feat(plugin): add chat.model hook for dynamic model routing for token savings

packages/opencode/src/session/prompt.ts

@@ -66,6 +66,13 @@ const STRUCTURED_OUTPUT_SYSTEM_PROMPT = `IMPORTANT: The user has requested struc
 export namespace SessionPrompt {
   const log = Log.create({ service: "session.prompt" })
 
+  // Tracks plugin-routed models per session for subagent inheritance
+  const routedModels: Record<string, { providerID: ProviderID; modelID: ModelID }> = {}
+
+  export function getRoutedModel(sessionID: string) {
+    return routedModels[sessionID]
+  }
+
   const state = Instance.state(
     () => {
       const data: Record<
@@ -966,7 +973,34 @@ export namespace SessionPrompt {
   async function createUserMessage(input: PromptInput) {
     const agent = await Agent.get(input.agent ?? (await Agent.defaultAgent()))
 
-    const model = input.model ?? agent.model ?? (await lastModel(input.sessionID))
+    // Resolve the proposed model before plugin can override
+    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: { providerID: proposedModel.providerID as string, modelID: proposedModel.modelID as string },
+      },
+      { model: undefined as { providerID: string; modelID: string } | undefined },
+    )
+
+    // 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) {
+      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,
+      })
+    }
     const full =
       !input.variant && agent.variant
         ? await Provider.getModel(model.providerID, model.modelID).catch(() => undefined)

packages/opencode/src/tool/task.ts

@@ -105,7 +105,8 @@ export const TaskTool = Tool.define("task", async (ctx) => {
       const msg = await MessageV2.get({ sessionID: ctx.sessionID, messageID: ctx.messageID })
       if (msg.info.role !== "assistant") throw new Error("Not an assistant message")
 
-      const model = agent.model ?? {
+      const routedModel = SessionPrompt.getRoutedModel(ctx.sessionID)
+      const model = agent.model ?? routedModel ?? {
         modelID: msg.info.modelID,
         providerID: msg.info.providerID,
       }

packages/plugin/src/index.ts

@@ -179,6 +179,21 @@ export interface Hooks {
     },
     output: { message: UserMessage; parts: Part[] },
   ) => Promise<void>
+  /**
+   * Called to dynamically route messages to different models.
+   * When a plugin sets output.model, OpenCode updates the session's
+   * active model so future turns and subagents inherit the routed model.
+   */
+  "chat.model"?: (
+    input: {
+      sessionID: string
+      agent: string
+      proposedModel: { providerID: string; modelID: string }
+    },
+    output: {
+      model?: { providerID: string; modelID: string }
+    },
+  ) => Promise<void>
   /**
    * Modify parameters sent to LLM
    */