diff --git a/.changeset/tidy-hashes-keep.md b/.changeset/tidy-hashes-keep.md
new file mode 100644
index 00000000..45e550c7
--- /dev/null
+++ b/.changeset/tidy-hashes-keep.md
@@ -0,0 +1,5 @@
+---
+"@fleet-sdk/crypto": patch
+---
+
+Fix `blake2b()` mutating the caller's options object: the `key`, `salt`, and `personalization` fields were converted from hex strings to `Uint8Array` in place, so a caller's options object came back changed (and a frozen options object threw a `TypeError`). The conversion now happens in a fresh options object, leaving the caller's object untouched; hash outputs are unchanged.
diff --git a/packages/crypto/src/hashes.spec.ts b/packages/crypto/src/hashes.spec.ts
index 6e482410..4b34301f 100644
--- a/packages/crypto/src/hashes.spec.ts
+++ b/packages/crypto/src/hashes.spec.ts
@@ -76,6 +76,49 @@ describe("Hashes smoke tests", () => {
     expect(sha256(byte)).to.be.deep.equal(sha256(hex));
   });
 
+  it("Should not mutate the caller's options object", () => {
+    const msg = utf8.decode("blake2b256");
+    const options = {
+      dkLen: 64,
+      key: "6a9059057f259d733766f6b32081b66c",
+      salt: "d877f8df03fd687ab7c0052e3ce88372",
+      personalization: "77616c6c65747320636865636b73756d" // "wallets checksum"
+    };
+    const snapshot = { ...options };
+
+    const result = blake2b(msg, options);
+
+    // the options object must be left exactly as the caller passed it:
+    // hex-string fields must not be replaced with Uint8Arrays
+    expect(options).to.be.deep.equal(snapshot);
+    expect(options.key).to.be.equal(snapshot.key);
+    expect(options.salt).to.be.equal(snapshot.salt);
+    expect(options.personalization).to.be.equal(snapshot.personalization);
+
+    // and the hash must match hashing with pre-decoded byte options
+    expect(result).to.be.deep.equal(
+      blake2b(msg, {
+        dkLen: 64,
+        key: hex.decode(snapshot.key),
+        salt: hex.decode(snapshot.salt),
+        personalization: hex.decode(snapshot.personalization)
+      })
+    );
+  });
+
+  it("Should accept a frozen options object", () => {
+    const msg = utf8.decode("blake2b256");
+    const options = Object.freeze({
+      key: "6a9059057f259d733766f6b32081b66c",
+      salt: "d877f8df03fd687ab7c0052e3ce88372"
+    });
+
+    expect(() => blake2b(msg, options)).not.to.throw();
+    expect(blake2b(msg, options)).to.be.deep.equal(
+      blake2b(msg, { key: hex.decode(options.key), salt: hex.decode(options.salt) })
+    );
+  });
+
   it("Should hash message using sha256", () => {
     expect(sha256(utf8.decode("sha256"))).to.be.deep.equal(
       hex.decode("5d5b09f6dcb2d53a5fffc60c4ac0d55fabdf556069d6631545f42aa6e3500f2e")
diff --git a/packages/crypto/src/hashes.ts b/packages/crypto/src/hashes.ts
index 29e521b6..4cc6a63a 100644
--- a/packages/crypto/src/hashes.ts
+++ b/packages/crypto/src/hashes.ts
@@ -18,10 +18,8 @@ export function ensureBytes(input: ByteInput): Uint8Array {
 }
 
 export function blake2b(message: ByteInput, options?: Blake2bOptions): Uint8Array {
-  if (options?.key) options.key = ensureBytes(options.key);
-  if (options?.salt) options.salt = ensureBytes(options.salt);
-  if (options?.personalization) options.personalization = ensureBytes(options.personalization);
-
+  // convert hex-string fields into a fresh options object; never mutate the
+  // caller's options object (it may be frozen or reused across calls)
   const opts: Blake2Opts = {
     key: options?.key ? ensureBytes(options?.key) : undefined,
     salt: options?.salt ? ensureBytes(options?.salt) : undefined,