diff --git a/arm/CUDly-CrossSubscription/template.json b/arm/CUDly-CrossSubscription/template.json
index 7a7a0c33e..b607ab07c 100644
--- a/arm/CUDly-CrossSubscription/template.json
+++ b/arm/CUDly-CrossSubscription/template.json
@@ -13,13 +13,6 @@
       "metadata": {
         "description": "Object ID of the CUDly Azure AD service principal. Obtain with: az ad sp show --id <appId> --query id -o tsv"
       }
-    },
-    "roleAssignmentGuidPrefix": {
-      "type": "string",
-      "defaultValue": "[newGuid()]",
-      "metadata": {
-        "description": "Base GUID used to derive deterministic role assignment names. Leave at default to auto-generate."
-      }
     }
   },
 
diff --git a/iac/arm_cross_subscription_template_test.go b/iac/arm_cross_subscription_template_test.go
new file mode 100644
index 000000000..c070a066d
--- /dev/null
+++ b/iac/arm_cross_subscription_template_test.go
@@ -0,0 +1,132 @@
+package iac
+
+import (
+	"encoding/json"
+	"os"
+	"regexp"
+	"strings"
+	"testing"
+)
+
+// These tests read the ARM template customers deploy into every onboarded
+// Azure subscription, and the shipped commands that deploy it, from the repo
+// tree (arm/ is not embedded; setup.sh and known-issues.md point customers at
+// the file itself).
+
+const crossSubscriptionTemplate = "../arm/CUDly-CrossSubscription/template.json"
+
+// crossSubscriptionDeployCommandSources are the shipped files that tell a
+// customer how to run `az deployment sub create` against the template.
+var crossSubscriptionDeployCommandSources = []string{
+	"../arm/CUDly-CrossSubscription/setup.sh",
+	"../known-issues.md",
+}
+
+// readCrossSubscriptionTemplate returns the template's declared parameters and
+// the rest of the template (everything but the parameters section) as JSON
+// text, so a parameter's own declaration cannot count as a use of it.
+func readCrossSubscriptionTemplate(t *testing.T) (map[string]json.RawMessage, string) {
+	t.Helper()
+	raw, err := os.ReadFile(crossSubscriptionTemplate)
+	if err != nil {
+		t.Fatalf("read %s: %v", crossSubscriptionTemplate, err)
+	}
+	var doc map[string]json.RawMessage
+	if err = json.Unmarshal(raw, &doc); err != nil {
+		t.Fatalf("parse %s: %v", crossSubscriptionTemplate, err)
+	}
+	var params map[string]json.RawMessage
+	if err = json.Unmarshal(doc["parameters"], &params); err != nil {
+		t.Fatalf("parse %s parameters: %v", crossSubscriptionTemplate, err)
+	}
+	if len(params) == 0 {
+		t.Fatalf("%s declares no parameters; expected at least servicePrincipalObjectId", crossSubscriptionTemplate)
+	}
+	delete(doc, "parameters")
+	body, err := json.Marshal(doc)
+	if err != nil {
+		t.Fatalf("re-marshal %s: %v", crossSubscriptionTemplate, err)
+	}
+	return params, string(body)
+}
+
+// TestCrossSubscriptionTemplateReadsEveryParameter guards issue #143: the
+// template declared roleAssignmentGuidPrefix, described as the base GUID for
+// deterministic role assignment names, and nothing read it. A parameter the
+// template ignores promises the customer a control that does nothing.
+func TestCrossSubscriptionTemplateReadsEveryParameter(t *testing.T) {
+	params, body := readCrossSubscriptionTemplate(t)
+	for name := range params {
+		// ARM parameter names are case-insensitive, as is the function name.
+		ref := regexp.MustCompile(`(?i)parameters\(\s*'` + regexp.QuoteMeta(name) + `'\s*\)`)
+		if !ref.MatchString(body) {
+			t.Errorf("parameter %q is declared but no variable, resource or output reads parameters('%s')", name, name)
+		}
+	}
+}
+
+// TestCrossSubscriptionTemplateHasNoNewGuidDefault keeps role assignment and
+// role definition names deterministic. A [newGuid()] default yields a fresh
+// value on every deployment, so feeding it into a guid(...) name seed would
+// make each redeploy try to create a second assignment of the same role to the
+// same principal at the same scope, which Azure refuses (RoleAssignmentExists).
+func TestCrossSubscriptionTemplateHasNoNewGuidDefault(t *testing.T) {
+	params, _ := readCrossSubscriptionTemplate(t)
+	newGUID := regexp.MustCompile(`(?i)newGuid\s*\(`)
+	for name, decl := range params {
+		if newGUID.Match(decl) {
+			t.Errorf("parameter %q has a newGuid() default: %s", name, decl)
+		}
+	}
+}
+
+// TestCrossSubscriptionDeployCommandsPassOnlyDeclaredParameters checks the
+// other side of removing a parameter: ARM fails a deployment that supplies a
+// parameter the template does not declare, so every `--parameters key=value`
+// in a shipped deploy command for this template must name a declared one.
+func TestCrossSubscriptionDeployCommandsPassOnlyDeclaredParameters(t *testing.T) {
+	params, _ := readCrossSubscriptionTemplate(t)
+	declared := make(map[string]bool, len(params))
+	for name := range params {
+		declared[strings.ToLower(name)] = true
+	}
+	paramArg := regexp.MustCompile(`--parameters\s+"?([A-Za-z0-9_]+)=`)
+
+	for _, src := range crossSubscriptionDeployCommandSources {
+		cmds := crossSubscriptionDeployCommands(t, src)
+		if len(cmds) == 0 {
+			t.Errorf("%s: found no `az deployment sub create` command for the cross-subscription template; update crossSubscriptionDeployCommandSources", src)
+			continue
+		}
+		for _, cmd := range cmds {
+			for _, m := range paramArg.FindAllStringSubmatch(cmd, -1) {
+				if !declared[strings.ToLower(m[1])] {
+					t.Errorf("%s: deploy command passes undeclared parameter %q, which ARM rejects:\n%s", src, m[1], cmd)
+				}
+			}
+		}
+	}
+}
+
+// crossSubscriptionDeployCommands returns each `az deployment sub create`
+// command in path that deploys the cross-subscription template, with its
+// backslash continuation lines joined.
+func crossSubscriptionDeployCommands(t *testing.T, path string) []string {
+	t.Helper()
+	raw, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatalf("read %s: %v", path, err)
+	}
+	joined := regexp.MustCompile(`\\\r?\n`).ReplaceAllString(string(raw), " ")
+	var cmds []string
+	for _, line := range strings.Split(joined, "\n") {
+		if !strings.Contains(line, "az deployment sub create") {
+			continue
+		}
+		if strings.Contains(line, "CUDly-CrossSubscription/template.json") ||
+			strings.Contains(line, "${SCRIPT_DIR}/template.json") {
+			cmds = append(cmds, line)
+		}
+	}
+	return cmds
+}