Plugins: `chat.params` hook cannot override the model (only sampling params) — model-switch plugins are forced to abort+resend, which breaks one-shot `opencode run` callers

Description

Plugins: chat.params hook cannot override the model (only sampling params) — model-switch plugins are forced to abort+resend, which breaks one-shot opencode run callers

Environment

• opencode: v1.18.31 (also observed on v1.18.30)
• Context: plugin ecosystem, specifically model-fallback plugins (opencode-auto-fallback v0.4.59)
• OS: Linux (Debian 13)

Summary

The chat.params plugin hook allows overriding sampling params only — temperature, topP, reasoningEffort, maxTokens, thinking — but not providerID/modelID of the upcoming request. Verified in the plugin's dist code: the hook handler maps exactly those fields onto the output; a model change is impossible from inside the hook.

Consequence: any plugin that needs to switch the model for an in-flight task (rate-limit fallback, context-overflow large-model switch, A/B routing) must abort the session and re-send a new prompt with body.model. There is no abort-free path.

Why that hurts: opencode run callers

For long-lived clients (TUI, HTTP POST /session/:id/message against a serve instance) abort+resend works: the process survives the abort, the plugin's follow-up prompt lands, the task continues on the new model.

But opencode run is a one-shot CLI: when its prompt is aborted, it prints Error: Aborted and exits — taking the plugin's in-flight follow-up prompt with it. The task is silently lost. Real-world case: a daily cron research job (opencode run -s <long-running-session> "<task>") failed 4 days in a row with Error: Aborted because the session context was over the model's threshold and the fallback plugin aborted to switch to a large-context model. Nothing executed; nothing logged on the plugin side that the continuation was lost.

Feature request

Enable plugins to change the model of the upcoming request without aborting the session, e.g.:

1. Extend the chat.params hook so providerID/modelID (and small-model/large-model variants) can be overridden for the request being built, or
2. Add a dedicated hook (e.g. chat.model) that runs before provider resolution and can return a model override.

This would allow abort-free model switching (fallback on rate limit, large-context switch on overflow) and fix the whole class of „scheduled task silently lost" failures. It also complements the native model-fallback discussions (#7602, stalled PR #26292): even with native fallback, plugin-level per-agent routing still needs a clean way to redirect the model.

Related

• #7602 (Native Model Fallback / Failover Support — open)
• #26292 (fallback PR, stalled)

Happy to test patches and provide detailed logs from the real-world failure case.

Plugins

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OpenCode version

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Steps to reproduce

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Screenshot and/or share link

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Operating System

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Terminal

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Trace the chat.params hook and the request path before provider resolution, focusing on how providerID/modelID are currently handled. Compare the proposed chat.params extension with a dedicated chat.model hook, and review related issues #7602 and #26292. Done means a plugin can switch the upcoming model without aborting the session, including for one-shot opencode run callers.