diff --git a/tests/test_grant_key_binding.py b/tests/test_grant_key_binding.py
new file mode 100644
index 00000000..f098edf1
--- /dev/null
+++ b/tests/test_grant_key_binding.py
@@ -0,0 +1,95 @@
+"""A controller's real grant cannot authorize an unrelated session key.
+
+Regression requirements from legacy PR #172, exercised against current
+Run Grants rather than the retired attested-peering protocol.
+"""
+
+from contextlib import closing
+import sqlite3
+
+import pytest
+from fastapi.testclient import TestClient
+
+from nandatown.coordinator import build_app
+from nandatown.identity_portable import (
+    IdentityError,
+    Keystore,
+    session_proof,
+    verify_grant,
+)
+from nandatown.profiles import PROFILES
+
+
+def _candidate(keystore, run_id, replace_public):
+    genuine = keystore.make_grant("buyer", run_id)
+    other = keystore.make_grant("other", run_id)
+    assert genuine["grant"]["session_public"] != other["grant"]["session_public"]
+    grant = dict(genuine["grant"])
+    if replace_public:
+        grant["session_public"] = other["grant"]["session_public"]
+    proof = session_proof(other["session_private"], run_id, "buyer")
+    return genuine, grant, proof
+
+
+@pytest.mark.parametrize("replace_public,reason", [
+    (False, "session proof"),
+    (True, "pinned controller key"),
+], ids=["copied-grant", "altered-session-key"])
+def test_grant_rejects_a_different_session_key(tmp_path, replace_public, reason):
+    keystore = Keystore(str(tmp_path / "keys"))
+    identity = keystore.new_identity("buyer")
+    keystore.new_identity("other")
+    genuine, grant, proof = _candidate(keystore, "run-1", replace_public)
+
+    with pytest.raises(IdentityError, match=reason):
+        verify_grant(grant, genuine["grant_signature"],
+                     identity["controller_public"], "run-1", "buyer", proof)
+
+    # The same genuine grant remains usable by its authorized session key.
+    verify_grant(genuine["grant"], genuine["grant_signature"],
+                 identity["controller_public"], "run-1", "buyer",
+                 session_proof(genuine["session_private"], "run-1", "buyer"))
+
+
+@pytest.mark.parametrize("replace_public", [False, True],
+                         ids=["copied-grant", "altered-session-key"])
+def test_rejected_key_substitution_leaves_join_unclaimed(tmp_path, replace_public):
+    keystore = Keystore(str(tmp_path / "keys"))
+    identity = keystore.new_identity("buyer")
+    keystore.new_identity("other")
+    db_path = str(tmp_path / "town.db")
+    admin = {"X-Town-Admin": "test-admin"}
+    with TestClient(build_app(db_path, admin_token="test-admin")) as client:
+        created = client.post("/runs", headers=admin, json={
+            "profile": PROFILES["quote-clean"].model_dump(),
+            "identities": {"buyer": identity},
+        })
+        assert created.status_code == 200, created.text
+        run_id = created.json()["run_id"]
+        genuine, grant, proof = _candidate(keystore, run_id, replace_public)
+        rejected = client.post(f"/runs/{run_id}/join", json={
+            "name": "buyer", "grant": grant,
+            "grant_signature": genuine["grant_signature"],
+            "session_proof": proof,
+        })
+        assert rejected.status_code == 403, rejected.text
+        with closing(sqlite3.connect(db_path)) as connection:
+            row = connection.execute(
+                "SELECT session, permissions_json FROM participants"
+                " WHERE run_id=? AND name=?", (run_id, "buyer")).fetchone()
+        assert row == (None, None), "a rejected key cannot obtain a session"
+        events = client.get(f"/runs/{run_id}/events", headers=admin)
+        assert events.status_code == 200, events.text
+        kinds = [event["kind"] for event in events.json()["events"]]
+        assert kinds.count("grant_rejected") == 1
+        assert "participant_joined" not in kinds
+        assert "portable_identity_verified" not in kinds
+
+        accepted = client.post(f"/runs/{run_id}/join", json={
+            "name": "buyer", "grant": genuine["grant"],
+            "grant_signature": genuine["grant_signature"],
+            "session_proof": session_proof(
+                genuine["session_private"], run_id, "buyer"),
+        })
+        assert accepted.status_code == 200, accepted.text
+        assert accepted.json()["session"]