A run's input is asked of a verb of the asker's seat, never read from its state (issue 1921, B0 review round 1)

The operator's answer: a build module reading the checker's state broke ADR 0596 §2. RunInput names the seat verb it is
served on (seat:<seat>.<verb>), RunInput.Read asks it and takes only the bytes named, and the seat
mesh-delivery-checker defines the verb input (RunInputAsk, RunInputAnswer).

Claude-Session: https://claude.ai/code/session_01NcbgfFSnn24M3GcqttoU7i

go/buildstep/stale.go

@@ -118,6 +118,12 @@ func AskWhat(a delivery.StepAsk) string {
 		}
 	case a.Compose != nil:
 		parts = append(parts, "compose", str(a.Compose.Fetch, "repository")+"@"+short(str(a.Compose.Fetch, "commit")))
+	case a.Run != nil:
+		parts = append(parts, "run "+a.Run.Program.Package, a.Run.Program.Repository+"@"+short(a.Run.Program.Commit))
+		if len(a.Run.Args) > 0 {
+			parts = append(parts, strings.Join(a.Run.Args, " "))
+		}
+		parts = append(parts, fmt.Sprintf("%d heads", len(a.Run.Heads)))
 	}
 	return strings.Join(parts, ", ")
 }

go/buildstep/stale_test.go

@@ -159,4 +159,9 @@ func TestAskWhat(t *testing.T) {
 	if got, want := AskWhat(build), "delivery ask, module mesh-x, fp abcdef012345"; got != want {
 		t.Errorf("build = %q", got)
 	}
+	// 1921: a run ask's step names the program, where it is built, its arguments and how many heads it reads.
+	run := delivery.StepAsk{Kind: "check", Run: &delivery.RunJob{Program: delivery.RunProgram{Repository: "novox/mesh-delivery", Commit: "fedcba9876543210", Package: "./cmd/mesh-checking-program"}, Heads: []delivery.RunHead{{}, {}}, Args: []string{"-verdict"}}}
+	if got, want := AskWhat(run), "check ask, run ./cmd/mesh-checking-program, novox/mesh-delivery@fedcba987654, -verdict, 2 heads"; got != want {
+		t.Errorf("run = %q", got)
+	}
 }

go/catalogue/checker_seat.go

@@ -3,8 +3,8 @@ package catalogue
 // CheckerSeat is the seat mesh-delivery-checker holds.
 const CheckerSeat = "mesh-delivery-checker"
 
-// checkerVerbs are the checker seat's verbs: ask a try, read one, end one by hand. Optional until the checker serves them
-// everywhere.
+// checkerVerbs are the checker seat's verbs: ask a try, read one, end one by hand, and answer a checking program's input.
+// Optional until the checker serves them everywhere.
 func checkerVerbs() []Verb {
 	return []Verb{
 		{Name: "try", Optional: true, Description: "Try a pushed head before its pull request is opened: the checker " +
@@ -28,5 +28,12 @@ func checkerVerbs() []Verb {
 			"id. A try that ended already is refused.",
 			Input: schema(map[string]string{"id": "the try's id, try-<digits>", "why": "why it is ended, in words"},
 				[]string{"id", "why"})},
+		{Name: "input", Optional: true, Description: "A checking program's input the checker keeps, by the sha256 of its " +
+			"exact bytes: the facts of a check its verdict is asked of, or the heads its seat list is asked of, as a run " +
+			"ask names it (its input's verb and digest). Answers the digest and the bytes, base64; a digest it keeps no " +
+			"input of is an error. Asked by the build module that runs the program, as its manifest's invokes grants it. " +
+			"Read-only.",
+			Input: schema(map[string]string{"digest": "the input's sha256, 64 lowercase hex digits, as the run ask names it"},
+				[]string{"digest"})},
 	}
 }

go/composer/composer.go

@@ -59,6 +59,11 @@ type Request struct {
 	// longer read: the composer judges no manifest (novox/hq ADR 0578 §5), the indexer's judge does, and its verdicts
 	// come in each case's Verdicts. Kept so an asker that still sends it is read.
 	Predefined catalogue.PredefinedList `json:"predefined,omitempty"`
+	// Seats are the seat definitions the manifest validator judges and the composer composes against, raw so that this
+	// package names no seat: inside a group's check, the one seat list the checking program made from the members' heads
+	// (novox/hq ADR 0636 §4, ADR 0644). A reader built before them ignores them; Decode, Fill and the request's
+	// marshalling keep them whole, so a build module that fills a request hands them on (issue 1921).
+	Seats []json.RawMessage `json:"seats,omitempty"`
 }
 
 // Case is one catalogue composed: every module's manifest by name, and per node the manifests kept at the build it runs.

go/composer/composer_test.go

@@ -12,7 +12,9 @@ const demo = `{"module": "demo", "version": "1", "claims": [{"name": "node-files
 // A manifest named by its digest goes on the wire as {"digest": ...}, and Fill puts the bytes the indexer keeps in its
 // place; a raw manifest goes as itself.
 func TestAManifestIsItsBytesOrItsDigest(t *testing.T) {
-	r := Request{V: Version, Cases: []Case{{Name: "change", Manifests: map[string]Manifest{
+	// 1921 (B0): the seat list rides whole through the wire, the decode and the fill.
+	seats := []json.RawMessage{json.RawMessage(`{"name":"node-files","verbs":["read"]}`)}
+	r := Request{V: Version, Seats: seats, Cases: []Case{{Name: "change", Manifests: map[string]Manifest{
 		"demo": {Digest: "d1"}, "raw": {Raw: json.RawMessage(`{"module":"raw","version":"1"}`)}},
 		Kept: map[string]map[string]Manifest{"laptop": {"demo": {Digest: "d1"}}}}}}
 	wire, err := json.Marshal(r)
@@ -36,6 +38,10 @@ func TestAManifestIsItsBytesOrItsDigest(t *testing.T) {
 	}); err != nil {
 		t.Fatal(err)
 	}
+	refilled, err := json.Marshal(back)
+	if err != nil || !strings.Contains(string(refilled), `"seats":[{"name":"node-files","verbs":["read"]}]`) {
+		t.Fatalf("the seats after the fill: %s %v", refilled, err)
+	}
 	if asked != 2 || string(back.Cases[0].Manifests["demo"].Raw) != demo || string(back.Cases[0].Kept["laptop"]["demo"].Raw) != demo {
 		t.Fatalf("filled %d times: %+v", asked, back.Cases[0])
 	}

go/delivery/builder.go

@@ -150,11 +150,6 @@ const (
 	// WaitsFor: a fetch found the module's lock naming a package version the registry does not hold yet; the builder
 	// writes the key queued with Waits (novox/hq ADR 0500 clause 6), and it is built once the package is published.
 	WaitsFor = "waits for: "
-	// Retake: a failure that passes by itself (a name lookup that timed out, a refused connection): the builder takes
-	// the key again after a backoff, at most MaxRetakes times (issue 1748). A build module opens every such failure with it.
-	Retake = "retakeable: "
-	// MaxRetakes is how many times the builder takes a key again by itself for a retakeable failure.
-	MaxRetakes = 3
 )
 
 // FailedUntilChanged says whether a failed key's error is one that is not built again until the module's fingerprint
@@ -164,12 +159,6 @@ func FailedUntilChanged(err string) bool {
 	return strings.HasPrefix(err, NotPinned) || strings.HasPrefix(err, VersionTaken) || strings.HasPrefix(err, RefusedKind)
 }
 
-// Retakeable says whether a failed key's error opens with Retake: the builder takes it again by itself after a backoff,
-// until BuilderBuild.Retakes reaches MaxRetakes. An error that is FailedUntilChanged, or opens with no prefix, is not.
-func Retakeable(err string) bool {
-	return strings.HasPrefix(err, Retake) && !FailedUntilChanged(err)
-}
-
 // ToolchainUsed is what a build used for one language (ADR 0405 clauses 2 and 7): the installed toolchain's version and
 // the build module's recipe version.
 type ToolchainUsed struct {
@@ -191,10 +180,6 @@ type BuilderBuild struct {
 	Artifacts  []BuiltArtifact          `json:"artifacts,omitempty"`
 	Toolchains map[string]ToolchainUsed `json:"toolchains,omitempty"`
 	Error      string                   `json:"error,omitempty"`
-	// Retakes is how many times the builder took this key again by itself after a retakeable failure (Retake); it is kept
-	// on the key, so a builder that starts again reads it and the count survives a restart (issue 1748). A reader that
-	// does not know the field reads 0.
-	Retakes int `json:"retakes,omitempty"`
 	// Waits is, on a queued key only, the package version `<name>@<version>` the module's lock names and the registry
 	// does not yet hold: the key is not claimed until that package's trunk key is done and published (novox/hq ADR 0500
 	// clause 6). A reader that does not know the field reads the key as queued.

go/delivery/builder_reference_test.go

@@ -86,38 +86,11 @@ func TestOnlyAPushFailureIsBuiltAgainBeforeTheFingerprintChanges(t *testing.T) {
 		{VersionTaken + "@novox/mesh-sdk@0.1.15 holds another file", false},
 		{RefusedKind + "a package in go", false},
 		{"go test: 2 failed", true},
-		{Retake + "lookup git.novox.be: i/o timeout", true},
 	} {
 		if got := FailedUntilChanged(c.err); got == c.again {
 			t.Errorf("FailedUntilChanged(%q) = %v", c.err, got)
 		}
 	}
-	// A retakeable error opens with Retake and is not one that waits for the fingerprint; the count is on the key's value (issue 1748).
-	for _, c := range []struct {
-		err  string
-		want bool
-	}{
-		{Retake + "lookup git.novox.be: i/o timeout", true},
-		{NotPinned + "x", false},
-		{Retake[:4] + " not the prefix", false},
-		{"go test: 2 failed", false},
-		{"", false},
-		{VersionTaken + Retake, false},
-	} {
-		if got := Retakeable(c.err); got != c.want {
-			t.Errorf("Retakeable(%q) = %v", c.err, got)
-		}
-	}
-	data, err := EncodeBuilderBuild(BuilderBuild{State: BuildFailed, Error: Retake + "x", Retakes: 2})
-	if err != nil {
-		t.Fatal(err)
-	}
-	if back, err := DecodeBuilderBuild(data); err != nil || back.Retakes != 2 {
-		t.Fatalf("decoded %+v, %v", back, err)
-	}
-	if Retake != "retakeable: " || MaxRetakes != 3 {
-		t.Fatal("the retake prefix or its bound changed")
-	}
 	if NotStored != "not stored: " || NotPinned != "not pinned: " || VersionTaken != "version taken: " || RefusedKind != "refused kind: " {
 		t.Fatal("a prefix changed its words")
 	}

go/delivery/checker.go

@@ -34,6 +34,10 @@ const (
 	CheckerKeptHistory = 1
 )
 
+// CheckerInputVerb is the verb of the seat mesh-delivery-checker that serves a checking program's input by its digest
+// (RunInput.Verb; asked RunInputAsk, answering RunInputAnswer; issue 1921, milestone 47/B0).
+const CheckerInputVerb = "input"
+
 // The test levels (ADR 0445 clause 2) and the states of a verdict.
 const (
 	LevelUnit        = "unit"
@@ -179,6 +183,40 @@ type Verdict struct {
 	// carried over, by repository (ADR 0548 §5): the verdict was rewritten for Trunk and Beside by running only the packages
 	// whose digest the moves changed. Absent from a verdict judged whole, and from one still running.
 	CarriedFrom map[string]string `json:"carried-from,omitempty"`
+	// JudgedBy names the checking program that judged the check: where it was built, the key its build is kept under and
+	// the heads it read (novox/hq ADR 0644, ADR 0656; design 47's bootstrap delivery, *Its tool*). Absent in a value
+	// written before it.
+	JudgedBy *JudgedBy `json:"judged-by,omitempty"`
+}
+
+// JudgedBy is the checking program's build that judged a check: built at Program, kept under Key (RunKey), Built when
+// this check's run built it rather than taking the kept one, over Heads.
+type JudgedBy struct {
+	Program RunProgram `json:"program"`
+	Key     string     `json:"key"`
+	Built   bool       `json:"built,omitempty"`
+	Heads   []RunHead  `json:"heads"`
+}
+
+func (j *JudgedBy) check() error {
+	if j == nil {
+		return nil
+	}
+	if err := checkRepository(j.Program.Repository); err != nil || !askCommitOK.MatchString(j.Program.Commit) || j.Program.Package == "" {
+		return fmt.Errorf("delivery: the verdict is judged by a checking program built at no repository, full commit and package (%+v)", j.Program)
+	}
+	if !strings.HasPrefix(j.Key, "program-") {
+		return fmt.Errorf("delivery: the verdict's checking program is kept under %q, which is no program key", j.Key)
+	}
+	if len(j.Heads) == 0 {
+		return fmt.Errorf("delivery: the verdict's checking program read no head")
+	}
+	for _, h := range j.Heads {
+		if err := checkRepository(h.Repository); err != nil || !askCommitOK.MatchString(h.Commit) {
+			return fmt.Errorf("delivery: the verdict's checking program read %s at %q, which is no full commit", h.Repository, h.Commit)
+		}
+	}
+	return nil
 }
 
 func (c *Composition) check() error {
@@ -254,6 +292,9 @@ func (v Verdict) check() error {
 	if err := v.Composition.check(); err != nil {
 		return err
 	}
+	if err := v.JudgedBy.check(); err != nil {
+		return err
+	}
 	seen := map[string]bool{}
 	for _, level := range v.OverBound {
 		if err := checkLevel(level); err != nil {

go/delivery/checker_test.go

@@ -3,6 +3,7 @@ package delivery
 import (
 	"encoding/json"
 	"errors"
+	"reflect"
 	"strconv"
 	"strings"
 	"testing"
@@ -314,6 +315,11 @@ func TestVerdictNewFieldsChecked(t *testing.T) {
 		"carried from an empty commit":  {func(v *Verdict) { v.CarriedFrom = map[string]string{"novox/mesh-sdk": ""} }, []string{"carried-from", "mesh-sdk"}},
 		"carried over no move":          {func(v *Verdict) { v.CarriedFrom = map[string]string{"novox/mesh-sdk": "aaaa"} }, []string{"nothing moved"}},
 		"a running verdict carried":     {func(v *Verdict) { v.State, v.CarriedFrom = VerdictRunning, map[string]string{"novox/mesh-sdk": "9999"} }, []string{"running", "carried"}},
+		// 1921 (B0): the checking program that judged it.
+		"judged by no program": {func(v *Verdict) { v.JudgedBy = &JudgedBy{Key: "program-0123456789abcdef", Heads: judgedBy().Heads} }, []string{"checking program", "full commit"}},
+		"judged under no key":  {func(v *Verdict) { j := judgedBy(); j.Key = "composer-1"; v.JudgedBy = &j }, []string{"composer-1"}},
+		"judged over no head":  {func(v *Verdict) { j := judgedBy(); j.Heads = nil; v.JudgedBy = &j }, []string{"no head"}},
+		"judged over a branch": {func(v *Verdict) { j := judgedBy(); j.Heads[0].Commit = "main"; v.JudgedBy = &j }, []string{"novox/mesh-delivery", "main"}},
 	}
 	for name, c := range cases {
 		v := passVerdict()
@@ -338,7 +344,7 @@ func TestVerdictOldAndNewReadersAgree(t *testing.T) {
 	if err != nil {
 		t.Fatalf("the new reader of an old value: %v", err)
 	}
-	if v.Composition != nil || v.OverBound != nil || v.Beside != nil || v.CarriedFrom != nil || v.Levels[LevelUnit].Skipped != 0 || v.Levels[LevelUnit].Run != 12 {
+	if v.Composition != nil || v.OverBound != nil || v.Beside != nil || v.CarriedFrom != nil || v.JudgedBy != nil || v.Levels[LevelUnit].Skipped != 0 || v.Levels[LevelUnit].Run != 12 {
 		t.Errorf("an old value grew fields: %+v", v)
 	}
 	again, err := EncodeVerdict(v)
@@ -352,10 +358,16 @@ func TestVerdictOldAndNewReadersAgree(t *testing.T) {
 	n.Beside = map[string]string{"novox/mesh-catalog": "cccc"}
 	n.CarriedFrom = map[string]string{"novox/mesh-sdk": "9999"}
 	n.Composition.FPSet = "fps1:00"
+	j := judgedBy()
+	n.JudgedBy = &j
 	data, err := EncodeVerdict(n)
 	if err != nil {
 		t.Fatal(err)
 	}
+	// 1921 (B0): who judged it reads back whole.
+	if back, err := DecodeVerdict(data); err != nil || back.JudgedBy == nil || !reflect.DeepEqual(*back.JudgedBy, j) {
+		t.Errorf("judged by, read back: %+v %v", back.JudgedBy, err)
+	}
 	var ov oldVerdict
 	if err := json.Unmarshal(data, &ov); err != nil || ov.Group != "feat/x" || ov.State != VerdictPass || ov.Levels[LevelUnit].Run != 12 || ov.SDK == "" {
 		t.Errorf("the old reader of a new value: %+v, %v", ov, err)
@@ -658,3 +670,8 @@ func TestVerdictNamesEveryFailure(t *testing.T) {
 		t.Error("a verdict naming more than MaxFailures and the rest was written")
 	}
 }
+
+func judgedBy() JudgedBy {
+	return JudgedBy{Program: RunProgram{Repository: "novox/mesh-delivery", Commit: askCommit, Dir: "modules/mesh-delivery-checker", Package: "p"},
+		Key: "program-0123456789abcdef", Built: true, Heads: []RunHead{{Repository: "novox/mesh-delivery", Commit: askCommit}}}
+}

go/delivery/history_records.go

@@ -175,9 +175,6 @@ type BuildLogEntry struct {
 	// sorted and each once, as the build module answered them (BuildAnswer.Imports; novox/hq ADR 0593 §2): the indexer
 	// keeps them on the module (KeptInputs2.Imports) when the build's fingerprint is the module's.
 	Imports []string `json:"imports,omitempty"`
-	// Retakes is, at a build's end, how many times the builder took the key again by itself after a retakeable failure
-	// (BuilderBuild.Retakes; issue 1748): builder_builds_since reads it.
-	Retakes int `json:"retakes,omitempty"`
 }
 
 // BuildsSince is the answer of BuildsSinceTool: every ended build since a time, oldest first, and whether the log held

go/delivery/stepasks.go

@@ -237,8 +237,9 @@ func checkAskID(id string) error {
 }
 
 // StepAsk is the message on a build seat's stream: who asked what, of which kind and language, and the one whole job.
-// Exactly one of Build, Check and Compose is set: a delivery ask is a build; a check ask a build (a candidate's
-// fingerprint), a check or, on the Go seat, a compose; a recheck ask a check; an image ask is always a build.
+// Exactly one of Build, Check, Compose and Run is set: a delivery ask is a build; a check ask a build (a candidate's
+// fingerprint), a check or, on the Go seat, a compose or a run (RunJob); a recheck ask a check; an image ask is always a
+// build.
 type StepAsk struct {
 	V        int    `json:"v"`
 	ID       string `json:"id"`
@@ -251,6 +252,7 @@ type StepAsk struct {
 	Build   *BuildJob   `json:"build,omitempty"`
 	Check   *CheckJob   `json:"check,omitempty"`
 	Compose *ComposeJob `json:"compose,omitempty"`
+	Run     *RunJob     `json:"run,omitempty"`
 	// Needs are what the node that runs the ask must have (AskNeeds); a check ask's alone. Empty: any node.
 	Needs []string `json:"needs,omitempty"`
 	// Packages are the version of each mesh package the build or the check builds against, by its module path (a Go
@@ -310,9 +312,19 @@ type ComposeAnswer struct {
 
 // ComposerKey is the key a build module keeps a built composer under: the repository, commit, directory and package it
 // was built from and the toolchain it was built with, hashed (`composer-` and 16 hex digits), so each head is built once
-// on a node (ADR 0482 clause 4).
-func ComposerKey(repository, commit, dir, pkg string, toolchain ToolchainUsed) string {
-	sum := sha256.Sum256([]byte(strings.Join([]string{repository, commit, dir, pkg, toolchain.Version, toolchain.Recipe}, "\n")))
+// on a node (ADR 0482 clause 4). The mesh package versions the ask names are an input of the build, so they are in the
+// key, sorted, when there are any; an ask that names none gets the key it always had (issue 1921).
+func ComposerKey(repository, commit, dir, pkg string, toolchain ToolchainUsed, versions PackageVersions) string {
+	parts := []string{repository, commit, dir, pkg, toolchain.Version, toolchain.Recipe}
+	pkgs := make([]string, 0, len(versions))
+	for p := range versions {
+		pkgs = append(pkgs, p)
+	}
+	sort.Strings(pkgs)
+	for _, p := range pkgs {
+		parts = append(parts, p+"="+versions[p])
+	}
+	sum := sha256.Sum256([]byte(strings.Join(parts, "\n")))
 	return "composer-" + hex.EncodeToString(sum[:8])
 }
 
@@ -399,16 +411,18 @@ func (a StepAsk) check() error {
 		return fmt.Errorf("delivery: the ask %s names no time", a.ID)
 	}
 	jobs := 0
-	for _, set := range []bool{a.Build != nil, a.Check != nil, a.Compose != nil} {
+	for _, set := range []bool{a.Build != nil, a.Check != nil, a.Compose != nil, a.Run != nil} {
 		if set {
 			jobs++
 		}
 	}
 	switch {
 	case jobs != 1:
-		return fmt.Errorf("delivery: the ask %s holds a build, a check or a compose, exactly one", a.ID)
+		return fmt.Errorf("delivery: the ask %s holds a build, a check, a compose or a run, exactly one", a.ID)
 	case a.Compose != nil && (a.Kind != AskCheck || a.Language != AskGo):
 		return fmt.Errorf("delivery: the ask %s is a compose, which the Go seat's check stream alone carries", a.ID)
+	case a.Run != nil && (a.Kind != AskCheck || a.Language != AskGo):
+		return fmt.Errorf("delivery: the ask %s is a run, which the Go seat's check stream alone carries", a.ID)
 	case a.Build != nil && a.Kind == AskRecheck:
 		return fmt.Errorf("delivery: the ask %s is a build on the recheck stream, which carries checks alone", a.ID)
 	case a.Check != nil && a.Kind == AskDelivery:
@@ -434,6 +448,9 @@ func (a StepAsk) check() error {
 	if a.Compose != nil {
 		return a.Compose.check(a.ID)
 	}
+	if a.Run != nil {
+		return a.Run.check(a.ID)
+	}
 	return a.Check.check(a.ID)
 }
 

go/delivery/stepasks_compose_test.go

@@ -2,6 +2,7 @@ package delivery
 
 import (
 	"encoding/json"
+	"errors"
 	"reflect"
 	"strings"
 	"testing"
@@ -16,20 +17,36 @@ func composeAsk() StepAsk {
 			Request: json.RawMessage(`{"v":1,"cases":[{"name":"change","manifests":{"demo":{"digest":"d1"}}}]}`), Reader: "mesh-sdk head"}}
 }
 
-// 1622: a compose ask encodes and decodes with every field.
+// runAsk is the checker's run of a checking program built at a mesh-delivery head, over two heads (issue 1921, B0).
+func runAsk() StepAsk {
+	in, err := RunInputOf("seat:mesh-delivery-checker."+CheckerInputVerb, []byte(`{"v":1}`))
+	if err != nil {
+		panic(err)
+	}
+	return StepAsk{ID: "mh3k2x9qa1b2c6", Kind: AskCheck, Language: AskGo, Asker: "mesh-delivery-checker",
+		At: time.Date(2026, 11, 6, 22, 0, 0, 0, time.UTC),
+		Run: &RunJob{Program: RunProgram{Repository: "novox/mesh-delivery", Commit: askCommit, Dir: "modules/mesh-delivery-checker",
+			Package: "git.novox.be/novox/mesh-delivery/modules/mesh-delivery-checker/cmd/mesh-checking-program"},
+			Heads: []RunHead{{Repository: "novox/mesh-delivery", Commit: askCommit}, {Repository: "novox/mesh-sdk", Commit: askCommit}},
+			Input: in, Args: []string{"-verdict"}, Reader: "checking program of mesh-delivery head"},
+		Packages: PackageVersions{"git.novox.be/novox/mesh-sdk/go": askCommit}}
+}
+
+// 1622: a compose ask encodes and decodes with every field; 1921 (B0): a run ask too.
 func TestAComposeAskRoundTrips(t *testing.T) {
-	a := composeAsk()
-	raw, err := EncodeStepAsk(a)
-	if err != nil {
-		t.Fatal(err)
-	}
-	got, err := DecodeStepAsk(raw)
-	if err != nil {
-		t.Fatal(err)
-	}
-	a.V = StepAskVersion
-	if !reflect.DeepEqual(got, a) {
-		t.Fatalf("round trip:\n got %+v\nwant %+v", got, a)
+	for _, a := range []StepAsk{composeAsk(), runAsk()} {
+		raw, err := EncodeStepAsk(a)
+		if err != nil {
+			t.Fatal(err)
+		}
+		got, err := DecodeStepAsk(raw)
+		if err != nil {
+			t.Fatal(err)
+		}
+		a.V = StepAskVersion
+		if !reflect.DeepEqual(got, a) {
+			t.Fatalf("round trip:\n got %+v\nwant %+v", got, a)
+		}
 	}
 }
 
@@ -54,23 +71,112 @@ func TestAComposeAskIsRefusedWhereItDoesNotBelong(t *testing.T) {
 			t.Errorf("a compose ask %s is encoded", name)
 		}
 	}
+	// 1921 (B0): a run ask likewise goes only on the Go seat's check stream, alone, builds a clean package at a full
+	// commit, reads each named head once at a full commit, and names an input kept by digest within its bound.
+	for name, change := range map[string]func(*StepAsk){
+		"on the delivery stream":        func(a *StepAsk) { a.Kind = AskDelivery },
+		"on the Node seat":              func(a *StepAsk) { a.Language = AskNode },
+		"beside a compose":              func(a *StepAsk) { a.Compose = composeAsk().Compose },
+		"with a short commit":           func(a *StepAsk) { a.Run.Program.Commit = "0123456" },
+		"outside the repository":        func(a *StepAsk) { a.Run.Program.Dir = "../x" },
+		"with no package":               func(a *StepAsk) { a.Run.Program.Package = "" },
+		"with no head":                  func(a *StepAsk) { a.Run.Heads = nil },
+		"with a head twice":             func(a *StepAsk) { a.Run.Heads = append(a.Run.Heads, a.Run.Heads[0]) },
+		"with a head at a branch":       func(a *StepAsk) { a.Run.Heads[1].Commit = "main" },
+		"with an input on a state":      func(a *StepAsk) { a.Run.Input.Verb = "mesh-delivery-checker.inputs" },
+		"with an input on one machine":  func(a *StepAsk) { a.Run.Input.Verb = "seat:mesh-delivery-checker.input@ace" },
+		"with an input by no digest":    func(a *StepAsk) { a.Run.Input.Digest = "d1" },
+		"with an input over its bound":  func(a *StepAsk) { a.Run.Input.Size = RunInputBound + 1 },
+		"with an argument of the shell": func(a *StepAsk) { a.Run.Args = []string{"-x; rm"} },
+	} {
+		a := runAsk()
+		change(&a)
+		if _, err := EncodeStepAsk(a); err == nil {
+			t.Errorf("a run ask %s is encoded", name)
+		}
+	}
+	// The asker refuses an input over its bound before it asks, naming its size; the build module refuses bytes that
+	// are not the input named.
+	if _, err := RunInputOf("seat:mesh-delivery-checker.input", make([]byte, RunInputBound+1)); err == nil || !strings.Contains(err.Error(), "262145 bytes") {
+		t.Errorf("an input over its bound: %v", err)
+	}
+	in := runAsk().Run.Input
+	if err := in.Verify([]byte(`{"v":1}`)); err != nil {
+		t.Errorf("the input named: %v", err)
+	}
+	if err := in.Verify([]byte(`{"v":2}`)); err == nil {
+		t.Error("another input read as the one named")
+	}
+	// The build module asks the verb the input names for its digest and takes only the bytes named (the operator's answer
+	// on B0: the input travels by a verb of the asker's seat, never a read of its state).
+	for name, c := range map[string]struct {
+		answer string
+		err    error
+		want   string
+	}{
+		"the bytes named":        {answer: `{"digest":"` + in.Digest + `","input":"eyJ2IjoxfQ=="}`},
+		"other bytes":            {answer: `{"digest":"` + in.Digest + `","input":"eyJ2IjoyfQ=="}`, want: "is not the one the ask names"},
+		"another digest":         {answer: `{"digest":"` + strings.Repeat("0", 64) + `","input":"eyJ2IjoxfQ=="}`, want: "answered the input"},
+		"no input":               {answer: `[1]`, want: "answered no input"},
+		"an error of the callee": {err: errors.New("keeps no input of that digest"), want: "keeps no input of that digest"},
+	} {
+		var asked string
+		var body any
+		got, err := in.Read(func(key string, b any) (json.RawMessage, error) {
+			asked, body = key, b
+			return json.RawMessage(c.answer), c.err
+		})
+		if asked != "seat:mesh-delivery-checker.input" || body != (RunInputAsk{Digest: in.Digest}) {
+			t.Errorf("%s: asked %s with %+v", name, asked, body)
+		}
+		switch {
+		case c.want == "" && (err != nil || string(got) != `{"v":1}`):
+			t.Errorf("%s: %q, %v", name, got, err)
+		case c.want != "" && (err == nil || !strings.Contains(err.Error(), c.want)):
+			t.Errorf("%s: %v, want %q", name, err, c.want)
+		}
+	}
 }
 
-// 1622/AC3: a composer is kept under one key per repository, commit, directory, package and toolchain, so two asks of
+// 1622/AC3, 1921: a composer is kept under one key per repository, commit, directory, package, toolchain and asked versions, so two asks of
 // one head build it once on a node, and another head or toolchain builds its own.
 func TestAComposerIsKeptByWhatItWasBuiltFrom(t *testing.T) {
 	tc := ToolchainUsed{Version: "go1.26.0", Recipe: "3"}
-	k := ComposerKey("novox/mesh-sdk", askCommit, "go", "p", tc)
-	if k != ComposerKey("novox/mesh-sdk", askCommit, "go", "p", tc) || !strings.HasPrefix(k, "composer-") || len(k) != len("composer-")+16 {
+	k := ComposerKey("novox/mesh-sdk", askCommit, "go", "p", tc, nil)
+	if k != ComposerKey("novox/mesh-sdk", askCommit, "go", "p", tc, nil) || !strings.HasPrefix(k, "composer-") || len(k) != len("composer-")+16 {
 		t.Fatalf("key = %q", k)
 	}
 	for _, other := range []string{
-		ComposerKey("novox/mesh-sdk", strings.Repeat("1", 40), "go", "p", tc),
-		ComposerKey("novox/mesh-controller", askCommit, ".", "p", tc),
-		ComposerKey("novox/mesh-sdk", askCommit, "go", "p", ToolchainUsed{Version: "go1.26.1", Recipe: "3"}),
+		ComposerKey("novox/mesh-sdk", strings.Repeat("1", 40), "go", "p", tc, nil),
+		ComposerKey("novox/mesh-controller", askCommit, ".", "p", tc, nil),
+		ComposerKey("novox/mesh-sdk", askCommit, "go", "p", ToolchainUsed{Version: "go1.26.1", Recipe: "3"}, nil),
+		ComposerKey("novox/mesh-sdk", askCommit, "go", "p", tc, PackageVersions{"git.novox.be/novox/mesh-sdk/go": strings.Repeat("2", 40)}),
 	} {
 		if other == k {
 			t.Errorf("two composers built from different inputs share the key %s", k)
 		}
 	}
+	// 1921: the asked versions are in the key, in any order of naming; none, or an empty ask, gives the key without them.
+	a, b := strings.Repeat("a", 40), strings.Repeat("b", 40)
+	k1 := ComposerKey("r", askCommit, "go", "p", tc, PackageVersions{"m/x": a, "m/y": b})
+	if k1 != ComposerKey("r", askCommit, "go", "p", tc, PackageVersions{"m/y": b, "m/x": a}) {
+		t.Error("the key depends on the order of the versions")
+	}
+	if k1 == ComposerKey("r", askCommit, "go", "p", tc, PackageVersions{"m/x": b, "m/y": a}) || k1 == ComposerKey("r", askCommit, "go", "p", tc, nil) {
+		t.Error("other versions, or none, share the key")
+	}
+	if ComposerKey("r", askCommit, "go", "p", tc, nil) != ComposerKey("r", askCommit, "go", "p", tc, PackageVersions{}) {
+		t.Error("an empty ask changed the key")
+	}
+	// 1921 (B0): a run's program is kept the same way, under a key of its own kind.
+	p := RunProgram{Repository: "novox/mesh-delivery", Commit: askCommit, Dir: "modules/mesh-delivery-checker", Package: "p"}
+	rk := RunKey(p, tc, nil)
+	if !strings.HasPrefix(rk, "program-") || len(rk) != len("program-")+16 || rk != RunKey(p, tc, nil) {
+		t.Errorf("run key = %q", rk)
+	}
+	q := p
+	q.Commit = strings.Repeat("1", 40)
+	if RunKey(q, tc, nil) == rk || RunKey(p, tc, PackageVersions{"m/x": a}) == rk {
+		t.Error("two programs built from different inputs share the key")
+	}
 }

go/delivery/stepasks_run.go

@@ -0,0 +1,201 @@
+package delivery
+
+import (
+	"crypto/sha256"
+	"encoding/hex"
+	"encoding/json"
+	"fmt"
+	"path"
+	"regexp"
+	"strings"
+
+	"git.novox.be/novox/mesh-sdk/go/calladdress"
+)
+
+// **A run ask has a program built from a change and run in the build seat's sandbox** (novox/hq ADR 0644, ADR 0656;
+// design 47, *What ADR 0656 leaves to this design*: the build seat's ask `run`; issue 1921). It goes on the Go seat's
+// check stream, a whole job pulled where there is capacity, like a compose: the build module fetches Program at its
+// commit and builds it once per RunKey, in a step that runs nothing but the build; fetches every head of Heads and lays
+// each read-only under the directory RunHeadsEnv names, at its repository's path (`<owner>/<name>`); reads the input
+// Input names, checks its digest and writes it to the file RunInputEnv names; then runs the program with Args, no
+// network and no bus account (ADR 0448 §2, §4), and answers RunAnswer on its step key.
+//
+// The input is carried by reference, as a compose carries its manifests: the asker keeps it by its digest, measured
+// against RunInputBound before it asks, and serves it on a verb of its seat (RunInput.Verb, answered RunInputAnswer); the
+// build module asks that verb as its manifest's `invokes` grants it, never reading the asker's state (ADR 0596 §2; the
+// operator's answer on milestone 47/B0), so the ask on the stream never carries it (issue 1930).
+
+// RunJob is a run ask's job.
+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 []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"`
+	// Args are the program's arguments (a checking program's `-seats` or `-verdict`), each a word or a flag.
+	Args []string `json:"args,omitempty"`
+	// Reader names, for a reader of the ask, what the run is for ("checking program of mesh-delivery head").
+	Reader string `json:"reader,omitempty"`
+}
+
+// RunProgram is where a run's program is built from.
+type RunProgram struct {
+	Repository string `json:"repository"`
+	Commit     string `json:"commit"`
+	Dir        string `json:"dir"`
+	Package    string `json:"package"`
+}
+
+// RunHead is one commit a run's program reads.
+type RunHead struct {
+	Repository string `json:"repository"`
+	Commit     string `json:"commit"`
+}
+
+// RunInput is a run's input by reference: the seat verb the asker serves it on, named as a manifest's `invokes` names it
+// (`seat:<seat>.<verb>`, asked of no one machine), its digest (the sha256 of its bytes, 64 lowercase hex digits) and its
+// size in bytes. The verb is asked RunInputAsk and answers RunInputAnswer.
+type RunInput struct {
+	Verb   string `json:"verb"`
+	Digest string `json:"digest"`
+	Size   int    `json:"size"`
+}
+
+// RunInputAsk is what a verb a RunInput names is asked: the input's digest.
+type RunInputAsk struct {
+	Digest string `json:"digest"`
+}
+
+// RunInputAnswer is that verb's answer: the digest asked and the input's exact bytes (base64 in JSON, so no encoding on
+// the way changes a byte the digest covers).
+type RunInputAnswer struct {
+	Digest string `json:"digest"`
+	Input  []byte `json:"input"`
+}
+
+// RunInputBound is the most bytes a run's input may hold: the bus's value limit the design already uses (256 KiB), so
+// the asker keeps it as one value (the lead's ruling on milestone 47/B0, 2026-11-06).
+const RunInputBound = 256 << 10
+
+// RunHeadsEnv names, for the program, the directory its heads are laid in; RunInputEnv the file its input is written to.
+const (
+	RunHeadsEnv = "MESH_RUN_HEADS"
+	RunInputEnv = "MESH_RUN_INPUT"
+)
+
+// RunInputOf is the reference to body served on verb (`seat:<seat>.<verb>`), refusing a body over RunInputBound, naming
+// its size, so an asker refuses it before asking.
+func RunInputOf(verb string, body []byte) (RunInput, error) {
+	if len(body) > RunInputBound {
+		return RunInput{}, fmt.Errorf("delivery: a run's input of %d bytes is over its bound of %d bytes", len(body), RunInputBound)
+	}
+	sum := sha256.Sum256(body)
+	in := RunInput{Verb: verb, Digest: hex.EncodeToString(sum[:]), Size: len(body)}
+	return in, in.check("")
+}
+
+// Verify refuses body when it is not the input i names: another size or another digest.
+func (i RunInput) Verify(body []byte) error {
+	sum := sha256.Sum256(body)
+	if len(body) != i.Size || hex.EncodeToString(sum[:]) != i.Digest {
+		return fmt.Errorf("delivery: the input %s answered for %s is not the one the ask names (%d bytes, %d named)", i.Verb, i.Digest, len(body), i.Size)
+	}
+	return nil
+}
+
+// Read asks the verb i names for its input with ask (a tools bundle's stdio.Ask, a process's Conn.Call) and answers its
+// bytes, refusing an answer for another digest or bytes that are not the input named.
+func (i RunInput) Read(ask func(key string, body any) (json.RawMessage, error)) ([]byte, error) {
+	raw, err := ask(i.Verb, RunInputAsk{Digest: i.Digest})
+	if err != nil {
+		return nil, fmt.Errorf("delivery: asking %s for the input %s: %w", i.Verb, i.Digest, err)
+	}
+	var a RunInputAnswer
+	if err := json.Unmarshal(raw, &a); err != nil {
+		return nil, fmt.Errorf("delivery: %s answered no input for %s: %w", i.Verb, i.Digest, err)
+	}
+	if a.Digest != i.Digest {
+		return nil, fmt.Errorf("delivery: %s answered the input %q, asked %s", i.Verb, a.Digest, i.Digest)
+	}
+	return a.Input, i.Verify(a.Input)
+}
+
+var (
+	runDigestOK = regexp.MustCompile(`^[0-9a-f]{64}$`)
+	runArgOK    = regexp.MustCompile(`^[-=_./a-zA-Z0-9]+$`)
+)
+
+func (i RunInput) check(id string) error {
+	switch {
+	case !runVerbOK(i.Verb):
+		return fmt.Errorf("delivery: the run ask %s serves its input on %q, which is no seat:<seat>.<verb> of no one machine", id, i.Verb)
+	case !runDigestOK.MatchString(i.Digest):
+		return fmt.Errorf("delivery: the run ask %s names its input by %q, which is no sha256 digest", id, i.Digest)
+	case i.Size <= 0 || i.Size > RunInputBound:
+		return fmt.Errorf("delivery: the run ask %s names an input of %d bytes, outside 1 to %d", id, i.Size, RunInputBound)
+	}
+	return nil
+}
+
+// runVerbOK says key names a seat's verb, asked of no one machine.
+func runVerbOK(key string) bool {
+	a, err := calladdress.Parse(key)
+	return err == nil && a.Seat != "" && a.Node == ""
+}
+
+func (r RunJob) check(id string) error {
+	p := r.Program
+	if err := checkRepository(p.Repository); err != nil {
+		return fmt.Errorf("the run ask %s's program: %w", id, err)
+	}
+	if !askCommitOK.MatchString(p.Commit) {
+		return fmt.Errorf("delivery: the run ask %s builds its program at no full commit", id)
+	}
+	if p.Dir == "" || p.Dir != path.Clean(p.Dir) || strings.HasPrefix(p.Dir, "/") || p.Dir == ".." || strings.HasPrefix(p.Dir, "../") {
+		return fmt.Errorf("delivery: the run ask %s names no clean directory inside the repository (%q)", id, p.Dir)
+	}
+	if p.Package == "" || strings.ContainsAny(p.Package, " \t\n") {
+		return fmt.Errorf("delivery: the run ask %s names no package to build (%q)", id, p.Package)
+	}
+	if len(r.Heads) == 0 {
+		return fmt.Errorf("delivery: the run ask %s names no head", id)
+	}
+	seen := map[string]bool{}
+	for _, h := range r.Heads {
+		if err := checkRepository(h.Repository); err != nil {
+			return fmt.Errorf("the run ask %s's head: %w", id, err)
+		}
+		if !askCommitOK.MatchString(h.Commit) {
+			return fmt.Errorf("delivery: the run ask %s names the head of %s at no full commit", id, h.Repository)
+		}
+		if seen[h.Repository] {
+			return fmt.Errorf("delivery: the run ask %s names %s twice", id, h.Repository)
+		}
+		seen[h.Repository] = true
+	}
+	for _, a := range r.Args {
+		if !runArgOK.MatchString(a) {
+			return fmt.Errorf("delivery: the run ask %s names the argument %q, which is no word or flag", id, a)
+		}
+	}
+	return r.Input.check(id)
+}
+
+// RunAnswer is a run ask's step result: whether this run built the program or took the one kept under its key, the
+// key, the program's exit, the last line of its standard output (Output, raw when it is JSON, else as a JSON string),
+// and Said: the first line of its standard error, or why the step could not run it.
+type RunAnswer struct {
+	Built  bool            `json:"built"`
+	Key    string          `json:"key"`
+	Exit   int             `json:"exit"`
+	Output json.RawMessage `json:"output,omitempty"`
+	Said   string          `json:"said,omitempty"`
+}
+
+// RunKey is the key a build module keeps a built program under, as ComposerKey keeps a composer: what it was built
+// from, the toolchain and the asked package versions, hashed (`program-` and 16 hex digits).
+func RunKey(p RunProgram, toolchain ToolchainUsed, versions PackageVersions) string {
+	return "program-" + strings.TrimPrefix(ComposerKey(p.Repository, p.Commit, p.Dir, p.Package, toolchain, versions), "composer-")
+}