diff --git a/TODO.md b/TODO.md
index 6c47bd86..b622a4e9 100644
--- a/TODO.md
+++ b/TODO.md
@@ -127,7 +127,22 @@ everything still open, in the order it should be tackled.
 
 ### Blockers before real secrets
 
-- [ ] **Independent security review of the TypeScript branch.** No one but the
+- [ ] **Independent security review of the TypeScript branch.** Review
+      strategy (decided 2026-08-18): two AI families plus vectors, because
+      they fail differently. (1) `/code-review ultra` — **blocked**: PR #989
+      spans main→port including beta's 150 commits (6795 files/720k lines),
+      over ultra's size limit; fallback options are a review-only PR
+      containing just the `js/` tree against main, or a narrower local-diff
+      target. (2) A GPT-family pass (Codex) over the four ranked targets in
+      the PR body, briefed to REFUTE the security claims in file comments,
+      not summarize them. (3) Third-party ground truth where possible: the
+      official BIP-85 spec vectors now run directly against the TS core
+      (test/bip85SpecVectors.test.ts; Python has had them all along), which
+      closes the both-implementations-agree-and-are-both-wrong hole for the
+      root derivation. Triage rule for findings: flagged by both families =
+      almost certainly real; flagged by one = verify against code, arbiter
+      is a reproduction test or the cross-impl suite, never the author's
+      opinion alone. No one but the
       authoring agent has read this code. Two earlier review rounds on this
       project each found criticals that self-review missed, so a green suite and
       an author's sign-off are not evidence of much. Scope it to
diff --git a/js/packages/core/src/derive/bip85.ts b/js/packages/core/src/derive/bip85.ts
index d7e09643..a9a99cc9 100644
--- a/js/packages/core/src/derive/bip85.ts
+++ b/js/packages/core/src/derive/bip85.ts
@@ -102,14 +102,25 @@ export interface DeriveEntropyOptions {
 export class Bip85 {
   private readonly root: HDKey;
 
-  constructor(seed: Uint8Array) {
-    this.root = HDKey.fromMasterSeed(seed);
+  constructor(seed: Uint8Array | HDKey) {
+    this.root = seed instanceof HDKey ? seed : HDKey.fromMasterSeed(seed);
   }
 
   static fromMnemonic(mnemonic: string, passphrase = ""): Bip85 {
     return new Bip85(mnemonicToSeedSync(canonicalizeMnemonic(mnemonic), passphrase));
   }
 
+  /**
+   * From a BIP-32 extended private key. Exists so the official BIP-85 spec
+   * test vectors — which start from an xprv, not a mnemonic — can be run
+   * directly against this implementation (third-party ground truth, immune
+   * to the failure mode where both of our implementations agree with each
+   * other and are both wrong).
+   */
+  static fromExtendedKey(xprv: string): Bip85 {
+    return new Bip85(HDKey.fromExtendedKey(xprv));
+  }
+
   deriveEntropy(opts: DeriveEntropyOptions): Uint8Array {
     const appNo = opts.appNo ?? 39;
     let path: string;
diff --git a/js/packages/core/test/bip85SpecVectors.test.ts b/js/packages/core/test/bip85SpecVectors.test.ts
new file mode 100644
index 00000000..ffa3c840
--- /dev/null
+++ b/js/packages/core/test/bip85SpecVectors.test.ts
@@ -0,0 +1,50 @@
+/**
+ * BIP-85 official spec test vectors, run directly against this implementation.
+ *
+ * Every other derivation check in this repo compares us against the Python
+ * implementation — which is cross-verification, not ground truth. These
+ * vectors come from the BIP-85 specification itself (also pinned in Python's
+ * test_bip85_vectors.py), so a shared misconception between our two
+ * implementations cannot pass here.
+ */
+
+import { describe, expect, it } from "vitest";
+import { Bip85, deriveTotpSecret, bytesToHex } from "@seedpass/core";
+
+// The spec's master key (BIP-85 test vectors section).
+const MASTER_XPRV =
+  "xprv9s21ZrQH143K2LBWUUQRFXhucrQqBpKdRRxNVq2zBqsx8HVqFk2uYo8kmbaLLHRdqtQpUm98uKfu3vca1LqdGhUtyoFnCNkfmXRyPXLjbKb";
+
+// Official BIP-39 application vectors (12 and 24 English words, index 0).
+const SPEC_12 =
+  "girl mad pet galaxy egg matter matrix prison refuse sense ordinary nose";
+const SPEC_24 =
+  "puppy ocean match cereal symbol another shed magic wrap hammer bulb intact gadget divorce twin tonight reason outdoor destroy simple truth cigar social volcano";
+
+// SeedPass-specific derivations from the same master, pinned identically in
+// Python's suite (not spec vectors, but a cross-family anchor for app 32 and
+// the TOTP path).
+const PY_SSH_ENTROPY =
+  "52405cd0dd21c5be78314a7c1a3c65ffd8d896536cc7dee3157db5824f0c92e2";
+const PY_TOTP_SECRET = "VQYTWDNEWYBY2G3LOGGCEKR4LZ3LNEYY";
+
+describe("BIP-85 spec vectors", () => {
+  const bip85 = Bip85.fromExtendedKey(MASTER_XPRV);
+
+  it("derives the spec's 12-word BIP-39 vector", () => {
+    expect(bip85.deriveMnemonic(0, 12)).toBe(SPEC_12);
+  });
+
+  it("derives the spec's 24-word BIP-39 vector", () => {
+    expect(bip85.deriveMnemonic(0, 24)).toBe(SPEC_24);
+  });
+
+  it("matches Python's pinned app-32 entropy from the spec master", () => {
+    const entropy = bip85.deriveEntropy({ index: 0, entropyBytes: 32, appNo: 32 });
+    expect(bytesToHex(entropy)).toBe(PY_SSH_ENTROPY);
+  });
+
+  it("matches Python's pinned TOTP secret derived from the spec's 24-word child", () => {
+    expect(deriveTotpSecret(SPEC_24, 0)).toBe(PY_TOTP_SECRET);
+  });
+});