diff --git a/tools/ci/project-shells.mjs b/tools/ci/project-shells.mjs
index 635c3c09..6dcdb972 100755
--- a/tools/ci/project-shells.mjs
+++ b/tools/ci/project-shells.mjs
@@ -4,6 +4,11 @@
  * a project and `POST` hands out a random id, so a fresh CI checkout has to put the shells with
  * the known ids in place itself. The playground is left out – no tool may touch it.
  *
+ * A file that is already there is left alone. Overwriting it used to cost a development machine
+ * the project's whole revision history and its name: the shell carries the key from PROJECTS as
+ * its name, and the seeders keep whatever name they find, so "Jump and Run" came back as
+ * "jumpRun". Only CI ever needs the shells, and there the directory is empty.
+ *
  * Usage: node tools/ci/project-shells.mjs [dataDir]
  */
 import { mkdir, writeFile } from 'node:fs/promises';
@@ -24,6 +29,11 @@ for (const [name, id] of Object.entries(PROJECTS)) {
     updatedAt: '1970-01-01T00:00:00Z',
     project: {},
   };
-  await writeFile(resolve(dir, `${id}.json`), `${JSON.stringify(doc)}\n`);
-  console.log(`shell ${name} ${id}`);
+  try {
+    await writeFile(resolve(dir, `${id}.json`), `${JSON.stringify(doc)}\n`, { flag: 'wx' });
+    console.log(`shell ${name} ${id}`);
+  } catch (err) {
+    if (err.code !== 'EEXIST') throw err;
+    console.log(`kept ${name} ${id}`);
+  }
 }