diff --git a/.gitignore b/.gitignore
index 7378666..a034613 100644
--- a/.gitignore
+++ b/.gitignore
@@ -5,3 +5,6 @@ out/
 
 # E2E throwaway sidecars (the harness copies the fixture to a tmpdir, but guard anyway)
 test/e2e/fixtures/workspace/.threads/
+
+# Sandbox playground churn (the EDH workspace - play freely, commit nothing)
+sandbox/.threads/
diff --git a/.vscode/launch.json b/.vscode/launch.json
index 3f863e5..68494a9 100644
--- a/.vscode/launch.json
+++ b/.vscode/launch.json
@@ -5,7 +5,7 @@
       "name": "Run Extension",
       "type": "extensionHost",
       "request": "launch",
-      "args": ["${workspaceFolder}", "--extensionDevelopmentPath=${workspaceFolder}"],
+      "args": ["${workspaceFolder}/sandbox", "--extensionDevelopmentPath=${workspaceFolder}"],
       "outFiles": ["${workspaceFolder}/out/**/*.cjs"],
       "preLaunchTask": "npm: build"
     }
diff --git a/README.md b/README.md
index cc8c202..e6bb5da 100644
--- a/README.md
+++ b/README.md
@@ -26,6 +26,9 @@ human/Claude attribution (Feature #6).
 1. `npm install`
 2. `npm run build`
 3. Press **F5** (or Run → "Run Extension") to launch the Extension Development Host.
+   It opens the committed **`sandbox/`** playground as its workspace (not the
+   repo itself — VS Code won't open one folder in two windows, #8); start with
+   `sandbox/playground.md`.
 4. In the new window: **Cmd/Ctrl+Shift+P** → **"Cowriting: Show Cline SDK Info"**.
 
 ## F2 — Region-anchored threads (Feature #4)
diff --git a/sandbox/playground.md b/sandbox/playground.md
new file mode 100644
index 0000000..234efdf
--- /dev/null
+++ b/sandbox/playground.md
@@ -0,0 +1,21 @@
+# Cowriting sandbox
+
+This folder is the Extension Development Host's workspace (see
+`.vscode/launch.json`): F5 opens it instead of the extension repo itself,
+because VS Code won't open one folder in two windows and the repo is already
+open in your dev window (#8).
+
+Play here:
+
+1. Type a sentence below — it renders with the human left-border as you type.
+2. Highlight it → right-click → **Ask Claude to Edit Selection** — the
+   replacement lands Claude-tinted.
+3. Edit inside the tinted span — it splits character-precisely.
+4. Save, then peek at `sandbox/.threads/playground.md.json` — the git-native
+   attribution record. Reopen the file: spans re-resolve.
+5. Highlight text → right-click → **Add Coauthoring Thread on Selection** for
+   an anchored discussion thread.
+
+---
+
+The quick brown fox jumps over the lazy dog.