diff --git a/modules/mesh-delivery-builder/builds/log.go b/modules/mesh-delivery-builder/builds/log.go
index 8bdc55e8..6bf530c5 100644
--- a/modules/mesh-delivery-builder/builds/log.go
+++ b/modules/mesh-delivery-builder/builds/log.go
@@ -72,6 +72,11 @@ type BuildLogEntry struct {
 	// writes, as the build module answered them (its test step's `out-of-step`): the indexer raises a finding naming them
 	// for a trunk build, and only a clean build of the same module clears it. A start or a narration names none.
 	OutOfStep []string `json:"out-of-step,omitempty"`
+	// Error and Step are, at a build's end, why it failed and the step it was at or did last, as the build's key holds them
+	// (BuilderBuild.Error, Step): a reader of the log sees a failed build's reason, never only its outcome. A start or a
+	// narration names neither.
+	Error string `json:"error,omitempty"`
+	Step  string `json:"step,omitempty"`
 	// Line is a narration's words, Why what it narrates (asked, waiting, stored, refused): a build.said entry's alone.
 	Line string `json:"line,omitempty"`
 	Why  string `json:"why,omitempty"`
@@ -118,8 +123,9 @@ func EncodeBuildLogEntry(e BuildLogEntry) ([]byte, error) {
 		switch {
 		case e.Module == "" || e.FP == "" || e.At.IsZero() || e.Line == "":
 			return nil, fmt.Errorf("builds: a build's narration names its module, fingerprint, line and time")
-		case e.Outcome != "" || len(e.Made) > 0 || e.Manifest != "" || len(e.Against) > 0 || len(e.Imports) > 0 || len(e.OutOfStep) > 0:
-			return nil, fmt.Errorf("builds: a build's narration names no outcome, manifest, base, artifact, import or file out of step")
+		case e.Outcome != "" || len(e.Made) > 0 || e.Manifest != "" || len(e.Against) > 0 || len(e.Imports) > 0 || len(e.OutOfStep) > 0 ||
+			e.Error != "" || e.Step != "":
+			return nil, fmt.Errorf("builds: a build's narration names no outcome, manifest, base, artifact, import, file out of step, error or step")
 		}
 		return json.Marshal(e)
 	case e.Event != BuildStartedEvent && e.Event != BuildEndedEvent:
@@ -131,8 +137,8 @@ func EncodeBuildLogEntry(e BuildLogEntry) ([]byte, error) {
 	case e.Event == BuildEndedEvent && e.Outcome == "":
 		return nil, fmt.Errorf("builds: a build's end names its outcome")
 	case e.Event == BuildStartedEvent && (e.Outcome != "" || len(e.Made) > 0 || e.Manifest != "" || len(e.Against) > 0 || len(e.Imports) > 0 ||
-		len(e.OutOfStep) > 0):
-		return nil, fmt.Errorf("builds: a build's start names no outcome, manifest, base, artifact, import or file out of step")
+		len(e.OutOfStep) > 0 || e.Error != "" || e.Step != ""):
+		return nil, fmt.Errorf("builds: a build's start names no outcome, manifest, base, artifact, import, file out of step, error or step")
 	}
 	if err := trunk.CheckImports(e.Imports); err != nil {
 		return nil, err
diff --git a/modules/mesh-delivery-builder/builds/log_test.go b/modules/mesh-delivery-builder/builds/log_test.go
index 4f62c5e0..93de78fc 100644
--- a/modules/mesh-delivery-builder/builds/log_test.go
+++ b/modules/mesh-delivery-builder/builds/log_test.go
@@ -13,11 +13,13 @@ func TestTheBuildersLogKeepsAStartAndAnEndAndTheirDuration(t *testing.T) {
 	end := start
 	end.Event, end.At, end.Outcome, end.Manifest, end.Against = BuildEndedEvent, at.Add(3*time.Minute), "done", "sha256:m", []string{"artifact-store://base/image@sha256:b"}
 	end.Made = []BuiltArtifact{{Name: "image", Kind: "image", Digest: "sha256:1", Reference: "artifact-store://letta/image@sha256:1"}}
-	for _, e := range []BuildLogEntry{start, end} {
+	failed := end
+	failed.Outcome, failed.Made, failed.Error, failed.Step = BuildFailed, nil, "go test: TestX failed", "test"
+	for _, e := range []BuildLogEntry{start, end, failed} {
 		raw, err := EncodeBuildLogEntry(e)
 		back, derr := DecodeBuildLogEntry(raw)
 		if err != nil || derr != nil || back.Event != e.Event || back.Outcome != e.Outcome || back.Directory != "modules/letta" ||
-			back.Repository != "novox/mesh-catalog" || back.Commit != "abc123" {
+			back.Repository != "novox/mesh-catalog" || back.Commit != "abc123" || back.Error != e.Error || back.Step != e.Step {
 			t.Errorf("%s read back as %+v, %v %v", e.Event, back, err, derr)
 		}
 	}
@@ -41,7 +43,13 @@ func TestTheBuildersLogKeepsAStartAndAnEndAndTheirDuration(t *testing.T) {
 		t.Error("a module with a dot was taken as a key")
 	}
 	for _, edit := range []func(*BuildLogEntry){func(e *BuildLogEntry) { e.Outcome = "" }, func(e *BuildLogEntry) { e.Commit = "" },
-		func(e *BuildLogEntry) { e.Event, e.Outcome, e.Against = BuildStartedEvent, "", nil }} {
+		func(e *BuildLogEntry) { e.Event, e.Outcome, e.Against = BuildStartedEvent, "", nil },
+		func(e *BuildLogEntry) {
+			e.Event, e.Outcome, e.Against, e.Made, e.Manifest, e.Error = BuildStartedEvent, "", nil, nil, "", "x"
+		},
+		func(e *BuildLogEntry) {
+			e.Event, e.Outcome, e.Against, e.Made, e.Manifest, e.Node, e.Line, e.Step = BuildSaidEvent, "", nil, nil, "", "", "asked", "test"
+		}} {
 		bad := end
 		edit(&bad)
 		if _, err := EncodeBuildLogEntry(bad); err == nil {
diff --git a/modules/mesh-delivery-builder/cmd/mesh-delivery-builder-tools/main.go b/modules/mesh-delivery-builder/cmd/mesh-delivery-builder-tools/main.go
index dc3ee1ba..cc24d6ec 100644
--- a/modules/mesh-delivery-builder/cmd/mesh-delivery-builder-tools/main.go
+++ b/modules/mesh-delivery-builder/cmd/mesh-delivery-builder-tools/main.go
@@ -196,7 +196,8 @@ func Tools(r build.StuckReader, askAgain func(key string, value []byte) error, q
 		{Name: builds.BuildsSinceTool,
 			Description: "Every build that ended since a time, oldest first, from the builder's log build-log (kept 30 days): per build its module, " +
 				"fingerprint, source revision, repository, directory, commit and ref, the build node, when it ended, its outcome (done, failed or " +
-				"queued), the manifest built, the bases it was built against and each artifact made with its digest and reference. more says the " +
+				"queued), the manifest built, the bases it was built against and each artifact made with its digest and reference; a failed " +
+				"build carries its error and the step it was at. more says the " +
 				"log held more than one answer carries: ask again from the last build's time. What the operator's command nox builds reads. Read-only.",
 			Input: map[string]any{"type": "object", "required": []string{"since"}, "properties": map[string]any{
 				"since": map[string]any{"type": "string", "description": "RFC 3339 time; the builds that ended at or after it"}}},
diff --git a/modules/mesh-delivery-builder/internal/build/imports_test.go b/modules/mesh-delivery-builder/internal/build/imports_test.go
index 6f81e2fe..20ee96e5 100644
--- a/modules/mesh-delivery-builder/internal/build/imports_test.go
+++ b/modules/mesh-delivery-builder/internal/build/imports_test.go
@@ -74,6 +74,11 @@ func outOfStepFilesABuildAnswersReachTheLog(t *testing.T) {
 			if len(logged) != 2 || strings.Join(logged[1].OutOfStep, ",") != row.want || len(logged[0].OutOfStep) != 0 {
 				t.Fatalf("log %+v, want the end naming %q", logged, row.want)
 			}
+			// A failed build's end carries its reason, so the log never hides why it failed; a passed one no error.
+			if end := logged[1]; row.fail != strings.Contains(end.Error, "out of step") || !row.fail && end.Error != "" ||
+				logged[0].Error != "" || logged[0].Step != "" {
+				t.Errorf("log %+v, want a failed build's end alone to carry its error", logged)
+			}
 		})
 	}
 }
diff --git a/modules/mesh-delivery-builder/internal/build/run.go b/modules/mesh-delivery-builder/internal/build/run.go
index fa76ab4e..4d38f634 100644
--- a/modules/mesh-delivery-builder/internal/build/run.go
+++ b/modules/mesh-delivery-builder/internal/build/run.go
@@ -1009,6 +1009,7 @@ func LogEnded(module string, m meshcatalog.ModulesByModuleValue, ref, by string,
 	}
 	e.Made = append([]builds.BuiltArtifact(nil), result.Artifacts...)
 	e.Imports = append([]string(nil), result.Imports...) // the indexer keeps them on the module (novox/hq ADR 0593 §2)
+	e.Error, e.Step = result.Error, result.Step          // a failed build's reason, not only its outcome
 	return e
 }