tweak: when using messages api for copilot, attach anthropic beta headers

packages/opencode/src/plugin/copilot.ts

@@ -277,6 +277,11 @@ export async function CopilotAuthPlugin(input: PluginInput): Promise<Hooks> {
     },
     "chat.headers": async (input, output) => {
       if (!input.model.providerID.includes("github-copilot")) return
+
+      if (input.model.api.npm === "@ai-sdk/anthropic") {
+        output.headers["anthropic-beta"] = "interleaved-thinking-2025-05-14"
+      }
+
       const session = await sdk.session
         .get({
           path: {