diff --git a/policy/diamond/policy/numtracker/numtracker.rego b/policy/diamond/policy/numtracker/numtracker.rego
new file mode 100644
index 0000000..739bf14
--- /dev/null
+++ b/policy/diamond/policy/numtracker/numtracker.rego
@@ -0,0 +1,20 @@
+package diamond.policy.numtracker
+
+import data.diamond.policy.session
+import data.diamond.policy.token
+
+import rego.v1
+
+default write_to_beamline_visit := false
+
+# User account check
+write_to_beamline_visit if {
+	session.access
+	input.beamline == session.beamline
+}
+
+# Service account check
+write_to_beamline_visit if {
+	input.beamline == token.claims.beamline
+	input.beamline == session.beamline
+}
diff --git a/policy/diamond/policy/numtracker/numtracker_test.rego b/policy/diamond/policy/numtracker/numtracker_test.rego
new file mode 100644
index 0000000..1c98703
--- /dev/null
+++ b/policy/diamond/policy/numtracker/numtracker_test.rego
@@ -0,0 +1,113 @@
+package diamond.policy.numtracker_test
+
+import data.diamond.policy.numtracker
+import rego.v1
+
+diamond_data := {
+	"subjects": {
+		"alice": {
+			"permissions": [],
+			"proposals": [1],
+			"sessions": [],
+		},
+		"bob": {
+			"permissions": ["b07_admin"],
+			"proposals": [],
+			"sessions": [11],
+		},
+		"carol": {
+			"permissions": ["super_admin"],
+			"proposals": [],
+			"sessions": [],
+		},
+		"desmond": {
+			"permissions": [],
+			"proposals": [2],
+			"sessions": [13],
+		},
+		"edna": {
+			"permissions": [],
+			"proposals": [2],
+			"sessions": [13, 14],
+		},
+		"oscar": {
+			"permissions": [],
+			"proposals": [],
+			"sessions": [],
+		},
+	},
+	"sessions": {
+		"11": {
+			"beamline": "i03",
+			"proposal_number": 1,
+			"visit_number": 1,
+		},
+		"12": {
+			"beamline": "b07",
+			"proposal_number": 1,
+			"visit_number": 2,
+		},
+		"13": {
+			"beamline": "b07",
+			"proposal_number": 2,
+			"visit_number": 1,
+		},
+		"14": {
+			"beamline": "b07",
+			"proposal_number": 2,
+			"visit_number": 2,
+		},
+	},
+	"proposals": {
+		"1": {"sessions": {
+			"1": 11,
+			"2": 12,
+		}},
+		"2": {"sessions": {
+			"1": 13,
+			"2": 14,
+		}},
+	},
+	"beamlines": {"i03": {"sessions": [11]}, "b07": {"sessions": [12, 13, 14]}},
+	"admin": {"b07_admin": ["b07"]},
+}
+
+# service account UDC path
+
+test_write_to_beamline_visit_service_account if {
+	numtracker.write_to_beamline_visit with data.diamond.data as diamond_data
+		with input as {"beamline": "i03", "proposal": 1, "visit": 1}
+		with data.diamond.policy.token.claims as {"beamline": "i03"}
+}
+
+test_write_to_beamline_visit_service_account_wrong_beamline if {
+	not numtracker.write_to_beamline_visit with data.diamond.data as diamond_data
+		with input as {"beamline": "i03", "proposal": 1, "visit": 1}
+		with data.diamond.policy.token.claims as {"beamline": "b07"}
+}
+
+test_write_to_beamline_visit_service_account_nonexistent_beamline if {
+	not numtracker.write_to_beamline_visit with data.diamond.data as diamond_data
+		with input as {"beamline": "i03", "proposal": 1, "visit": 1}
+		with data.diamond.policy.token.claims as {"beamline": "i99"}
+}
+
+# user fedid path
+
+test_write_to_beamline_visit_user if {
+	numtracker.write_to_beamline_visit with data.diamond.data as diamond_data
+		with input as {"beamline": "b07", "proposal": 1, "visit": 2}
+		with data.diamond.policy.token.claims as {"fedid": "alice"}
+}
+
+test_write_to_beamline_visit_user_no_access if {
+	not numtracker.write_to_beamline_visit with data.diamond.data as diamond_data
+		with input as {"beamline": "i03", "proposal": 1, "visit": 1}
+		with data.diamond.policy.token.claims as {"fedid": "oscar"}
+}
+
+test_write_to_beamline_visit_user_wrong_beamline if {
+	not numtracker.write_to_beamline_visit with data.diamond.data as diamond_data
+		with input as {"beamline": "i03", "proposal": 1, "visit": 2}
+		with data.diamond.policy.token.claims as {"fedid": "alice"}
+}