feat(agentic): route PlaintextFollowup Write through post-write Read

src/crates/core/src/agentic/agents/prompts/agentic_mode.md

@@ -66,10 +66,11 @@ The user will primarily request you perform software engineering tasks. This inc
 - For security-sensitive tasks, support defensive analysis and remediation only. Refuse malicious code, exploit workflows, credential harvesting, or instructions that would facilitate abuse.
 - Edit reliability discipline:
   - Read a file in this session before Edit. Partial range reads are allowed, but the Read range must include every line you will copy into `old_string`.
-  - Base `old_string` on the latest Read result for that file (or exact content from a successful prior Edit/Write on the same file).
+  - Base `old_string` on the latest Read result for that file; after Write, use the post-write Read result returned by the system.
   - Read output uses cat -n format: spaces, line number, tab, then file content. Copy only the text after the tab into `old_string` and `new_string`.
   - Do not reformat HTML/CSS/JS when constructing Edit strings; match indentation and blank lines exactly.
-  - Treat Read output as stale after a successful edit to the same file; re-read before the next Edit unless you are continuing from the updated content in the prior tool result.
+  - Treat Read output as stale after a successful edit to the same file; re-read before the next Edit unless you are continuing from the exact text returned by that Edit or by the post-write Read result.
+  - Use Write only to create a new file or intentionally replace a whole file in one step. After Write succeeds for a path, do not call Write again for that path to continue, refine, or patch it; use Edit against the latest Read content.
   - Use 2-4 adjacent lines with stable surrounding context when that is enough to make `old_string` unique.
   - Use `replace_all` only when every occurrence should change.
   - If Edit fails because text was not found or matched multiple locations, Read the target lines again and retry with freshly copied text — do not adjust the failed string from memory.

src/crates/core/src/agentic/execution/execution_engine.rs

@@ -539,10 +539,6 @@ impl ExecutionEngine {
             .unwrap_or_else(|| "auto".to_string())
     }
 
-    fn should_use_fast_auto_model(turn_index: usize, original_user_input: &str) -> bool {
-        turn_index == 0 && original_user_input.chars().count() <= 10
-    }
-
     fn collect_unlocked_collapsed_tools(
         messages: &[Message],
         collapsed_tools: &[String],
@@ -822,9 +818,11 @@ impl ExecutionEngine {
         let resolved_configured_model_id =
             Self::resolve_configured_model_id(&ai_config, &configured_model_id);
 
-        let model_id = if configured_model_id == "auto" || resolved_configured_model_id == "auto" {
-            let use_fast_model = Self::should_use_fast_auto_model(turn_index, original_user_input);
-            let fallback_model = if use_fast_model { "fast" } else { "primary" };
+        let model_id = if configured_model_id == "auto"
+            || configured_model_id == "default"
+            || resolved_configured_model_id == "auto"
+        {
+            let fallback_model = "primary";
             let resolved_model_id = ai_config.resolve_model_selection(fallback_model);
 
             if let Some(resolved_model_id) = resolved_model_id {
@@ -3169,25 +3167,6 @@ mod tests {
         }
     }
 
-    #[test]
-    fn auto_model_uses_fast_for_short_first_message() {
-        assert!(ExecutionEngine::should_use_fast_auto_model(0, "你好"));
-        assert!(ExecutionEngine::should_use_fast_auto_model(0, "1234567890"));
-    }
-
-    #[test]
-    fn auto_model_uses_primary_for_long_first_message() {
-        assert!(!ExecutionEngine::should_use_fast_auto_model(
-            0,
-            "12345678901"
-        ));
-    }
-
-    #[test]
-    fn auto_model_uses_primary_after_first_turn() {
-        assert!(!ExecutionEngine::should_use_fast_auto_model(1, "短消息"));
-    }
-
     #[test]
     fn resolve_configured_fast_model_falls_back_to_primary_when_fast_is_stale() {
         let mut ai_config = AIConfig::default();