Only send 1M context beta header for supported models

Author: charlesvien

packages/agent/src/adapters/claude/claude-agent.ts

@@ -66,6 +66,7 @@ import {
   DEFAULT_MODEL,
   getEffortOptions,
   resolveModelPreference,
+  supports1MContext,
   toSdkModelId,
 } from "./session/models";
 import {
@@ -932,6 +933,10 @@ export class ClaudeAcpAgent extends BaseAcpAgent {
       await this.session.query.setModel(resolvedSdkModel);
     }
 
+    if (supports1MContext(resolvedModelId)) {
+      options.betas = ["context-1m-2025-08-07"];
+    }
+
     const availableModes = getAvailableModes();
     const modes: SessionModeState = {
       currentModeId: permissionMode,