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diff --git a/packages/meshbay-common/tests/test_version_negotiation.py b/packages/meshbay-common/tests/test_version_negotiation.py
new file mode 100644
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+++ b/packages/meshbay-common/tests/test_version_negotiation.py
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+"""
+The version range, checked rather than merely written.
+
+Before MNP 1.0 every message carried a `v` that no one read, so a mismatch
+surfaced as a *missing field*: an 0.x client reading a 1.0 handshake ack finds no
+`enabled_apps` and applies its documented fallback — show every app — which is a
+wrong answer rather than an error. The three failure modes 1.0's flag day was
+called for all present that way, which is why negotiation ships in the same
+deployment rather than after it (phase 15.6, decision D2).
+"""
+
+import pytest
+from meshbay_common import MNP_VERSION
+from meshbay_common.handshake import (
+    MNP_MIN_SUPPORTED,
+    HandshakeError,
+    check_version,
+    parse_version,
+)
+
+
+def test_this_build_accepts_itself():
+    check_version(MNP_VERSION, MNP_MIN_SUPPORTED)
+
+
+def test_a_peer_that_declares_no_minimum_is_read_as_speaking_only_its_own():
+    """
+    Which is the right reading of every 0.x peer: none of them declared a range,
+    because none of them checked one.
+    """
+    check_version(MNP_VERSION)
+
+
+def test_an_older_peer_is_refused_with_a_code():
+    with pytest.raises(HandshakeError) as caught:
+        check_version("0.15")
+    assert caught.value.code == "version_too_old"
+    # The text is for a human and may be reworded; the client matches the code.
+    assert "0.15" in str(caught.value)
+
+
+def test_a_peer_requiring_more_than_we_speak_is_refused_with_a_code():
+    ours = parse_version(MNP_VERSION)
+    future = f"{ours[0] + 1}.0"
+    with pytest.raises(HandshakeError) as caught:
+        check_version(future, future)
+    assert caught.value.code == "version_too_new"
+
+
+def test_a_newer_peer_that_still_accepts_us_is_allowed():
+    """
+    The point of a range rather than an equality: a 1.4 node that still speaks to
+    1.0 clients must not refuse one.
+    """
+    ours = parse_version(MNP_VERSION)
+    check_version(f"{ours[0]}.{ours[1] + 4}", MNP_MIN_SUPPORTED)
+
+
+@pytest.mark.parametrize("bad", ["", "one.two", "1", None, "1.0.0", "v1.0"])
+def test_an_unreadable_version_is_refused_not_guessed(bad):
+    with pytest.raises(HandshakeError) as caught:
+        check_version(bad)
+    assert caught.value.code == "version_unreadable"
+
+
+def test_versions_order_numerically_not_lexically():
+    """`"0.9" < "0.15"` as strings, and the opposite as versions."""
+    assert parse_version("0.9") < parse_version("0.15") < parse_version("1.0")
+
+
+def test_mnp_1_0_is_a_major_bump():
+    """
+    Recorded as a test because the number is the only thing that says "this one is
+    different". 1.0 seals the index and the ack under the group key: no 0.x peer
+    can open either, and there is nothing to be compatible with.
+    """
+    assert parse_version(MNP_VERSION) >= (1, 0)
+    assert parse_version(MNP_MIN_SUPPORTED) >= (1, 0)