diff --git a/js/packages/cli/src/api/routes.ts b/js/packages/cli/src/api/routes.ts
index cec88e84..95ae2507 100644
--- a/js/packages/cli/src/api/routes.ts
+++ b/js/packages/cli/src/api/routes.ts
@@ -745,7 +745,7 @@ export function registerRoutes(server: ApiServer, ctx: ApiContext): void {
       const vault = await readVault(ctx);
       const body = bodyObject(req);
       const encrypt = body["plaintext"] !== true;
-      const wrapper = await exportBackup(vault.index as unknown as Record<string, unknown>, {
+      const wrapper = await exportBackup(vault.index, {
         mnemonic,
         fingerprint: generateFingerprint(mnemonic),
         encrypt,
diff --git a/js/packages/cli/src/program.ts b/js/packages/cli/src/program.ts
index 436bbe06..cf7f8f25 100644
--- a/js/packages/cli/src/program.ts
+++ b/js/packages/cli/src/program.ts
@@ -2425,7 +2425,7 @@ export function buildProgram(io: ProgramIo = defaultIo): Command {
         existsSync(resolved) && statSync(resolved).isDirectory()
           ? join(resolved, defaultBackupFilename(fingerprint))
           : resolved;
-      const wrapper = await exportBackup(vault.index as Record<string, unknown>, {
+      const wrapper = await exportBackup(vault.index, {
         mnemonic: vault.mnemonic,
         fingerprint,
         encrypt: !o.plaintext,
diff --git a/js/packages/cli/src/tui/menus.ts b/js/packages/cli/src/tui/menus.ts
index a7951df5..d2c50602 100644
--- a/js/packages/cli/src/tui/menus.ts
+++ b/js/packages/cli/src/tui/menus.ts
@@ -1505,7 +1505,7 @@ async function exportDatabase(s: Session): Promise<void> {
   } else if (!basename(dest).includes(".")) {
     dest = `${dest}${BACKUP_EXTENSION}`;
   }
-  const payload = await exportBackup(s.vault.index as unknown as Record<string, unknown>, {
+  const payload = await exportBackup(s.vault.index, {
     mnemonic: s.vault.mnemonic,
     fingerprint: s.fingerprint,
   });
diff --git a/js/packages/core/src/vault/index0.ts b/js/packages/core/src/vault/index0.ts
index 37ca04fc..184a2653 100644
--- a/js/packages/core/src/vault/index0.ts
+++ b/js/packages/core/src/vault/index0.ts
@@ -343,7 +343,13 @@ export function normalizeViewManifest(raw: unknown): Dict {
   };
 }
 
-export function recomputeIndex0Stats(index0: Dict): Dict {
+// Typed to exactly what it reads. It used to take a bare Dict, so every one
+// of its five callers — all holding a real Index0 — had to launder it through
+// `as unknown as Dict` to get in. A Pick says what the function needs, admits
+// any Index0 without ceremony, and still accepts a partial one.
+export function recomputeIndex0Stats(
+  index0: Pick<Index0, "events" | "checkpoints" | "heads">,
+): Dict {
   const events = normalizeMapping(index0["events"]);
   const checkpoints = normalizeMapping(index0["checkpoints"]);
   const heads = normalizeMapping(index0["heads"]);
@@ -394,7 +400,7 @@ export function normalizeIndex0(raw: unknown): Index0 {
     const normalizedHead = normalizeHead(head);
     if (normalizedHead) normalized.heads[String(writerId)] = normalizedHead;
   }
-  normalized.stats = recomputeIndex0Stats(normalized as unknown as Dict);
+  normalized.stats = recomputeIndex0Stats(normalized);
   return normalized;
 }
 
@@ -544,7 +550,7 @@ export function appendIndex0Event(payload: unknown, options: AppendEventOptions)
     head_hash: computeHeadHash(event),
     modified_ts: event["modified_ts"] as number,
   };
-  system.stats = recomputeIndex0Stats(system as unknown as Dict);
+  system.stats = recomputeIndex0Stats(system);
   return out;
 }
 
@@ -658,7 +664,7 @@ export function compactIndex0(
     normalized as unknown as Dict,
     options,
   );
-  normalized.stats = recomputeIndex0Stats(normalized as unknown as Dict);
+  normalized.stats = recomputeIndex0Stats(normalized);
   return normalized;
 }
 
@@ -850,7 +856,7 @@ export function rebuildCanonicalViewsPayload(
   const sortedViews: Record<string, Dict> = {};
   for (const key of Object.keys(views).sort(compareCodePoints)) sortedViews[key] = views[key]!;
   index0.canonical_views = sortedViews;
-  index0.stats = recomputeIndex0Stats(index0 as unknown as Dict);
+  index0.stats = recomputeIndex0Stats(index0);
   (out["_system"] as Dict)["index0"] = index0;
   return out;
 }
@@ -1038,6 +1044,6 @@ export function mergeSystemIndex0(current: unknown, incoming: unknown): Index0 {
     heads: sortRecords(heads),
     stats: {},
   };
-  merged.stats = recomputeIndex0Stats(merged as unknown as Dict);
+  merged.stats = recomputeIndex0Stats(merged);
   return merged;
 }
diff --git a/js/packages/core/src/vault/portableBackup.ts b/js/packages/core/src/vault/portableBackup.ts
index 532bc8a4..be0c2c11 100644
--- a/js/packages/core/src/vault/portableBackup.ts
+++ b/js/packages/core/src/vault/portableBackup.ts
@@ -12,6 +12,7 @@ import { base64 } from "@scure/base";
 import { z } from "zod";
 import { canonicalJson, canonicalHash } from "../sync/canonical.js";
 import { deriveIndexKeyBytes } from "./indexKey.js";
+import type { VaultIndex } from "../schema/entries.js";
 import { decryptPayload } from "./payload.js";
 import { encryptV3 } from "./aead.js";
 import { utf8 } from "../util/bytes.js";
@@ -102,7 +103,11 @@ export async function importBackup(
 
 /** Export an index as a portable backup wrapper (format_version 1). */
 export async function exportBackup(
-  index: Record<string, unknown>,
+  // A live VaultIndex or a round-tripped backup payload: both are passed, and
+  // the function treats either as an opaque JSON document to canonicalize and
+  // encrypt. Naming both is what lets a caller holding a real index hand it
+  // over without laundering it through `as unknown as Record<string, unknown>`.
+  index: VaultIndex | Record<string, unknown>,
   options: {
     mnemonic: string;
     fingerprint: string;