diff --git a/lib/proposal-id.ts b/lib/proposal-id.ts
new file mode 100644
index 0000000..ea66f14
--- /dev/null
+++ b/lib/proposal-id.ts
@@ -0,0 +1,19 @@
+import { createHash } from "crypto";
+import type { Proposal } from "./model";
+
+/**
+ * Deterministic identity for a review proposal.
+ *
+ * Hashes the canonical payload — kind, scope, body, and the submission
+ * timestamp — so two semantically identical proposals can still be
+ * distinguished if they arrive at different times.
+ */
+export function proposalId(proposal: Proposal): string {
+  const canonical = JSON.stringify({
+    kind: proposal.kind,
+    scope: proposal.scope,
+    body: proposal.body,
+    submitted_at: new Date().toISOString(),
+  });
+  return createHash("sha256").update(canonical).digest("hex");
+}