diff --git a/modules/mesh-delivery-checker/cmd/mesh-checking-program/main.go b/modules/mesh-delivery-checker/cmd/mesh-checking-program/main.go
new file mode 100644
index 00000000..82e45fc6
--- /dev/null
+++ b/modules/mesh-delivery-checker/cmd/mesh-checking-program/main.go
@@ -0,0 +1,59 @@
+// Command mesh-checking-program is the checker's checking program (novox/hq ADR 0644, ADR 0656; design 47's bootstrap
+// delivery): built from a check's heads and run in the build seat's sandbox through the seat node-build-go's ask run,
+// with no network and no bus account, it reads the check's facts from the file MESH_RUN_INPUT names and answers the
+// verdict the judging rules give them, as one JSON line on its standard output (rules.Answer).
+//
+//	mesh-checking-program -verdict
+//
+// It refuses facts of a version it does not read, naming it, with exit 2.
+package main
+
+import (
+	"encoding/json"
+	"flag"
+	"fmt"
+	"io"
+	"os"
+
+	"git.novox.be/novox/mesh-sdk/go/delivery"
+	"mesh-delivery-checker/rules"
+)
+
+func main() {
+	os.Exit(run(os.Args[1:], os.Getenv, os.Stdout, os.Stderr))
+}
+
+// run is the command over its arguments, environment and outputs, answering its exit.
+func run(args []string, getenv func(string) string, stdout, stderr io.Writer) int {
+	fs := flag.NewFlagSet("mesh-checking-program", flag.ContinueOnError)
+	fs.SetOutput(stderr)
+	verdict := fs.Bool("verdict", false, "answer the verdict of the facts in the input")
+	if err := fs.Parse(args); err != nil {
+		return 2
+	}
+	refuse := func(format string, a ...any) int {
+		fmt.Fprintf(stderr, "mesh-checking-program: "+format+"\n", a...)
+		return 2
+	}
+	if !*verdict {
+		return refuse("nothing asked: give -verdict")
+	}
+	file := getenv(delivery.RunInputEnv)
+	if file == "" {
+		return refuse("no input: %s names no file", delivery.RunInputEnv)
+	}
+	data, err := os.ReadFile(file)
+	if err != nil {
+		return refuse("the input cannot be read: %v", err)
+	}
+	facts, err := rules.DecodeFacts(data)
+	if err != nil {
+		return refuse("%v", err)
+	}
+	line, err := json.Marshal(rules.Answer{V: rules.AnswerVersion, Verdict: rules.Judge(facts)})
+	if err != nil {
+		return refuse("the verdict cannot be written: %v", err)
+	}
+	fmt.Fprintf(stdout, "%s\n", line)
+	return 0
+}
diff --git a/modules/mesh-delivery-checker/cmd/mesh-checking-program/main_test.go b/modules/mesh-delivery-checker/cmd/mesh-checking-program/main_test.go
new file mode 100644
index 00000000..b45af7c6
--- /dev/null
+++ b/modules/mesh-delivery-checker/cmd/mesh-checking-program/main_test.go
@@ -0,0 +1,74 @@
+package main
+
+import (
+	"bytes"
+	"encoding/json"
+	"os"
+	"path/filepath"
+	"reflect"
+	"strings"
+	"testing"
+
+	"git.novox.be/novox/mesh-sdk/go/delivery"
+	"mesh-delivery-checker/rules"
+)
+
+// 1921 (B0, D2): the checking program answers, on its last line, the verdict the rules give the facts in its input, and
+// refuses facts of a version it does not read, an input it is not given, and a question it is not asked, naming each.
+func TestTheCheckingProgramAnswersTheVerdictOfItsFacts(t *testing.T) {
+	failing, _ := json.Marshal(rules.Tested{Level: delivery.LevelUnit, Tests: 3, Failed: []rules.Failure{{Package: "x/a", Test: "TestA", Cause: "a_test.go:9: boom"}}})
+	facts := rules.Facts{V: rules.FactsVersion, Group: "feat/x", Heads: "0123456789ab", Members: []rules.Member{{Repository: "novox/x", Number: 3, Head: "h"}},
+		Steps: []rules.Step{
+			{Repository: "novox/x", Number: 3, Module: "mesh-build-go", Node: "laptop", Level: &delivery.LevelAnswer{Level: delivery.LevelUnit, Tests: []json.RawMessage{failing}}},
+			{Repository: "novox/x", Number: 3, Failed: &delivery.FailedFirst{Machine: "laptop", Words: "novox/x#3 has files in conflict: go.mod"}},
+		}}
+	dir := t.TempDir()
+	write := func(name string, v any) string {
+		data, _ := json.Marshal(v)
+		p := filepath.Join(dir, name)
+		if err := os.WriteFile(p, data, 0o600); err != nil {
+			t.Fatal(err)
+		}
+		return p
+	}
+	v1 := write("v1.json", facts)
+	later := facts
+	later.V = rules.FactsVersion + 1
+	v2 := write("v2.json", later)
+	for name, c := range map[string]struct {
+		args  []string
+		input string
+		exit  int
+		said  string
+	}{
+		"the facts":         {[]string{"-verdict"}, v1, 0, ""},
+		"a later version":   {[]string{"-verdict"}, v2, 2, "facts of version 2, this checking program reads version 1"},
+		"no input":          {[]string{"-verdict"}, "", 2, delivery.RunInputEnv + " names no file"},
+		"nothing asked":     {nil, v1, 2, "nothing asked"},
+		"an unknown asking": {[]string{"-judge"}, v1, 2, "-judge"},
+	} {
+		var out, errs bytes.Buffer
+		exit := run(c.args, func(k string) string {
+			if k == delivery.RunInputEnv {
+				return c.input
+			}
+			return ""
+		}, &out, &errs)
+		if exit != c.exit || !strings.Contains(errs.String(), c.said) {
+			t.Errorf("%s: exit %d, said %q; want %d and %q", name, exit, errs.String(), c.exit, c.said)
+			continue
+		}
+		if c.exit != 0 {
+			continue
+		}
+		lines := strings.Split(strings.TrimSpace(out.String()), "\n")
+		var a rules.Answer
+		if err := json.Unmarshal([]byte(lines[len(lines)-1]), &a); err != nil || a.V != rules.AnswerVersion {
+			t.Fatalf("%s: the last line %q: %v", name, lines[len(lines)-1], err)
+		}
+		if want := rules.Judge(facts); !reflect.DeepEqual(a.Verdict, want) || a.Verdict.State != delivery.VerdictFail || len(a.Verdict.Failures) != 2 ||
+			a.Verdict.Failures[1].Words != "novox/x#3 has files in conflict: go.mod" || a.Verdict.Machines[0] != "laptop" || a.Verdict.Levels[delivery.LevelUnit].Run != 3 {
+			t.Errorf("%s: the verdict %+v, want %+v", name, a.Verdict, want)
+		}
+	}
+}
diff --git a/modules/mesh-delivery-checker/rules/facts.go b/modules/mesh-delivery-checker/rules/facts.go
index d903706e..b3f33e28 100644
--- a/modules/mesh-delivery-checker/rules/facts.go
+++ b/modules/mesh-delivery-checker/rules/facts.go
@@ -207,3 +207,12 @@ func Slowest(tally Tally) []string {
 func failAt(machine, words string) *delivery.Verdict {
 	return &delivery.Verdict{State: delivery.VerdictFail, Failed: delivery.FailedFirst{Machine: machine, Words: words}}
 }
+
+// AnswerVersion is the version of Answer this program writes.
+const AnswerVersion = 1
+
+// Answer is what the checking program answers on the last line of its standard output: its version and the verdict.
+type Answer struct {
+	V       int              `json:"v"`
+	Verdict delivery.Verdict `json:"verdict"`
+}