filled in the composer.
The user can change the model, harness (Supervised / Auto-accept / Full access), or edit the prompt before manually submitting.

Why this matters

Model Swapping between planning and implementation is often a good way for Token Budgeting and making use of each Model's superpowers.

Smallest useful scope

Allow model / harness change when implementing the plan in a new thread.

Alternatives considered

No response

Risks or tradeoffs

No response

Examples or references

No response

Contribution

• I would be open to helping implement this.

[thehorse2000]

Opened a PR at #1877