F3 SLICE-5 review fixes: fence-aware replacement extraction + editSelection guards (#6)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/extension.ts

@@ -86,7 +86,12 @@ export function activate(context: vscode.ExtensionContext): CowritingApi | undef
   context.subscriptions.push(
     vscode.commands.registerCommand("cowriting.editSelection", async () => {
       const editor = vscode.window.activeTextEditor;
-      if (!editor || editor.selection.isEmpty || !editor.document.uri.fsPath.startsWith(root)) {
+      if (
+        !editor ||
+        editor.selection.isEmpty ||
+        editor.document.uri.scheme !== "file" ||
+        !editor.document.uri.fsPath.startsWith(root)
+      ) {
         void vscode.window.showWarningMessage("Cowriting: select some text in a workspace document first.");
         return;
       }
@@ -95,6 +100,10 @@ export function activate(context: vscode.ExtensionContext): CowritingApi | undef
         placeHolder: "e.g. tighten this paragraph",
       });
       if (!instruction) return;
+      if (editor.selection.isEmpty) {
+        void vscode.window.showWarningMessage("Cowriting: select some text in a workspace document first.");
+        return;
+      }
       const document = editor.document;
       const selection = editor.selection;
       const expectedVersion = document.version;
@@ -106,6 +115,12 @@ export function activate(context: vscode.ExtensionContext): CowritingApi | undef
           async () => {
             const { runEditTurn } = await import("./liveTurn");
             const turn = await runEditTurn(instruction, selectedText);
+            if (turn.replacement === "") {
+              void vscode.window.showWarningMessage(
+                "Cowriting: Claude returned an empty replacement — nothing was applied.",
+              );
+              return;
+            }
             const ok = await attributionController.applyAgentEdit(
               document,
               new vscode.Range(selection.start, selection.end),
@@ -119,7 +134,7 @@ export function activate(context: vscode.ExtensionContext): CowritingApi | undef
             );
             if (!ok) {
               void vscode.window.showWarningMessage(
-                "Cowriting: the document changed while Claude was editing — nothing was applied.",
+                "Cowriting: the edit could not be applied (the document changed during the turn, or the editor rejected the edit) — nothing was changed.",
               );
             }
           },

src/liveTurn.ts

@@ -24,8 +24,15 @@ const SYSTEM_PROMPT = [
   "unless the instruction says otherwise.",
 ].join(" ");
 
-/** Strip a wrapping markdown code fence if the model added one anyway. */
-export function extractReplacement(outputText: string): string {
+/**
+ * Strip a wrapping markdown code fence if the model added one anyway.
+ * If `selectedText` itself was fence-wrapped (i.e. the user's selection was a
+ * fenced block), the model mirroring that format is correct — return
+ * `outputText` unchanged so the fence is preserved.
+ */
+export function extractReplacement(outputText: string, selectedText: string): string {
+  const fencePattern = /^\s*```[^\n]*\n[\s\S]*?\n?```\s*$/;
+  if (fencePattern.test(selectedText)) return outputText;
   const m = outputText.match(/^\s*```[^\n]*\n([\s\S]*?)\n?```\s*$/);
   return m ? m[1] : outputText;
 }
@@ -53,5 +60,5 @@ export async function runEditTurn(
         "(is Claude Code installed and signed in?)",
     );
   }
-  return { replacement: extractReplacement(result.outputText), model: modelId, sessionId: result.runId };
+  return { replacement: extractReplacement(result.outputText, selectedText), model: modelId, sessionId: result.runId };
 }