Keep Anthropic beta flags out of JSON body

rust/crates/api/tests/client_integration.rs

@@ -97,10 +97,7 @@ async fn send_message_posts_json_and_parses_response() {
     assert!(body.get("stream").is_none());
     assert_eq!(body["tools"][0]["name"], json!("get_weather"));
     assert_eq!(body["tool_choice"]["type"], json!("auto"));
-    assert_eq!(
-        body["betas"],
-        json!(["claude-code-20250219", "prompt-caching-scope-2026-01-05"])
-    );
+    assert!(body.get("betas").is_none());
 }
 
 #[tokio::test]
@@ -156,14 +153,7 @@ async fn send_message_applies_request_profile_and_records_telemetry() {
     let body: serde_json::Value =
         serde_json::from_str(&request.body).expect("request body should be json");
     assert_eq!(body["metadata"]["source"], json!("clawd-code"));
-    assert_eq!(
-        body["betas"],
-        json!([
-            "claude-code-20250219",
-            "prompt-caching-scope-2026-01-05",
-            "tools-2026-04-01"
-        ])
-    );
+    assert!(body.get("betas").is_none());
 
     let events = sink.events();
     assert_eq!(events.len(), 6);

rust/crates/telemetry/src/lib.rs

@@ -115,12 +115,7 @@ impl AnthropicRequestProfile {
         for (key, value) in &self.extra_body {
             object.insert(key.clone(), value.clone());
         }
-        if !self.betas.is_empty() {
-            object.insert(
-                "betas".to_string(),
-                Value::Array(self.betas.iter().cloned().map(Value::String).collect()),
-            );
-        }
+        // Anthropic expects beta flags in the `anthropic-beta` header, not as a JSON body field.
         Ok(body)
     }
 }
@@ -432,7 +427,7 @@ mod tests {
     use super::*;
 
     #[test]
-    fn request_profile_emits_headers_and_merges_body() {
+    fn request_profile_emits_headers_and_merges_body_without_json_betas() {
         let profile = AnthropicRequestProfile::new(
             ClientIdentity::new("claude-code", "1.2.3").with_runtime("rust-cli"),
         )
@@ -462,14 +457,7 @@ mod tests {
             body["metadata"]["source"],
             Value::String("test".to_string())
         );
-        assert_eq!(
-            body["betas"],
-            serde_json::json!([
-                "claude-code-20250219",
-                "prompt-caching-scope-2026-01-05",
-                "tools-2026-04-01"
-            ])
-        );
+        assert!(body.get("betas").is_none());
     }
 
     #[test]