New Session uses the wrong model when an agent is selected

[ValentinFrima]

OpenCode Desktop 1.18.4

When creating a new session, selecting an agent that has a configured model does not apply that model. Instead, the session starts with the previously selected model. The correct model is only applied after reselecting the agent from within the active session.

Steps to reproduce

• Configure two agents with different models.
• Open the New Session dialog.
• Select an agent with a custom model.
• Start the session.
• Send a prompt.

Expected

The session starts with the model configured for the selected agent.
The model selector reflects the agent's configured model.

Actual

The New Session dialog keeps the previously selected model.
The new session starts using that incorrect model.
The correct model is only applied after reselecting the agent from within the active session.

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• Web: switching sessions overwrites user-selected model with agent default #35899: Web UI switching sessions overwrites user-selected model with agent default (similar symptom: agent model not correctly applied to session)
• Model selection silently reverts after answering a question #34207: Model selection silently reverts after answering a question (model gets overwritten by agent default in similar circumstances)

If your issue is specifically about the New Session dialog not applying the agent's configured model when first starting a session, it may be distinct enough to stand on its own — but it's worth checking those issues first to see if they cover the same root cause.

[LLawliet444]

I’d like to take this. I reproduced the likely state-priority issue in the new-session composer: the persisted prompt model is preferred over the newly selected agent’s configured model. I’ll add a regression test ensuring an agent selection initializes the new session with that agent’s model while preserving an explicit subsequent model choice.

[Dominic-Leidenfrost]

Hit this on 1.18.13 and dug into it — posting the root cause, since it's a bit more specific than the title suggests and I don't think #38372 as it stands fully closes it.

Two things combine.

packages/app/src/pages/session/composer/prompt-model-selection.ts orders the persisted draft model ahead of the agent's:

const key = [prompt.model.current(), input.agent()?.model, configured(), recent(), fallback()].find(
  (item): item is ModelKey => !!item && valid(item),
)

And openNewTab in packages/app/src/components/titlebar.tsx seeds a new draft with the previous tab's model:

const model = tabs.stateValue<PromptSession>(sessionTab, "prompt")?.model.current()
tabs.newDraft({ server: sessionTab.server, directory: activeSession.directory }, "", model)

So the wrong model is already in the draft before the agent is ever consulted. #38372 hooks the agent select callback, which covers step 3 of the repro here — but a new tab opened without touching the agent picker still starts on the inherited model, because the select never fires.

Worth flagging: #39319 is the same code path from the opposite side, where the agent's model overwriting a manual pick is the complaint. A fix that just makes the agent model win harder will reopen that one.

What worked for me is scoping rather than reordering: tag the persisted selection with the agent it was made under, and only honour it while that agent is active. Values without a tag — pre-existing drafts, and models inherited via openNewTab — lose to a configured agent model, but are still honoured for agents that have no model configured, so the inheritance behaviour stays intact where it's unambiguous.

That gives:

• new session and agent switch land on the agent's model (this issue)
• a manual pick survives across turns while you stay on that agent (Manually selected model reverts to the agent's configured model after every turn #39319)
• switching away and back restores it

I've been running this against 1.18.13 as a patched bundle rather than a source build. Happy to open a PR against dev if that direction looks right.