diff --git a/src/attributionController.ts b/src/attributionController.ts
index 470ef21..8744c22 100644
--- a/src/attributionController.ts
+++ b/src/attributionController.ts
@@ -17,6 +17,7 @@ import { gitUserEmail } from "./identity";
 import { applyChange, coalesce, type LiveSpan } from "./attributionTracker";
 import { minimizeReplace, PendingEditRegistry } from "./pendingEdits";
 import type { VersionGuard } from "./versionGuard";
+import { isUnderRoot } from "./workspacePath";
 
 /** Test-facing snapshot of live attribution state for a document. */
 export interface RenderedSpan {
@@ -87,7 +88,7 @@ export class AttributionController implements vscode.Disposable {
   }
 
   private isTracked(document: vscode.TextDocument): boolean {
-    return document.uri.scheme === "file" && document.uri.fsPath.startsWith(this.rootDir);
+    return document.uri.scheme === "file" && isUnderRoot(document.uri.fsPath, this.rootDir);
   }
   private docPathOf(uri: vscode.Uri): string {
     return vscode.workspace.asRelativePath(uri, false);
diff --git a/src/extension.ts b/src/extension.ts
index be53c2c..2bd71bd 100644
--- a/src/extension.ts
+++ b/src/extension.ts
@@ -9,6 +9,7 @@ import { VersionGuard } from "./versionGuard";
 import { BaselineStore } from "./baselineStore";
 import { DiffViewController } from "./diffViewController";
 import { TrackChangesPreviewController } from "./trackChangesPreview";
+import { isUnderRoot, selectionRejection } from "./workspacePath";
 
 const CHANNEL_NAME = "Cowriting (Cline SDK)";
 
@@ -187,22 +188,28 @@ 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.scheme !== "file" ||
-        !editor.document.uri.fsPath.startsWith(root)
-      ) {
-        void vscode.window.showWarningMessage("Cowriting: select some text in a workspace document first.");
+      // Each failure names its real reason (no editor / no selection / unsaved /
+      // outside the workspace folder) — not always "select some text" (#bug:
+      // an out-of-workspace file was rejected with the no-selection message).
+      const reason = selectionRejection({
+        hasEditor: !!editor,
+        selectionEmpty: editor?.selection.isEmpty ?? true,
+        scheme: editor?.document.uri.scheme ?? "",
+        fsPath: editor?.document.uri.fsPath ?? "",
+        root,
+      });
+      if (reason) {
+        void vscode.window.showWarningMessage(reason);
         return;
       }
+      if (!editor) return; // unreachable once reason is null, but narrows the type
       const instruction = await vscode.window.showInputBox({
         prompt: "What should Claude do with the selection?",
         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.");
+        void vscode.window.showWarningMessage("Cowriting: select some text to send to Claude first.");
         return;
       }
       const document = editor.document;
@@ -262,7 +269,7 @@ export function activate(context: vscode.ExtensionContext): CowritingApi | undef
 
   // Render threads + attributions for already-open editors, and on future opens.
   const renderIfOpen = (doc: vscode.TextDocument) => {
-    if (doc.uri.scheme === "file" && doc.uri.fsPath.startsWith(root)) {
+    if (doc.uri.scheme === "file" && isUnderRoot(doc.uri.fsPath, root)) {
       threadController.renderAll(doc);
       attributionController.loadAll(doc);
       proposalController.renderAll(doc);
diff --git a/src/proposalController.ts b/src/proposalController.ts
index 6ae714b..6b15570 100644
--- a/src/proposalController.ts
+++ b/src/proposalController.ts
@@ -16,6 +16,7 @@ import { resolve, shift, type OffsetRange } from "./anchorer";
 import { addProposal, proposalBody, removeProposal } from "./proposalModel";
 import type { AttributionController } from "./attributionController";
 import type { VersionGuard } from "./versionGuard";
+import { isUnderRoot } from "./workspacePath";
 
 /** Test-facing snapshot of what is currently rendered for a document. */
 export interface RenderedProposal {
@@ -70,7 +71,7 @@ export class ProposalController implements vscode.Disposable {
   }
 
   private isTracked(document: vscode.TextDocument): boolean {
-    return document.uri.scheme === "file" && document.uri.fsPath.startsWith(this.rootDir);
+    return document.uri.scheme === "file" && isUnderRoot(document.uri.fsPath, this.rootDir);
   }
   private docPathOf(uri: vscode.Uri): string {
     return vscode.workspace.asRelativePath(uri, false);
diff --git a/src/threadController.ts b/src/threadController.ts
index 005e1a5..515b54b 100644
--- a/src/threadController.ts
+++ b/src/threadController.ts
@@ -14,6 +14,7 @@ import { buildFingerprint, resolve, shift, type OffsetRange } from "./anchorer";
 import { gitUserEmail } from "./identity";
 import { addThread, appendMessage, setStatus } from "./threadModel";
 import type { VersionGuard } from "./versionGuard";
+import { isUnderRoot } from "./workspacePath";
 
 /** Test-facing snapshot of what is currently rendered for a document. */
 export interface RenderedThread {
@@ -69,7 +70,7 @@ export class ThreadController implements vscode.Disposable {
   }
 
   private isInRoot(uri: vscode.Uri): boolean {
-    return uri.scheme === "file" && uri.fsPath.startsWith(this.rootDir);
+    return uri.scheme === "file" && isUnderRoot(uri.fsPath, this.rootDir);
   }
 
   private docPathOf(uri: vscode.Uri): string {
diff --git a/src/workspacePath.ts b/src/workspacePath.ts
new file mode 100644
index 0000000..5c61292
--- /dev/null
+++ b/src/workspacePath.ts
@@ -0,0 +1,56 @@
+/**
+ * workspacePath — pure, vscode-free helpers for the "is this document one
+ * Cowriting can anchor to?" decision (F2/F3/F4 require a SAVED file under the
+ * workspace folder, because threads/attribution/proposals persist to a
+ * `.threads/` sidecar beside that file).
+ *
+ * Split out so the membership test and the per-condition warning are
+ * deterministic and unit-testable with no editor — see `test/workspacePath.test.ts`.
+ */
+import * as path from "node:path";
+
+/**
+ * Is `fsPath` the workspace root or a path strictly inside it?
+ *
+ * Uses a path-separator boundary, NOT a bare `startsWith(root)` — otherwise a
+ * sibling whose name merely begins with the root's name would falsely match
+ * (e.g. `.../vscode-cowriting-plugin-content/x` vs root `.../vscode-cowriting-plugin`).
+ */
+export function isUnderRoot(fsPath: string, root: string): boolean {
+  return fsPath === root || fsPath.startsWith(root + path.sep);
+}
+
+export interface SelectionContext {
+  /** Is there an active text editor at all? */
+  hasEditor: boolean;
+  /** Is the active editor's selection empty (no highlight)? */
+  selectionEmpty: boolean;
+  /** The active document's URI scheme (`file`, `untitled`, …). */
+  scheme: string;
+  /** The active document's filesystem path (empty if none). */
+  fsPath: string;
+  /** The workspace folder Cowriting is anchored to. */
+  root: string;
+}
+
+/**
+ * Why a selection can't be sent to Claude — or `null` if it can. Each condition
+ * gets its OWN message so the user learns the real reason instead of always
+ * being told to "select some text" (the reported bug: an out-of-workspace file
+ * was rejected with the no-selection message even though text was selected).
+ */
+export function selectionRejection(ctx: SelectionContext): string | null {
+  if (!ctx.hasEditor) {
+    return "Cowriting: focus a text editor with a selection first.";
+  }
+  if (ctx.selectionEmpty) {
+    return "Cowriting: select some text to send to Claude first.";
+  }
+  if (ctx.scheme !== "file") {
+    return "Cowriting: save this document to a file first — Cowriting anchors edits to a saved workspace file.";
+  }
+  if (!isUnderRoot(ctx.fsPath, ctx.root)) {
+    return "Cowriting: this file is outside your workspace folder — open a file inside it to ask Claude to edit (Cowriting anchors edits and attribution to files in the workspace).";
+  }
+  return null;
+}
diff --git a/test/workspacePath.test.ts b/test/workspacePath.test.ts
new file mode 100644
index 0000000..55b3a74
--- /dev/null
+++ b/test/workspacePath.test.ts
@@ -0,0 +1,55 @@
+import { describe, it, expect } from "vitest";
+import { isUnderRoot, selectionRejection } from "../src/workspacePath";
+
+describe("isUnderRoot", () => {
+  const root = "/a/vscode-cowriting-plugin/sandbox";
+
+  it("accepts the root itself and files inside it", () => {
+    expect(isUnderRoot(root, root)).toBe(true);
+    expect(isUnderRoot(`${root}/docs/x.md`, root)).toBe(true);
+  });
+
+  it("rejects files outside the root", () => {
+    expect(isUnderRoot("/a/other/x.md", root)).toBe(false);
+  });
+
+  it("rejects a SIBLING whose path is a string-prefix of the root's parent (the prefix-collision bug)", () => {
+    // The reported case: EDH root is .../vscode-cowriting-plugin/sandbox, the file
+    // is in the sibling content repo. Plain startsWith on the plugin-repo prefix
+    // would falsely match; the separator boundary must reject it.
+    const pluginRoot = "/a/vscode-cowriting-plugin";
+    const contentFile = "/a/vscode-cowriting-plugin-content/issues/x.md";
+    expect(contentFile.startsWith(pluginRoot)).toBe(true); // the latent bug
+    expect(isUnderRoot(contentFile, pluginRoot)).toBe(false); // fixed
+  });
+});
+
+describe("selectionRejection", () => {
+  const root = "/ws";
+  const ok = { hasEditor: true, selectionEmpty: false, scheme: "file", fsPath: "/ws/a.md", root };
+
+  it("returns null when everything is valid", () => {
+    expect(selectionRejection(ok)).toBeNull();
+  });
+
+  it("names the missing editor", () => {
+    const msg = selectionRejection({ ...ok, hasEditor: false });
+    expect(msg).toMatch(/focus a text editor/i);
+  });
+
+  it("names the empty selection (only when there IS an editor)", () => {
+    const msg = selectionRejection({ ...ok, selectionEmpty: true });
+    expect(msg).toMatch(/select some text/i);
+  });
+
+  it("names an unsaved/non-file document", () => {
+    const msg = selectionRejection({ ...ok, scheme: "untitled" });
+    expect(msg).toMatch(/save this document/i);
+  });
+
+  it("names an out-of-workspace file (the reported bug — NOT a selection problem)", () => {
+    const msg = selectionRejection({ ...ok, fsPath: "/elsewhere/a.md" });
+    expect(msg).toMatch(/outside your workspace folder/i);
+    expect(msg).not.toMatch(/select some text/i);
+  });
+});