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

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

esbuild.mjs

@@ -9,7 +9,7 @@ const options = {
   bundle: true,
   platform: "node",
   format: "cjs",
-  target: "node20",
+  target: "node22",
   sourcemap: true,
   // vscode is provided by the host; @cline/sdk is ESM-only and is loaded at
   // runtime via dynamic import() from node_modules, so keep both external.

scripts/smoke-live-turn.mjs

@@ -17,6 +17,6 @@ try {
   console.log(`elapsed:     ${((Date.now() - t0) / 1000).toFixed(1)}s`);
   process.exit(0);
 } catch (err) {
-  console.error(`live turn failed (expected when Claude Code is absent/signed out): ${err.message}`);
+  console.error(`live turn failed (expected when Claude Code is absent/signed out): ${err instanceof Error ? err.message : String(err)}`);
   process.exit(1);
 }

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 };
 }

test/liveTurn.test.ts

@@ -3,16 +3,24 @@ import { extractReplacement } from "../src/liveTurn";
 
 describe("extractReplacement", () => {
   it("returns plain text untouched", () => {
-    expect(extractReplacement("Hello world.")).toBe("Hello world.");
+    expect(extractReplacement("Hello world.", "x")).toBe("Hello world.");
   });
   it("strips a wrapping fence", () => {
-    expect(extractReplacement("```\nHello world.\n```")).toBe("Hello world.");
+    expect(extractReplacement("```\nHello world.\n```", "x")).toBe("Hello world.");
   });
   it("strips a language-tagged fence", () => {
-    expect(extractReplacement("```markdown\nHello.\n```\n")).toBe("Hello.");
+    expect(extractReplacement("```markdown\nHello.\n```\n", "x")).toBe("Hello.");
   });
   it("leaves inner fences alone", () => {
     const inner = "before\n```js\ncode\n```\nafter";
-    expect(extractReplacement(inner)).toBe(inner);
+    expect(extractReplacement(inner, "x")).toBe(inner);
+  });
+  it("keeps the fence when the original selection was itself fence-wrapped", () => {
+    const sel = "```python\nold()\n```";
+    const out = "```python\nnew()\n```";
+    expect(extractReplacement(out, sel)).toBe(out);
+  });
+  it("still strips when the selection was plain but the model added a fence", () => {
+    expect(extractReplacement("```\nplain\n```", "plain old text")).toBe("plain");
   });
 });