diff --git a/src/tests/test_manager_seed_setup.py b/src/tests/test_manager_seed_setup.py
new file mode 100644
index 0000000..8adfb95
--- /dev/null
+++ b/src/tests/test_manager_seed_setup.py
@@ -0,0 +1,58 @@
+import builtins
+from mnemonic import Mnemonic
+from password_manager.manager import PasswordManager
+
+
+def test_validate_bip85_seed_invalid_word():
+    pm = PasswordManager.__new__(PasswordManager)
+    bad_phrase = "abandon " * 11 + "zzzz"
+    assert not pm.validate_bip85_seed(bad_phrase)
+
+
+def test_validate_bip85_seed_checksum_failure():
+    pm = PasswordManager.__new__(PasswordManager)
+    m = Mnemonic("english")
+    phrase = m.generate(strength=128)
+    words = phrase.split()
+    words[-1] = "abandon" if words[-1] != "abandon" else "about"
+    bad_phrase = " ".join(words)
+    assert not pm.validate_bip85_seed(bad_phrase)
+
+
+def test_setup_existing_seed_words(monkeypatch):
+    m = Mnemonic("english")
+    phrase = m.generate(strength=128)
+    words = phrase.split()
+    inputs = iter(words + ["y"] * len(words))
+    monkeypatch.setattr(builtins, "input", lambda *_: next(inputs))
+
+    pm = PasswordManager.__new__(PasswordManager)
+    monkeypatch.setattr(pm, "_finalize_existing_seed", lambda seed: seed)
+
+    result = pm.setup_existing_seed(method="words")
+    assert result == phrase
+
+
+def test_setup_existing_seed_paste(monkeypatch):
+    m = Mnemonic("english")
+    phrase = m.generate(strength=128)
+
+    called = {}
+
+    def fake_masked_input(prompt: str) -> str:
+        called["prompt"] = prompt
+        return phrase
+
+    monkeypatch.setattr("password_manager.manager.masked_input", fake_masked_input)
+    monkeypatch.setattr(
+        builtins,
+        "input",
+        lambda *_: (_ for _ in ()).throw(RuntimeError("input called")),
+    )
+
+    pm = PasswordManager.__new__(PasswordManager)
+    monkeypatch.setattr(pm, "_finalize_existing_seed", lambda seed: seed)
+
+    result = pm.setup_existing_seed(method="paste")
+    assert result == phrase
+    assert called["prompt"].startswith("Enter your 12-word BIP-85 seed")