diff --git a/placement/go/clients/meshmoduleassignerclient/client_test.go b/placement/go/clients/meshmoduleassignerclient/client_test.go
index 09ec425..06453e0 100644
--- a/placement/go/clients/meshmoduleassignerclient/client_test.go
+++ b/placement/go/clients/meshmoduleassignerclient/client_test.go
@@ -12,6 +12,13 @@ import (
 )
 
 func TestPlacementClient(t *testing.T) {
+	t.Run("proposal id", func(t *testing.T) {
+		for id, ok := range map[string]bool{"p-20261028t183000-1a2b3c": true, "abc": true, "ab": false, "A-bc": false, "-abc": false, "a.bc": false, "": false} {
+			if (CheckProposalID(id) == nil) != ok {
+				t.Errorf("CheckProposalID(%q) = %v", id, CheckProposalID(id))
+			}
+		}
+	})
 	t.Run("key and file", func(t *testing.T) {
 		// Research 072 row 37: one node key and one key per module, a legacy key read beside them, any other refused.
 		for key, want := range map[string]struct {
diff --git a/placement/go/clients/meshmoduleassignerclient/proposal.go b/placement/go/clients/meshmoduleassignerclient/proposal.go
new file mode 100644
index 0000000..284a358
--- /dev/null
+++ b/placement/go/clients/meshmoduleassignerclient/proposal.go
@@ -0,0 +1,17 @@
+package meshmoduleassignerclient
+
+import (
+	"fmt"
+	"regexp"
+)
+
+var proposalIDOK = regexp.MustCompile(`^[a-z0-9][a-z0-9-]{2,62}$`)
+
+// CheckProposalID refuses an id that is no proposal's: lower-case letters, digits and dashes, 3 to 63 of them, so it is one
+// key token. The holder writes proposals under it, and the keys of the asks about a proposal (`placement.<id>`) carry it.
+func CheckProposalID(id string) error {
+	if !proposalIDOK.MatchString(id) {
+		return fmt.Errorf("mesh-module-assigner: %q is no proposal id (lower-case letters, digits and dashes, one key token)", id)
+	}
+	return nil
+}