Use Claude Opus 4.6 with 1M context window as default Anthropic model

Switch default model from claude-sonnet-4-5 to claude-opus-4-6 across
run and serve commands. Enable the 1M token context window via the
context-1m-2025-08-07 beta header for opus-4-6 requests.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

crates/agent/src/profiles/anthropic.rs

@@ -138,17 +138,29 @@ in the project. Keep changes minimal and focused on the task.";
     }
 
     fn capabilities(&self) -> ProfileCapabilities {
+        let context_window_size = if self.model().contains("opus-4-6") {
+            1_000_000
+        } else {
+            200_000
+        };
         ProfileCapabilities {
             supports_reasoning: true,
             supports_streaming: true,
             supports_parallel_tool_calls: true,
-            context_window_size: 200_000,
+            context_window_size,
         }
     }
 
     fn provider_options(&self) -> Option<serde_json::Value> {
         let model = self.model();
-        if model.contains("opus-4-6") || model.contains("sonnet-4-6") {
+        if model.contains("opus-4-6") {
+            Some(serde_json::json!({
+                "anthropic": {
+                    "thinking": {"type": "adaptive"},
+                    "beta_headers": ["context-1m-2025-08-07"]
+                }
+            }))
+        } else if model.contains("sonnet-4-6") {
             Some(serde_json::json!({
                 "anthropic": {
                     "thinking": {"type": "adaptive"}
@@ -185,6 +197,12 @@ mod tests {
         assert_eq!(profile.context_window_size(), 200_000);
     }
 
+    #[test]
+    fn anthropic_opus_4_6_has_1m_context_window() {
+        let profile = AnthropicProfile::new("claude-opus-4-6");
+        assert_eq!(profile.context_window_size(), 1_000_000);
+    }
+
     #[test]
     fn anthropic_system_prompt_contains_env_context() {
         let profile = AnthropicProfile::new("claude-sonnet-4-20250514");
@@ -284,6 +302,10 @@ mod tests {
             Some("adaptive"),
             "thinking type should be adaptive"
         );
+        let beta_headers = options["anthropic"]["beta_headers"]
+            .as_array()
+            .expect("beta_headers should be an array");
+        assert_eq!(beta_headers[0].as_str(), Some("context-1m-2025-08-07"));
     }
 
     #[test]

crates/attractor/src/cli/run.rs

@@ -120,7 +120,7 @@ pub async fn run_command(args: RunArgs, styles: &'static Styles) -> anyhow::Resu
         .unwrap_or_else(|| match provider.as_deref() {
             Some("openai") => "gpt-5.2".to_string(),
             Some("gemini") => "gemini-3.1-pro-preview".to_string(),
-            _ => "claude-sonnet-4-5".to_string(),
+            _ => "claude-opus-4-6".to_string(),
         });
 
     // 6. Build engine

crates/attractor/src/cli/serve.rs

@@ -44,7 +44,7 @@ pub async fn serve_command(args: ServeArgs, styles: &'static Styles) -> anyhow::
     let model = args.model.unwrap_or_else(|| match provider.as_deref() {
         Some("openai") => "gpt-5.2".to_string(),
         Some("gemini") => "gemini-3.1-pro-preview".to_string(),
-        _ => "claude-sonnet-4-5".to_string(),
+        _ => "claude-opus-4-6".to_string(),
     });
 
     // Build registry factory

crates/llm/src/catalog.json

@@ -3,7 +3,7 @@
     "id": "claude-opus-4-6",
     "provider": "anthropic",
     "display_name": "Claude Opus 4.6",
-    "context_window": 200000,
+    "context_window": 1000000,
     "max_output": 128000,
     "supports_tools": true,
     "supports_vision": true,