diff --git a/apps/hub/src/builds/app-snapshot.test.ts b/apps/hub/src/builds/app-snapshot.test.ts
index e53cc5df..242c6be1 100644
--- a/apps/hub/src/builds/app-snapshot.test.ts
+++ b/apps/hub/src/builds/app-snapshot.test.ts
@@ -71,6 +71,7 @@ test("the collected app keeps binaries and byte-order marks, honors .gitignore a
       "server/data.sqlite3": png,
       "debug.log": "left out",
       "node_modules/react/index.js": "left out",
+      ".scratch/shots/new-dialog.png": "left out",
       "../home/secret.txt": "home\n",
     };
     for (const [path, content] of Object.entries(files)) {
diff --git a/apps/hub/src/builds/app-snapshot.ts b/apps/hub/src/builds/app-snapshot.ts
index c60cb79e..85ed9693 100644
--- a/apps/hub/src/builds/app-snapshot.ts
+++ b/apps/hub/src/builds/app-snapshot.ts
@@ -16,13 +16,14 @@ import { WorkflowHttpError, workflowHttpJson } from "../workflows/http-json.ts";
 import { INPUTS_DIRECTORY } from "../config/inputs.ts";
 import { spliceDesignScreens } from "./starter.ts";
 
-// Installs and builds are rebuilt by setup and check-app, and .builder and the inputs come from
-// filed documents.
+// Installs and builds are rebuilt by setup and check-app, .builder and the inputs come from filed
+// documents, and .scratch holds the coder's throwaway files, such as the screenshots it takes.
 export const EXCLUDED_DIRECTORIES: ReadonlySet<string> = new Set([
   "node_modules",
   "dist",
   ".git",
   ".builder",
+  ".scratch",
   INPUTS_DIRECTORY,
 ]);
 // Secrets (environment files, keys, registry credentials) and local databases never leave the
diff --git a/workflows/coder/starter/.gitignore b/workflows/coder/starter/.gitignore
index 0a9bee15..2d11a0b7 100644
--- a/workflows/coder/starter/.gitignore
+++ b/workflows/coder/starter/.gitignore
@@ -1,5 +1,6 @@
 node_modules/
 dist/
+.scratch/
 server/data/
 .env
 .env.*
diff --git a/workflows/coder/starter/AGENTS.md b/workflows/coder/starter/AGENTS.md
index 7177d4bc..3672b7ce 100644
--- a/workflows/coder/starter/AGENTS.md
+++ b/workflows/coder/starter/AGENTS.md
@@ -45,7 +45,7 @@ Commands, run from the root:
 
 It does not try features: confirm each requirement in the running app, against the acceptance checklist and the design.
 
-The app's dev server runs while you build and reloads as files change; `app_status` gives its address and the server log. Use the browser tools on that address to try features the way a person would.
+The app's dev server runs while you build and reloads as files change; `app_status` gives its address and the server log. Use the browser tools on that address to try features the way a person would. Keep throwaway files, such as the screenshots you take while trying it, under `.scratch/`; nothing there is part of the app.
 
 ## The app's documents