fix: use branded ProviderID/ModelID types for routed model storage

Convert plain strings from plugin hook to branded types using
ProviderID.make() and ModelID.make(). Cast branded types to plain
strings when sending to the hook interface. Fixes TS2345/TS2322
typecheck errors.

Author: rjmorales13

packages/opencode/src/session/prompt.ts

@@ -67,7 +67,7 @@ export namespace SessionPrompt {
   const log = Log.create({ service: "session.prompt" })
 
   // Tracks plugin-routed models per session for subagent inheritance
-  const routedModels: Record<string, { providerID: string; modelID: string }> = {}
+  const routedModels: Record<string, { providerID: ProviderID; modelID: ModelID }> = {}
 
   export function getRoutedModel(sessionID: string) {
     return routedModels[sessionID]
@@ -977,23 +977,28 @@ export namespace SessionPrompt {
     const proposedModel = input.model ?? agent.model ?? (await lastModel(input.sessionID))
 
     // Fire chat.model hook to allow plugins to dynamically route to different models
+    // Plugin types use plain strings; cast branded types for the hook interface
     const modelOverride = await Plugin.trigger(
       "chat.model",
       {
         sessionID: input.sessionID,
         agent: agent.name,
-        proposedModel,
+        proposedModel: { providerID: proposedModel.providerID as string, modelID: proposedModel.modelID as string },
       },
       { model: undefined as { providerID: string; modelID: string } | undefined },
     )
 
-    // If plugin set a model, persist it for subagent inheritance
-    const model = modelOverride.model ?? proposedModel
+    // If plugin set a model, convert plain strings back to branded types and persist
+    let model = proposedModel as { providerID: ProviderID; modelID: ModelID }
     if (modelOverride.model) {
-      routedModels[input.sessionID] = modelOverride.model
+      model = {
+        providerID: ProviderID.make(modelOverride.model.providerID),
+        modelID: ModelID.make(modelOverride.model.modelID),
+      }
+      routedModels[input.sessionID] = model
       log.info("plugin routed model", {
         sessionID: input.sessionID,
-        model: modelOverride.model,
+        model,
       })
     }
     const full =