refactor: refine system prompt and runtime context

Author: linyuqiao

YCode.CLI/Program.cs

@@ -50,6 +50,11 @@
 var system = $"""
     You are **YCode**, a senior coding agent operating INSIDE the user's repository at: {config.WorkDir}
 
+    ## Response style
+    - Do exactly what the user asks; nothing more, nothing less.
+    - Be concise and direct. Avoid preambles, postambles, and unnecessary explanations.
+    - If you changed a file, briefly confirm completion unless the user asked for details.
+
     ## Core operating mode (ReAct)
     Use this loop on every turn:
     1) **Reason**: quickly classify intent, risk, and missing facts.
@@ -61,6 +66,7 @@ Keep reasoning internal. Do not expose long chain-of-thought. Output only concis
 
     ## Tool-routing policy (must be precise)
     - File/system inspection or execution -> MCP tools (`read_file`, `run`, etc.).
+    - For filesystem-heavy operations (bulk create/move/copy, scaffolding, template generation), prefer the shell via `run` and CLI tooling appropriate to the current OS/shell. Use file tools for targeted edits.
     - Multi-step work -> `TodoWriter` (keep exactly one `in_progress`).
     - Save durable memory -> `MemoryWriter`:
       - `profile`: stable user preferences/habits.
@@ -82,14 +88,23 @@ Keep reasoning internal. Do not expose long chain-of-thought. Output only concis
     ## Execution rules
     - Never invent file paths; discover first.
     - Apply minimal safe edits.
+    - Prefer editing existing files; avoid creating new files unless required. Do not create documentation files unless the user explicitly requests them.
     - Avoid destructive/privileged shell operations.
     - Keep answers short, structured, and test-oriented.
 
+    ## Subagent usage (Task)
+    - Use `Task` when you need isolated, focused work (deep scan, plan, or implementation) without polluting the main thread.
+    - Provide: `description` (3–5 words), `prompt` (explicit goals + constraints), `agent_type` (must match the list below).
+    - Prefer `Task` for complex research, large refactors, or multi-file changes that need a dedicated pass.
+
     ## Available subAgents
     {agentManager.GetDescription()}
 
     ## Available skills
     {skills.GetDescription()}
+
+    ## Runtime environment
+    - OS: {config.OsPlatform} ({config.OsDescription})
 """;
 
 var tools = await toolManager.Register();

YCode.CLI/ServiceRegistration.cs

@@ -12,8 +12,16 @@ public static IServiceProvider Register(this IServiceCollection services)
             var uri = Environment.GetEnvironmentVariable("YCODE_API_BASE_URI")!;
             var model = Environment.GetEnvironmentVariable("YCODE_MODEL")!;
             var workDir = Directory.GetCurrentDirectory();
+            var osDescription = System.Runtime.InteropServices.RuntimeInformation.OSDescription;
+            var osPlatform = System.Runtime.InteropServices.RuntimeInformation.IsOSPlatform(System.Runtime.InteropServices.OSPlatform.Windows)
+                ? "Windows"
+                : System.Runtime.InteropServices.RuntimeInformation.IsOSPlatform(System.Runtime.InteropServices.OSPlatform.Linux)
+                    ? "Linux"
+                    : System.Runtime.InteropServices.RuntimeInformation.IsOSPlatform(System.Runtime.InteropServices.OSPlatform.OSX)
+                        ? "macOS"
+                        : "Unknown";
 
-            services.AddSingleton(new AppConfig(key, uri, model, workDir));
+            services.AddSingleton(new AppConfig(key, uri, model, workDir, osPlatform, osDescription));
 
             services.RegisterAttributedServices();
 
@@ -56,7 +64,7 @@ private static void RegisterAttributedServices(this IServiceCollection services)
         }
     }
 
-    internal sealed record AppConfig(string Key, string Uri, string Model, string WorkDir);
+    internal sealed record AppConfig(string Key, string Uri, string Model, string WorkDir, string OsPlatform, string OsDescription);
 
     [AttributeUsage(AttributeTargets.Class, AllowMultiple = false)]
     internal class InjectAttribute : Attribute