diff --git a/modules/mesh-delivery-checker/internal/check/failure_words_test.go b/modules/mesh-delivery-checker/internal/check/failure_words_test.go
new file mode 100644
index 0000000..ef95b47
--- /dev/null
+++ b/modules/mesh-delivery-checker/internal/check/failure_words_test.go
@@ -0,0 +1,73 @@
+package check
+
+import (
+	"encoding/json"
+	"strings"
+	"testing"
+
+	"git.novox.be/novox/mesh-sdk/go/delivery"
+)
+
+// The failure the build module reported for group r4g on 2026-10-23: the test logged a line, then failed with a multi-line
+// t.Fatalf. Its output is what the build module now sends (its last lines, with the error line as Cause).
+func fatalAfterLog() Failure {
+	return Failure{
+		Package: "github.com/novox/mesh-controller/internal/broker", Test: "TestTheCataloguesDerivedSubjectsMeet",
+		Cause: "3 subscription(s) derive to a subject no emitter owns:",
+		Output: "    agreement_catalogue_test.go:130: judged against /var/lib/mesh-build-go-work/id/beside/mesh-catalog as cloned\n" +
+			"    agreement_catalogue_test.go:186: 3 subscription(s) derive to a subject no emitter owns:\n" +
+			"          a.b.c\n          d.e.f\n          g.h.i\n--- FAIL: TestTheCataloguesDerivedSubjectsMeet (0.00s)",
+	}
+}
+
+func TestAFailedTestsVerdictKeepsItsWholeFailureMessage(t *testing.T) { // 1901/AC1
+	v := levelFailure(&target{}, "unit", Tested{Failed: []Failure{fatalAfterLog()}})
+	for _, want := range []string{"3 subscription(s) derive to a subject no emitter owns:", "a.b.c", "d.e.f", "g.h.i"} {
+		if !strings.Contains(v.Failed.Words, want) {
+			t.Errorf("words lack %q: %q", want, v.Failed.Words)
+		}
+	}
+	if len(v.Failed.Words) > MaxWords+200 {
+		t.Errorf("words are %d bytes, bound %d", len(v.Failed.Words), MaxWords)
+	}
+}
+
+func TestTheStatusLineNamesTheFailureNotTheFirstLoggedLine(t *testing.T) { // 1901/AC2
+	v := levelFailure(&target{}, "unit", Tested{Failed: []Failure{fatalAfterLog()}})
+	got := firstFailed(*v)
+	if !strings.Contains(got, "3 subscription(s) derive to a subject no emitter owns:") || strings.Contains(got, "judged against") || strings.Contains(got, "\n") {
+		t.Errorf("firstFailed = %q", got)
+	}
+	// With no cause told apart (an older build module), the words still start at the first line, as before.
+	f := fatalAfterLog()
+	f.Cause = ""
+	v = levelFailure(&target{}, "unit", Tested{Failed: []Failure{f}})
+	if !strings.HasPrefix(v.Failed.Words, "agreement_catalogue_test.go:130") {
+		t.Errorf("words = %q", v.Failed.Words)
+	}
+}
+
+func TestWordsLongerThanTheBoundKeepTheirLastLines(t *testing.T) { // 1901/AC1
+	f := Failure{Output: strings.Repeat("log line of some length\n", 500) + "the cause is here"}
+	v := levelFailure(&target{}, "unit", Tested{Failed: []Failure{f}})
+	if !strings.HasSuffix(v.Failed.Words, "the cause is here") || len(v.Failed.Words) > MaxWords+200 {
+		t.Errorf("%d bytes, ends %q", len(v.Failed.Words), v.Failed.Words[len(v.Failed.Words)-30:])
+	}
+	_ = delivery.VerdictFail
+}
+
+// An older build module sends no "cause": its answer is still accepted and the verdict's words are its output, as before.
+func TestAnOlderBuildModulesFailureIsAcceptedWithItsWholeOutput(t *testing.T) { // 1901/AC3
+	old := `{"package":"p","test":"TestX","output":"x_test.go:1: logged\nx_test.go:2: the old kept lines"}`
+	var f Failure
+	if err := json.Unmarshal([]byte(old), &f); err != nil || f.Cause != "" {
+		t.Fatalf("1901/AC3: err = %v, cause = %q", err, f.Cause)
+	}
+	v := levelFailure(&target{}, "unit", Tested{Failed: []Failure{f}})
+	if v.Failed.Words != "x_test.go:1: logged\nx_test.go:2: the old kept lines" {
+		t.Errorf("1901/AC3: words = %q", v.Failed.Words)
+	}
+	if got := firstFailed(*v); !strings.Contains(got, "logged") || strings.Contains(got, "\n") {
+		t.Errorf("1901/AC3: status = %q", got)
+	}
+}
diff --git a/modules/mesh-delivery-checker/internal/check/judge.go b/modules/mesh-delivery-checker/internal/check/judge.go
index a6f134c..079d688 100644
--- a/modules/mesh-delivery-checker/internal/check/judge.go
+++ b/modules/mesh-delivery-checker/internal/check/judge.go
@@ -525,13 +525,35 @@ func failedName(f Failure) string {
 	return f.Package + "." + f.Test
 }
 
-// firstLine is the first non-empty line of a failure's output, shortened.
+// MaxWords is the most a failed test's words in a verdict hold (issue 1901). **The words keep the failure's last lines**,
+// where a test says why it failed: the first line is often something it logged before (the group r4g verdict kept a
+// "judged against" line and lost the t.Fatalf after it), and the 120 characters kept before cut a docker command's echo
+// before docker's own error. Four kilobytes holds a long message and is small beside a verdict's other parts.
+const MaxWords = 4096
+
+// failureWords is a failure's words: its output's last lines up to MaxWords, with the line the build module told apart as
+// the test's error first when the output does not start with it, so the first line of the words is the cause and the
+// status line, which is cut short, names it.
+func failureWords(f Failure) string {
+	out := strings.TrimSpace(f.Output)
+	if len(out) > MaxWords {
+		out = out[len(out)-MaxWords:]
+		// Start on a whole line, and a whole character.
+		if i := strings.IndexByte(out, '\n'); i >= 0 {
+			out = "... " + strings.TrimLeft(out[i:], "\n")
+		}
+		out = strings.ToValidUTF8(out, "")
+	}
+	if c := strings.TrimSpace(f.Cause); c != "" && !strings.HasPrefix(firstLine(out), c) {
+		out = c + "\n" + out
+	}
+	return out
+}
+
+// firstLine is the first non-empty line of a failure's words.
 func firstLine(s string) string {
 	for _, l := range strings.Split(s, "\n") {
 		if l = strings.TrimSpace(l); l != "" {
-			if len(l) > 120 {
-				l = l[:120]
-			}
 			return l
 		}
 	}
@@ -548,7 +570,7 @@ func levelFailure(t *target, level string, res Tested) *delivery.Verdict {
 		if t.module == buildrun.NodeBuildModule && f.Package == "" {
 			name = res.Module
 		}
-		v.Failed = delivery.FailedFirst{Test: level + ": " + name, Words: firstLine(f.Output)}
+		v.Failed = delivery.FailedFirst{Test: level + ": " + name, Words: failureWords(f)}
 	default:
 		why := res.Why
 		if why == "" {
diff --git a/modules/mesh-delivery-checker/internal/check/line.go b/modules/mesh-delivery-checker/internal/check/line.go
index 3e5afc0..f847823 100644
--- a/modules/mesh-delivery-checker/internal/check/line.go
+++ b/modules/mesh-delivery-checker/internal/check/line.go
@@ -37,18 +37,21 @@ func firstFailed(v delivery.Verdict) string {
 	f := v.Failed
 	switch {
 	case f.Test != "":
-		if f.Words != "" {
-			return f.Test + ": " + f.Words
+		if w := firstWordsLine(f.Words); w != "" {
+			return f.Test + ": " + w
 		}
 		return f.Test
-	case f.Machine != "" && f.Words != "":
-		return f.Machine + ": " + f.Words
+	case f.Machine != "" && firstWordsLine(f.Words) != "":
+		return f.Machine + ": " + firstWordsLine(f.Words)
 	case f.Machine != "":
 		return f.Machine
 	}
-	return f.Words
+	return firstWordsLine(f.Words)
 }
 
+// firstWordsLine is the first line of a failure's words: the words are several lines (issue 1901) and the status line one.
+func firstWordsLine(w string) string { return firstLine(w) }
+
 // pieces are the parts of a line, from the most to the least needed.
 type pieces struct {
 	head, commits, counts, shortCounts, times, shortTimes, over, builds, composition, shortComposition, group string
diff --git a/modules/mesh-delivery-checker/internal/check/results.go b/modules/mesh-delivery-checker/internal/check/results.go
index 8336247..275e821 100644
--- a/modules/mesh-delivery-checker/internal/check/results.go
+++ b/modules/mesh-delivery-checker/internal/check/results.go
@@ -59,6 +59,8 @@ type Failure struct {
 	Package string `json:"package"`
 	Test    string `json:"test,omitempty"`
 	Output  string `json:"output,omitempty"`
+	// Cause is the first line of what the test reported as its error, when the build module told it apart (issue 1901).
+	Cause string `json:"cause,omitempty"`
 }
 
 // TestTime is one test and the seconds it took.