diff --git a/modules/mesh-delivery-sdk/buildstep/ask_test.go b/modules/mesh-delivery-sdk/buildstep/ask_test.go
index f63c248b..7676d0f4 100644
--- a/modules/mesh-delivery-sdk/buildstep/ask_test.go
+++ b/modules/mesh-delivery-sdk/buildstep/ask_test.go
@@ -59,7 +59,9 @@ func stepAsk() StepAsk {
 // The ask the three build seats share is one neutral form: it round-trips, and every ill-formed ask is refused,
 // naming what is wrong (the rules of the old delivery.StepAsk, now the shared runner's).
 func TestAskCheckAndRoundTrip(t *testing.T) {
-	for _, a := range []StepAsk{goodBuild(), goodCheck(), runAsk(), stepAsk()} {
+	headless := runAsk() // a run whose program reads only its input names no head (design 47 G §712, A-238)
+	headless.Run.Heads = nil
+	for _, a := range []StepAsk{goodBuild(), goodCheck(), runAsk(), headless, stepAsk()} {
 		raw, err := EncodeStepAsk(a)
 		if err != nil {
 			t.Fatal(err)
@@ -106,7 +108,6 @@ func TestAskCheckAndRoundTrip(t *testing.T) {
 		{"run at a short commit", func(a *StepAsk) { a.Run.Program.Commit = "0123456" }, "no full commit"},
 		{"run outside the repository", func(a *StepAsk) { a.Run.Program.Dir = "../x" }, "no clean directory"},
 		{"run of no package", func(a *StepAsk) { a.Run.Program.Package = "" }, "no package"},
-		{"run over no head", func(a *StepAsk) { a.Run.Heads = nil }, "no head"},
 		{"run over a head twice", func(a *StepAsk) { a.Run.Heads = append(a.Run.Heads, a.Run.Heads[0]) }, "twice"},
 		{"run over a branch", func(a *StepAsk) { a.Run.Heads[1].Commit = "main" }, "novox/mesh-sdk at no full commit"},
 		{"input on a state", func(a *StepAsk) { a.Run.Input.Verb = "mesh-delivery-checker.inputs" }, "no seat:<seat>.<verb>"},
diff --git a/modules/mesh-delivery-sdk/buildstep/run.go b/modules/mesh-delivery-sdk/buildstep/run.go
index e975cdf4..c98c6262 100644
--- a/modules/mesh-delivery-sdk/buildstep/run.go
+++ b/modules/mesh-delivery-sdk/buildstep/run.go
@@ -31,7 +31,9 @@ type RunJob struct {
 	// Program is where the program is built: the repository and full commit fetched, the Go module's directory in it and
 	// the program's import path. The ask's Packages are the mesh package versions it builds against (ADR 0573 §2, §4).
 	Program RunProgram `json:"program"`
-	// Heads are the commits the program reads, each laid read-only at its repository's path; a repository at most once.
+	// Heads are the commits the program reads, each laid read-only at its repository's path; a repository at most once. None
+	// is a program that reads only its input (the indexer's manifest validator, design 47 G §712, A-238): the directory
+	// RunHeadsEnv names is then empty.
 	Heads []RunHead `json:"heads"`
 	// Input names the program's input, kept by the asker by its digest and served on a verb of its seat.
 	Input RunInput `json:"input"`
@@ -150,9 +152,6 @@ func (r RunJob) check(id string) error {
 	if p.Package == "" || strings.ContainsAny(p.Package, " \t\n") {
 		return fmt.Errorf("buildstep: the run ask %s names no package to build (%q)", id, p.Package)
 	}
-	if len(r.Heads) == 0 {
-		return fmt.Errorf("buildstep: the run ask %s names no head", id)
-	}
 	seen := map[string]bool{}
 	for _, h := range r.Heads {
 		if err := checkRunRepository(id, "a head of", h.Repository); err != nil {