Simplify coding-agent-loop: remove redundant constructors, use let-else, add doc comments

- Remove History::new() (redundant with #[derive(Default)]), replace all callers with History::default()
- Replace match with let-else in extract_signatures_from_assistant for clearer happy path
- Add doc comments to Turn::System and Turn::Steering explaining their LLM role mapping
- Verified: Arc import in openai.rs is used in production code, mutex .expect() messages are consistent

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

crates/coding-agent-loop/src/history.rs

@@ -7,10 +7,6 @@ pub struct History {
 }
 
 impl History {
-    pub fn new() -> Self {
-        Self { turns: Vec::new() }
-    }
-
     pub fn push(&mut self, turn: Turn) {
         self.turns.push(turn);
     }
@@ -87,14 +83,14 @@ mod tests {
 
     #[test]
     fn empty_history_produces_empty_messages() {
-        let history = History::new();
+        let history = History::default();
         assert!(history.convert_to_messages().is_empty());
         assert_eq!(history.turns().len(), 0);
     }
 
     #[test]
     fn user_turn_maps_to_user_message() {
-        let mut history = History::new();
+        let mut history = History::default();
         history.push(Turn::User {
             content: "Hello".into(),
             timestamp: SystemTime::now(),
@@ -107,7 +103,7 @@ mod tests {
 
     #[test]
     fn assistant_turn_maps_to_assistant_message() {
-        let mut history = History::new();
+        let mut history = History::default();
         history.push(Turn::Assistant {
             content: "Hi there".into(),
             tool_calls: vec![],
@@ -124,7 +120,7 @@ mod tests {
 
     #[test]
     fn assistant_turn_with_tool_calls() {
-        let mut history = History::new();
+        let mut history = History::default();
         let tc = ToolCall::new("call_1", "read_file", serde_json::json!({"path": "foo.rs"}));
         history.push(Turn::Assistant {
             content: "Let me read that".into(),
@@ -146,7 +142,7 @@ mod tests {
 
     #[test]
     fn assistant_turn_with_reasoning() {
-        let mut history = History::new();
+        let mut history = History::default();
         history.push(Turn::Assistant {
             content: "The answer is 42".into(),
             tool_calls: vec![],
@@ -166,7 +162,7 @@ mod tests {
 
     #[test]
     fn tool_results_turn_maps_to_tool_message() {
-        let mut history = History::new();
+        let mut history = History::default();
         let result = ToolResult {
             tool_call_id: "call_1".into(),
             content: serde_json::json!("file contents here"),
@@ -186,7 +182,7 @@ mod tests {
 
     #[test]
     fn system_turn_maps_to_system_message() {
-        let mut history = History::new();
+        let mut history = History::default();
         history.push(Turn::System {
             content: "You are a coding assistant".into(),
             timestamp: SystemTime::now(),
@@ -199,7 +195,7 @@ mod tests {
 
     #[test]
     fn steering_turn_maps_to_user_message() {
-        let mut history = History::new();
+        let mut history = History::default();
         history.push(Turn::Steering {
             content: "Focus on the main task".into(),
             timestamp: SystemTime::now(),
@@ -212,7 +208,7 @@ mod tests {
 
     #[test]
     fn turns_len_matches_push_count() {
-        let mut history = History::new();
+        let mut history = History::default();
         assert_eq!(history.turns().len(), 0);
         history.push(Turn::User {
             content: "First".into(),
@@ -232,7 +228,7 @@ mod tests {
 
     #[test]
     fn round_trip_preserves_content() {
-        let mut history = History::new();
+        let mut history = History::default();
         history.push(Turn::User {
             content: "Hello".into(),
             timestamp: SystemTime::now(),

crates/coding-agent-loop/src/loop_detection.rs

@@ -12,13 +12,13 @@ fn tool_call_signature(name: &str, arguments: &serde_json::Value) -> u64 {
 }
 
 fn extract_signatures_from_assistant(turn: &Turn) -> Vec<u64> {
-    match turn {
-        Turn::Assistant { tool_calls, .. } => tool_calls
-            .iter()
-            .map(|tc| tool_call_signature(&tc.name, &tc.arguments))
-            .collect(),
-        _ => vec![],
-    }
+    let Turn::Assistant { tool_calls, .. } = turn else {
+        return vec![];
+    };
+    tool_calls
+        .iter()
+        .map(|tc| tool_call_signature(&tc.name, &tc.arguments))
+        .collect()
 }
 
 pub fn detect_loop(history: &History, window_size: usize) -> bool {
@@ -109,20 +109,20 @@ mod tests {
 
     #[test]
     fn too_few_turns_returns_false() {
-        let history = History::new();
+        let history = History::default();
         assert!(!detect_loop(&history, 10));
     }
 
     #[test]
     fn single_turn_returns_false() {
-        let mut history = History::new();
+        let mut history = History::default();
         history.push(assistant_with_tool("shell", serde_json::json!({"cmd": "ls"})));
         assert!(!detect_loop(&history, 10));
     }
 
     #[test]
     fn pattern_1_repeating_detected() {
-        let mut history = History::new();
+        let mut history = History::default();
         // Same tool call repeated 3 times
         history.push(assistant_with_tool("shell", serde_json::json!({"cmd": "ls"})));
         history.push(assistant_with_tool("shell", serde_json::json!({"cmd": "ls"})));
@@ -132,7 +132,7 @@ mod tests {
 
     #[test]
     fn pattern_2_repeating_detected() {
-        let mut history = History::new();
+        let mut history = History::default();
         // A-B-A-B pattern
         history.push(assistant_with_tool("shell", serde_json::json!({"cmd": "ls"})));
         history.push(assistant_with_tool("read_file", serde_json::json!({"path": "foo.rs"})));
@@ -143,7 +143,7 @@ mod tests {
 
     #[test]
     fn pattern_3_repeating_detected() {
-        let mut history = History::new();
+        let mut history = History::default();
         // A-B-C-A-B-C pattern
         history.push(assistant_with_tool("shell", serde_json::json!({"cmd": "ls"})));
         history.push(assistant_with_tool("read_file", serde_json::json!({"path": "a.rs"})));
@@ -156,7 +156,7 @@ mod tests {
 
     #[test]
     fn non_repeating_returns_false() {
-        let mut history = History::new();
+        let mut history = History::default();
         history.push(assistant_with_tool("shell", serde_json::json!({"cmd": "ls"})));
         history.push(assistant_with_tool("read_file", serde_json::json!({"path": "a.rs"})));
         history.push(assistant_with_tool("grep", serde_json::json!({"pattern": "fn"})));
@@ -166,7 +166,7 @@ mod tests {
 
     #[test]
     fn same_name_different_args_are_different() {
-        let mut history = History::new();
+        let mut history = History::default();
         history.push(assistant_with_tool("shell", serde_json::json!({"cmd": "ls"})));
         history.push(assistant_with_tool("shell", serde_json::json!({"cmd": "pwd"})));
         history.push(assistant_with_tool("shell", serde_json::json!({"cmd": "cat"})));
@@ -196,7 +196,7 @@ mod tests {
 
     #[test]
     fn user_turns_are_ignored() {
-        let mut history = History::new();
+        let mut history = History::default();
         history.push(Turn::User {
             content: "hello".into(),
             timestamp: SystemTime::now(),
@@ -214,7 +214,7 @@ mod tests {
 
     #[test]
     fn window_size_limits_lookback() {
-        let mut history = History::new();
+        let mut history = History::default();
         // Add non-repeating turns first
         history.push(assistant_with_tool("shell", serde_json::json!({"cmd": "unique1"})));
         history.push(assistant_with_tool("shell", serde_json::json!({"cmd": "unique2"})));

crates/coding-agent-loop/src/session.rs

@@ -45,7 +45,7 @@ impl Session {
         Self {
             id: uuid::Uuid::new_v4().to_string(),
             config,
-            history: History::new(),
+            history: History::default(),
             event_emitter: EventEmitter::new(),
             state: SessionState::Idle,
             llm_client,

crates/coding-agent-loop/src/types.rs

@@ -19,10 +19,13 @@ pub enum Turn {
         results: Vec<ToolResult>,
         timestamp: SystemTime,
     },
+    /// Injected content sent as a system-role message to the LLM (maps to `Role::System`).
     System {
         content: String,
         timestamp: SystemTime,
     },
+    /// Injected steering content sent as a user-role message to the LLM (maps to `Role::User`).
+    /// Used to guide the assistant's behavior mid-conversation without appearing as actual user input.
     Steering {
         content: String,
         timestamp: SystemTime,