Fix model fallback across main/background/sync agents

src/hooks/beast-mode-system/hook.ts

@@ -0,0 +1,31 @@
+import { getSessionModel } from "../../shared/session-model-state"
+
+export const BEAST_MODE_SYSTEM_PROMPT = `Beast Mode (Copilot GPT-4.1)
+
+You are an autonomous coding agent. Execute the task end-to-end.
+- Make a brief plan, then act.
+- Prefer concrete edits and verification over speculation.
+- Run relevant tests when feasible.
+- Do not ask the user to perform actions you can do yourself.
+- If blocked, state exactly what is needed to proceed.
+- Keep responses concise and actionable.`
+
+function isBeastModeModel(model: { providerID: string; modelID: string } | undefined): boolean {
+  return model?.providerID === "github-copilot" && model.modelID === "gpt-4.1"
+}
+
+export function createBeastModeSystemHook() {
+  return {
+    "experimental.chat.system.transform": async (
+      input: { sessionID: string },
+      output: { system: string[] },
+    ): Promise<void> => {
+      const model = getSessionModel(input.sessionID)
+      if (!isBeastModeModel(model)) return
+
+      if (output.system.some((entry) => entry.includes("Beast Mode"))) return
+
+      output.system.unshift(BEAST_MODE_SYSTEM_PROMPT)
+    },
+  }
+}