diff --git a/policy-site/src/ui/policy-document.ts b/policy-site/src/ui/policy-document.ts
index 7e310d64..d31373a6 100644
--- a/policy-site/src/ui/policy-document.ts
+++ b/policy-site/src/ui/policy-document.ts
@@ -90,6 +90,17 @@ export function joinDocument(
   return { value: ordered(isObject(loaded) ? loaded : {}, fields) };
 }
 
+/**
+ * The engine a new policy starts in, where this build has it: Cedar, whose
+ * documents are text rather than JSON.
+ */
+export const NEW_POLICY_ENGINE = "bot.did.policy#cedar";
+
+/** The engine a new policy starts in, out of the ones this build carries. */
+export function defaultEngine(engines: string[]): string | undefined {
+  return engines.includes(NEW_POLICY_ENGINE) ? NEW_POLICY_ENGINE : engines[0];
+}
+
 /** An empty document of `type`, as editor text. */
 export function emptyDocument(type: string): DocumentText {
   const engine = DOCUMENT_BODIES[type];
diff --git a/policy-site/src/ui/policy-editor.ts b/policy-site/src/ui/policy-editor.ts
index bea40857..eea4734c 100644
--- a/policy-site/src/ui/policy-editor.ts
+++ b/policy-site/src/ui/policy-editor.ts
@@ -10,6 +10,7 @@ import { checkboxes, debounce, labelled, renderReport, stamp, type EditorTarget
 import {
   actionsOf,
   assemblePolicy,
+  defaultEngine,
   emptyDocument,
   joinDocument,
   languageOf,
@@ -50,8 +51,8 @@ export function policyEditor(checks: Checks, target: EditorTarget): PolicyEditor
   const createdAt = h("input", { attrs: { readonly: "" } });
   createdAt.value = typeof loaded.createdAt === "string" ? loaded.createdAt : "";
 
-  // A new policy with no document starts as the first engine's, empty.
-  const first = vocabulary.engines[0];
+  // A new policy with no document starts empty, in the default engine.
+  const first = defaultEngine(vocabulary.engines);
   let held: DocumentText =
     loaded.document === undefined && first ? emptyDocument(first) : start.document;
   const modeOf = (doc: DocumentText): EditorMode =>
diff --git a/policy-site/tests/check-flows.mjs b/policy-site/tests/check-flows.mjs
index 6e8f678e..ce50455f 100644
--- a/policy-site/tests/check-flows.mjs
+++ b/policy-site/tests/check-flows.mjs
@@ -216,6 +216,16 @@ function expectWrites(name, batch, writes) {
   }
 }
 
+// A new policy starts in the engine whose documents are Cedar text.
+{
+  const { defaultEngine, emptyDocument } = await import("../src/ui/policy-document.ts");
+  const engines = ["bot.did.policy#regex", "bot.did.policy#cedar"];
+  if (defaultEngine(engines) !== "bot.did.policy#cedar") fail(`a new policy starts as ${defaultEngine(engines)}`);
+  const started = emptyDocument(defaultEngine(engines));
+  if (started.text !== "" || started.whole) fail(`a new Cedar policy starts as ${JSON.stringify(started)}`);
+  if (defaultEngine(["bot.did.policy#regex"]) !== "bot.did.policy#regex") fail("a build without Cedar does not fall back");
+}
+
 // A sign-in comes back only to one of the page's own addresses.
 {
   const back = "/recommended/no-posts/customize/edit";