diff --git a/modules/mesh-delivery-builder/cmd/mesh-delivery-builder/bus.go b/modules/mesh-delivery-builder/cmd/mesh-delivery-builder/bus.go
index 5798b3a..d92318e 100644
--- a/modules/mesh-delivery-builder/cmd/mesh-delivery-builder/bus.go
+++ b/modules/mesh-delivery-builder/cmd/mesh-delivery-builder/bus.go
@@ -49,6 +49,14 @@ func (s kvStore) Update(ctx context.Context, key string, value []byte, revision
 	return rev, conflict(err)
 }
 
+func (s kvStore) Keys(ctx context.Context) ([]string, error) {
+	keys, err := s.kv.Keys(ctx)
+	if errors.Is(err, jetstream.ErrNoKeysFound) {
+		return nil, nil
+	}
+	return keys, err
+}
+
 func opOf(e jetstream.KeyValueEntry) bool { return e.Operation() == jetstream.KeyValuePut }
 
 // settle reads the entries a watcher holds now, up to its mark of the last values, and returns the watcher for the
diff --git a/modules/mesh-delivery-builder/cmd/mesh-delivery-builder/integration_test.go b/modules/mesh-delivery-builder/cmd/mesh-delivery-builder/integration_test.go
index 780387e..305ed79 100644
--- a/modules/mesh-delivery-builder/cmd/mesh-delivery-builder/integration_test.go
+++ b/modules/mesh-delivery-builder/cmd/mesh-delivery-builder/integration_test.go
@@ -504,3 +504,21 @@ func (b *bed) moduleAt(t *testing.T, name, fixture, path, goVersion, at string)
 	t.Helper()
 	b.moduleCommit(t, name, fixture, path, map[string]delivery.ToolchainUsed{"go": {Version: goVersion, Recipe: "1"}}, at)
 }
+
+// TestAKeyLeftBuildingForAModuleThatIsGoneIsClosedAtStart: the builds bucket holds a key left building by another node for
+// a module the indexer no longer has; the program, at start, fails it naming why (issue 1660, ADR 0478 clause 6).
+func TestAKeyLeftBuildingForAModuleThatIsGoneIsClosedAtStart(t *testing.T) {
+	b := startBed(t)
+	data, err := delivery.EncodeBuilderBuild(delivery.BuilderBuild{State: delivery.BuildBuilding, By: "n2.mesh-delivery-builder", Manifest: strings.Repeat("a", 64)})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, err := b.builds.Put(b.ctx, mustKey(t, "ghost"), data); err != nil {
+		t.Fatal(err)
+	}
+	b.start(t, "n1", nil)
+	v := b.await(t, "ghost", isState(delivery.BuildFailed))
+	if !strings.Contains(v.Error, "no longer has this fingerprint") || !strings.Contains(v.Error, "n2.mesh-delivery-builder") {
+		t.Errorf("build = %+v, want it closed naming who left it", v)
+	}
+}
diff --git a/modules/mesh-delivery-builder/cmd/mesh-delivery-builder/main.go b/modules/mesh-delivery-builder/cmd/mesh-delivery-builder/main.go
index f93a061..9db5436 100644
--- a/modules/mesh-delivery-builder/cmd/mesh-delivery-builder/main.go
+++ b/modules/mesh-delivery-builder/cmd/mesh-delivery-builder/main.go
@@ -158,9 +158,9 @@ func run(ctx context.Context, getenv func(string) string, logOut io.Writer) erro
 			return fmt.Errorf("MESH_BUILDER_STEP_BOUND: %w", err)
 		}
 	}
-	capacity, err := build.ParseCapacity(getenv("MESH_BUILDER_CAPACITY"))
+	capacity, err := capacityFrom(strings.TrimSpace(getenv("MESH_BUILDER_SETTINGS")), getenv("MESH_BUILDER_CAPACITY"))
 	if err != nil {
-		return fmt.Errorf("MESH_BUILDER_CAPACITY: %w", err)
+		return fmt.Errorf("the build capacity setting: %w", err)
 	}
 	tools := &build.ToolchainSet{}
 	presence := &build.PresenceSet{}
@@ -177,7 +177,7 @@ func run(ctx context.Context, getenv func(string) string, logOut io.Writer) erro
 		Logf:       logger.Printf,
 	}
 	// The toolchains first, so a build that starts from the first modules value finds the nodes.
-	watch := newToolchainWatch(ctx, js, tools, b.Reconsider, logger)
+	watch := newToolchainWatch(ctx, js, tools, b.NodesChanged, logger)
 	if err := watch.start(); err != nil {
 		return err
 	}
@@ -214,7 +214,7 @@ func run(ctx context.Context, getenv func(string) string, logOut io.Writer) erro
 		go func() {
 			presenceErrs <- follow(ctx, pw, func(e jetstream.KeyValueEntry) {
 				if presence.Put(e.Key(), e.Value(), !opOf(e)) {
-					b.Reconsider(ctx) // a node came online or went: a build that waited for one may go on
+					b.NodesChanged(ctx) // a node came online or went: a build that waited for one may go on
 				}
 			})
 		}()
@@ -235,7 +235,7 @@ func run(ctx context.Context, getenv func(string) string, logOut io.Writer) erro
 		go func() {
 			haveErrs <- follow(ctx, hw, func(e jetstream.KeyValueEntry) {
 				if have.Put(e.Key(), e.Value(), !opOf(e)) {
-					b.Reconsider(ctx) // a build module became healthy or stopped being: a build that waited may go on
+					b.NodesChanged(ctx) // a build module became healthy or stopped being: a build that waited may go on
 				}
 			})
 		}()
@@ -251,6 +251,10 @@ func run(ctx context.Context, getenv func(string) string, logOut io.Writer) erro
 	if err != nil {
 		return err
 	}
+	// A key left building for a fingerprint its module no longer has is closed (issue 1660); the modules are all known now.
+	if err := b.Sweep(ctx); err != nil {
+		return err
+	}
 	logger.Printf("started on %s", cred.Node)
 	errs := make(chan error, 1)
 	go func() { errs <- follow(ctx, w, each) }()
diff --git a/modules/mesh-delivery-builder/cmd/mesh-delivery-builder/settings.go b/modules/mesh-delivery-builder/cmd/mesh-delivery-builder/settings.go
new file mode 100644
index 0000000..36ead03
--- /dev/null
+++ b/modules/mesh-delivery-builder/cmd/mesh-delivery-builder/settings.go
@@ -0,0 +1,29 @@
+package main
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+
+	"mesh-delivery-builder/internal/build"
+)
+
+// capacityFrom is the build capacity of each node (ADR 0478 clause 3): the module's setting `capacity`, `<node>=<steps at
+// once>` pairs separated by commas, laid into the settings file the controller composes (issue 1661); empty is one step at a
+// time on every node. With no settings file named, fallback is read instead, for a run outside the module's launch.
+func capacityFrom(path, fallback string) (map[string]int, error) {
+	if path == "" {
+		return build.ParseCapacity(fallback)
+	}
+	raw, err := os.ReadFile(path)
+	if err != nil {
+		return nil, fmt.Errorf("the settings file: %w", err)
+	}
+	var s struct {
+		Capacity string `json:"capacity"`
+	}
+	if err := json.Unmarshal(raw, &s); err != nil {
+		return nil, fmt.Errorf("the settings file %s is not JSON: %w", path, err)
+	}
+	return build.ParseCapacity(s.Capacity)
+}
diff --git a/modules/mesh-delivery-builder/cmd/mesh-delivery-builder/settings_test.go b/modules/mesh-delivery-builder/cmd/mesh-delivery-builder/settings_test.go
new file mode 100644
index 0000000..8b52444
--- /dev/null
+++ b/modules/mesh-delivery-builder/cmd/mesh-delivery-builder/settings_test.go
@@ -0,0 +1,68 @@
+package main
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"mesh-delivery-builder/internal/build"
+)
+
+func settingsFile(t *testing.T, body string) string {
+	t.Helper()
+	p := filepath.Join(t.TempDir(), "settings.json")
+	if err := os.WriteFile(p, []byte(body), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	return p
+}
+
+// 1661: the build capacity is the module's setting `capacity`, laid into the settings file: empty is one step at a time everywhere.
+func TestCapacityIsReadFromTheSettingsFile(t *testing.T) {
+	for body, want := range map[string]map[string]int{
+		`{"capacity": "shanks=3,ace=2"}`: {"shanks": 3, "ace": 2},
+		`{"capacity": ""}`:               {},
+		`{}`:                             {},
+	} {
+		got, err := capacityFrom(settingsFile(t, body), "")
+		if err != nil || len(got) != len(want) || got["shanks"] != want["shanks"] || got["ace"] != want["ace"] {
+			t.Errorf("%s: capacity %v, %v, want %v", body, got, err, want)
+		}
+	}
+	p := &build.Places{Capacity: mustCapacity(t, settingsFile(t, `{"capacity": ""}`))}
+	if p.CapacityOf("shanks") != 1 || p.CapacityOf("ace") != 1 {
+		t.Errorf("an empty setting gives %d and %d, want 1 everywhere", p.CapacityOf("shanks"), p.CapacityOf("ace"))
+	}
+}
+
+func mustCapacity(t *testing.T, path string) map[string]int {
+	t.Helper()
+	c, err := capacityFrom(path, "")
+	if err != nil {
+		t.Fatal(err)
+	}
+	return c
+}
+
+func TestACapacitySettingThatIsNotOneIsRefusedNamingIt(t *testing.T) {
+	if _, err := capacityFrom(settingsFile(t, `{"capacity": "shanks=-1"}`), ""); err == nil {
+		t.Error("shanks=-1 accepted")
+	}
+	if c, err := capacityFrom(settingsFile(t, `{"capacity": "shanks=3,ace=2,g14=0,novox=0"}`), ""); err != nil || c["g14"] != 0 || len(c) != 4 {
+		t.Errorf("the starting value: %v, %v", c, err)
+	}
+	if _, err := capacityFrom(settingsFile(t, `not json`), ""); err == nil {
+		t.Error("a file that is not JSON accepted")
+	}
+	if _, err := capacityFrom(filepath.Join(t.TempDir(), "none"), ""); err == nil {
+		t.Error("a settings file that is not there accepted")
+	}
+}
+
+// With no settings file named (a run outside the module's launch, as the tests do), the capacity is the environment's.
+func TestWithNoSettingsFileTheCapacityIsTheEnvironments(t *testing.T) {
+	got, err := capacityFrom("", "ace=2")
+	if err != nil || got["ace"] != 2 {
+		t.Errorf("capacity %v, %v", got, err)
+	}
+}
diff --git a/modules/mesh-delivery-builder/files/settings.json b/modules/mesh-delivery-builder/files/settings.json
new file mode 100644
index 0000000..3175fe3
--- /dev/null
+++ b/modules/mesh-delivery-builder/files/settings.json
@@ -0,0 +1,3 @@
+{
+  "capacity": "shanks=3,ace=2,g14=0,novox=0"
+}
diff --git a/modules/mesh-delivery-builder/internal/build/choose.go b/modules/mesh-delivery-builder/internal/build/choose.go
index a71c61c..7f87b88 100644
--- a/modules/mesh-delivery-builder/internal/build/choose.go
+++ b/modules/mesh-delivery-builder/internal/build/choose.go
@@ -34,6 +34,9 @@ func ChooseAnyNodes(language string, entries []delivery.NodeToolchain, places *P
 }
 
 func choose(language string, entries []delivery.NodeToolchain, places *Places, qualifies func(node string) bool, want int) ([]string, error) {
+	if places == nil {
+		places = &Places{}
+	}
 	nodes := make([]string, 0, len(entries))
 	var reporting []string
 	seen := map[string]bool{}
@@ -43,16 +46,13 @@ func choose(language string, entries []delivery.NodeToolchain, places *Places, q
 		}
 		seen[e.Node] = true
 		reporting = append(reporting, e.Node)
-		if qualifies == nil || qualifies(e.Node) {
+		if places.CapacityOf(e.Node) > 0 && (qualifies == nil || qualifies(e.Node)) { // a capacity of 0 keeps the builder off a node
 			nodes = append(nodes, e.Node)
 		}
 	}
 	if len(nodes) == 0 {
 		sort.Strings(reporting)
-		return nil, fmt.Errorf("no %s build node qualifies: %s report the toolchain, and none is online with a healthy build module", language, strings.Join(reporting, ", "))
-	}
-	if places == nil {
-		places = &Places{}
+		return nil, fmt.Errorf("no %s build node qualifies: %s report the toolchain, and none is online with a healthy build module and a build capacity", language, strings.Join(reporting, ", "))
 	}
 	return places.choose(nodes, want), nil
 }
diff --git a/modules/mesh-delivery-builder/internal/build/claim.go b/modules/mesh-delivery-builder/internal/build/claim.go
index 2875d57..6d16939 100644
--- a/modules/mesh-delivery-builder/internal/build/claim.go
+++ b/modules/mesh-delivery-builder/internal/build/claim.go
@@ -18,6 +18,8 @@ type Store interface {
 	Get(ctx context.Context, key string) (value []byte, revision uint64, found bool, err error)
 	Create(ctx context.Context, key string, value []byte) (uint64, error)
 	Update(ctx context.Context, key string, value []byte, revision uint64) (uint64, error)
+	// Keys are every key of the bucket.
+	Keys(ctx context.Context) ([]string, error)
 }
 
 // Claimer claims fingerprints for the builder instance By (ADR 0429 clause 5: a builder claims a fingerprint by creating
@@ -57,7 +59,8 @@ type ClaimOptions struct {
 	// time, and a build left building by a builder that is gone, on this node or another, is nobody's (ADR 0478 clause 6).
 	TakeOver bool
 	// MoreNodes says whether one of these build modules, which built a done key on one node only, has a second node that
-	// qualifies now. Nil means none does.
+	// qualifies now and the key may be built again (it is not held back after a failed second build, issue 1664). Nil means
+	// none does.
 	MoreNodes func(modules []string) bool
 }
 
diff --git a/modules/mesh-delivery-builder/internal/build/claim_test.go b/modules/mesh-delivery-builder/internal/build/claim_test.go
index 644b586..a5b2cb9 100644
--- a/modules/mesh-delivery-builder/internal/build/claim_test.go
+++ b/modules/mesh-delivery-builder/internal/build/claim_test.go
@@ -29,6 +29,16 @@ func (m *memStore) Get(_ context.Context, key string) ([]byte, uint64, bool, err
 	return v, m.revs[key], ok, nil
 }
 
+func (m *memStore) Keys(_ context.Context) ([]string, error) {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	var keys []string
+	for k := range m.vals {
+		keys = append(keys, k)
+	}
+	return keys, nil
+}
+
 func (m *memStore) hook() {
 	if h := m.before; h != nil {
 		m.before = nil
diff --git a/modules/mesh-delivery-builder/internal/build/followups_test.go b/modules/mesh-delivery-builder/internal/build/followups_test.go
new file mode 100644
index 0000000..6128d0c
--- /dev/null
+++ b/modules/mesh-delivery-builder/internal/build/followups_test.go
@@ -0,0 +1,83 @@
+package build
+
+import (
+	"context"
+	"strings"
+	"testing"
+
+	"git.novox.be/novox/mesh-sdk/go/delivery"
+)
+
+// 1664: a done key built on one node is not built again, over and over, while the second node's build keeps failing.
+func TestASecondNodeBuildThatFailsIsNotRetriedUntilTheNodesChange(t *testing.T) { // issue 1664
+	r := newRig(t, goBundle)
+	r.pres.Put("ace", presenceValue(t, false), false) // only novox qualifies
+	first := r.consider(t, "modules/m")
+	r.bm.fails["ace/build_go_fetch"] = "ace is out of disk"
+	r.pres.Put("ace", presenceValue(t, true), false)
+	r.b.NodesChanged(context.Background()) // ace came online: one more try
+	r.b.Wait()
+	tries := len(r.bm.callsOf("build_go_fetch"))
+	if tries < 2 {
+		t.Fatalf("%d fetches: the second node was never tried", tries)
+	}
+	if v := valueOf(t, r.st, r.key); !delivery.SameBuilderBuild(v, first) {
+		t.Fatalf("the done key changed: %+v", v)
+	}
+	// Nothing about the nodes changed: a modules value, a Reconsider and a start of looking again build nothing.
+	for i := 0; i < 3; i++ {
+		r.b.Module(context.Background(), "m", moduleValue(t, "modules/m"))
+		r.b.Reconsider(context.Background())
+		r.b.Wait()
+	}
+	if n := len(r.bm.callsOf("build_go_fetch")); n != tries {
+		t.Errorf("%d fetches after looking again, was %d: the failed second build was retried in a loop", n, tries)
+	}
+	// A node changing (ace goes and comes back) is a reason to try again, and ace now builds.
+	delete(r.bm.fails, "ace/build_go_fetch")
+	r.pres.Put("ace", presenceValue(t, false), false)
+	r.b.NodesChanged(context.Background())
+	r.pres.Put("ace", presenceValue(t, true), false)
+	r.b.NodesChanged(context.Background())
+	r.b.Wait()
+	if v := valueOf(t, r.st, r.key); v.State != delivery.BuildDone || strings.HasPrefix(v.Step, OneNodePrefix) || !strings.Contains(v.Step, "ace and novox") {
+		t.Errorf("build = %+v, want it built on both nodes once ace could", v)
+	}
+}
+
+// 1660: at start the builder closes a key left building for a fingerprint the module no longer has.
+func TestAKeyLeftBuildingForAFingerprintTheModuleNoLongerHasIsClosedAtStart(t *testing.T) { // issue 1660
+	r := newRig(t, goBundle)
+	stale := seed(t, r.st, delivery.BuilderBuild{State: delivery.BuildBuilding, By: "ace.builder", Manifest: manifestD}) // m at fp1
+	other, _ := delivery.BuilderBuildKey("gone", fp1)
+	data, _ := delivery.EncodeBuilderBuild(delivery.BuilderBuild{State: delivery.BuildBuilding, By: "ace.builder", Manifest: manifestD})
+	r.st.put(other, data)
+	// The module m has moved on to another fingerprint.
+	moved := delivery.Module{Repository: "novox/mesh-catalog", Commit: "7a74cee83f88e6585a5511c3cc53bdf0f5303d20", FP: "src1:" + hex64('f'),
+		FP2: "src2:" + hex64('7'), Manifest: manifestD, Path: "modules/m", Toolchains: agreedToolchains}
+	enc, _ := delivery.EncodeModule(moved)
+	r.b.Module(context.Background(), "m", enc)
+	r.b.Wait()
+	if err := r.b.Sweep(context.Background()); err != nil {
+		t.Fatal(err)
+	}
+	for _, k := range []string{stale, other} {
+		v := valueOf(t, r.st, k)
+		if v.State != delivery.BuildFailed || !strings.Contains(v.Error, "no longer") || !strings.Contains(v.Error, "ace.builder") {
+			t.Errorf("%s = %+v, want it closed as failed, naming why and who left it", k, v)
+		}
+	}
+}
+
+func TestSweepLeavesAKeyOfTheModulesCurrentFingerprintForTheBuildToTakeOver(t *testing.T) { // issue 1660
+	r := newRig(t, goBundle)
+	seed(t, r.st, delivery.BuilderBuild{State: delivery.BuildBuilding, By: "ace.builder", Manifest: manifestD})
+	r.b.Module(context.Background(), "m", moduleValue(t, "modules/m")) // takes it over and builds it
+	r.b.Wait()
+	if err := r.b.Sweep(context.Background()); err != nil {
+		t.Fatal(err)
+	}
+	if v := valueOf(t, r.st, r.key); v.State != delivery.BuildDone || v.By != "novox.builder" {
+		t.Errorf("build = %+v, want done by novox.builder", v)
+	}
+}
diff --git a/modules/mesh-delivery-builder/internal/build/places.go b/modules/mesh-delivery-builder/internal/build/places.go
index 18e2c30..65d41fb 100644
--- a/modules/mesh-delivery-builder/internal/build/places.go
+++ b/modules/mesh-delivery-builder/internal/build/places.go
@@ -14,7 +14,7 @@ const DefaultCapacity = 1
 
 // ParseCapacity reads the builder's capacity setting: `node=steps` pairs separated by commas, such as
 // `shanks=3,ace=2`. A node without a pair has DefaultCapacity. A pair with no node, a count that is not a whole number of
-// at least 1, and a node named twice are refused.
+// at least 0, and a node named twice are refused. A count of 0 keeps the builder off the node: it is never chosen.
 func ParseCapacity(s string) (map[string]int, error) {
 	out := map[string]int{}
 	for _, pair := range strings.Split(s, ",") {
@@ -25,8 +25,8 @@ func ParseCapacity(s string) (map[string]int, error) {
 		node, count, ok := strings.Cut(pair, "=")
 		node, count = strings.TrimSpace(node), strings.TrimSpace(count)
 		n, err := strconv.Atoi(count)
-		if !ok || node == "" || err != nil || n < 1 {
-			return nil, fmt.Errorf("the capacity %q is not <node>=<steps at once, at least 1>", pair)
+		if !ok || node == "" || err != nil || n < 0 {
+			return nil, fmt.Errorf("the capacity %q is not <node>=<steps at once, 0 or more>", pair)
 		}
 		if _, dup := out[node]; dup {
 			return nil, fmt.Errorf("the capacity names %s twice", node)
@@ -52,9 +52,10 @@ type Places struct {
 	changed  chan struct{}  // closed, and replaced, each time a lane ends
 }
 
-// CapacityOf is how many steps the builder runs on the node at once.
+// CapacityOf is how many steps the builder runs on the node at once: DefaultCapacity for a node the setting does not
+// name, and 0 for one it names so, where the builder never builds.
 func (p *Places) CapacityOf(node string) int {
-	if n, ok := p.Capacity[node]; ok && n >= 1 {
+	if n, ok := p.Capacity[node]; ok && n >= 0 {
 		return n
 	}
 	return DefaultCapacity
diff --git a/modules/mesh-delivery-builder/internal/build/places_test.go b/modules/mesh-delivery-builder/internal/build/places_test.go
index 5cb3d24..27c7d8d 100644
--- a/modules/mesh-delivery-builder/internal/build/places_test.go
+++ b/modules/mesh-delivery-builder/internal/build/places_test.go
@@ -19,7 +19,7 @@ func TestParseCapacityReadsNodeEqualsCountPairs(t *testing.T) {
 	if c, err := ParseCapacity(""); err != nil || len(c) != 0 {
 		t.Errorf("empty: %v, %v", c, err)
 	}
-	for _, bad := range []string{"shanks", "shanks=0", "shanks=x", "=2", "a=1,a=2", "shanks=-1"} {
+	for _, bad := range []string{"shanks", "shanks=x", "=2", "a=1,a=2", "shanks=-1"} {
 		if _, err := ParseCapacity(bad); err == nil {
 			t.Errorf("%q accepted", bad)
 		}
@@ -138,3 +138,49 @@ func TestTwoNodeAgreementStillTakesTwoDifferentNodesWhenCapacitiesAreSet(t *test
 		t.Errorf("build = %+v, want done on ace and novox", v)
 	}
 }
+
+// The operator's decision of 2026-10-21 13:40: the builder builds on shanks and ace only; a capacity of 0 keeps it off a node.
+func TestACapacityOfZeroKeepsTheBuilderOffTheNodeAndANodeNotNamedKeepsOne(t *testing.T) { // issue 1661
+	c, err := ParseCapacity("shanks=3,ace=2,g14=0,novox=0")
+	if err != nil || c["g14"] != 0 || c["novox"] != 0 || c["shanks"] != 3 || len(c) != 4 {
+		t.Fatalf("capacity = %v, %v", c, err)
+	}
+	p := &Places{Capacity: c}
+	if p.CapacityOf("g14") != 0 || p.CapacityOf("novox") != 0 || p.CapacityOf("ace") != 2 || p.CapacityOf("other") != DefaultCapacity {
+		t.Errorf("capacities = %d %d %d %d, want 0 0 2 and the default for a node not named", p.CapacityOf("g14"), p.CapacityOf("novox"), p.CapacityOf("ace"), p.CapacityOf("other"))
+	}
+	if _, err := ParseCapacity("shanks=-1"); err == nil {
+		t.Error("a negative capacity accepted")
+	}
+	entries := tcs("go", "go1.27.1", "ace", "g14", "novox", "shanks", "other")
+	for i := 0; i < 6; i++ {
+		nodes, _, err := ChooseNodes("go", entries, p, nil, 2)
+		if err != nil {
+			t.Fatal(err)
+		}
+		for _, n := range nodes {
+			if n == "g14" || n == "novox" {
+				t.Fatalf("choice %d: %v includes a node of capacity 0", i, nodes)
+			}
+		}
+	}
+	// Only nodes of capacity 0 report: nothing qualifies, and the build says why.
+	_, _, err = ChooseNodes("go", tcs("go", "go1.27.1", "g14", "novox"), p, nil, 2)
+	if err == nil || !strings.Contains(err.Error(), "capacity") {
+		t.Errorf("only capacity-0 nodes: error = %v, want it to name the capacity", err)
+	}
+}
+
+func TestABuildNeverRunsOnANodeOfCapacityZero(t *testing.T) { // issue 1661
+	r := newRig(t, catalogue.Artifact{Name: "tool", Kind: "bundle", Language: "go"}) // ace and novox report
+	r.b.Places = &Places{Capacity: map[string]int{"novox": 0}}
+	v := r.consider(t, "modules/m")
+	if v.State != "done" || !strings.Contains(v.Step, "on one node, ace") {
+		t.Fatalf("build = %+v, want done on ace alone", v)
+	}
+	for _, c := range r.bm.calls {
+		if c.node == "novox" {
+			t.Errorf("a step was called on novox, whose capacity is 0: %+v", c)
+		}
+	}
+}
diff --git a/modules/mesh-delivery-builder/internal/build/run.go b/modules/mesh-delivery-builder/internal/build/run.go
index f7ade56..53de028 100644
--- a/modules/mesh-delivery-builder/internal/build/run.go
+++ b/modules/mesh-delivery-builder/internal/build/run.go
@@ -3,6 +3,7 @@ package build
 import (
 	"context"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"sort"
 	"strings"
@@ -53,6 +54,10 @@ type Builder struct {
 	again    map[string]delivery.Module // by build key: the newest value that arrived while the key was being built
 	plans    map[string]planned         // by manifest digest
 	tried    map[string]bool            // by build key: looked at since this process started
+	// epoch counts the changes of the nodes (online, healthy, a toolchain) seen since this process started; a done key
+	// whose second-node build failed is held back until it moves (issue 1664).
+	epoch    uint64
+	secondAt map[string]uint64 // by build key: the epoch a second-node build of a one-node key failed at
 }
 
 type planned struct {
@@ -68,6 +73,7 @@ func (b *Builder) init() {
 		}
 		b.sem = make(chan struct{}, limit)
 		b.known, b.inflight, b.again, b.plans, b.tried = map[string]delivery.Module{}, map[string]bool{}, map[string]delivery.Module{}, map[string]planned{}, map[string]bool{}
+		b.secondAt = map[string]uint64{}
 		if b.Places == nil {
 			b.Places = &Places{}
 		}
@@ -98,6 +104,63 @@ func (b *Builder) Module(ctx context.Context, key string, value []byte) {
 	b.consider(ctx, key, m)
 }
 
+// NodesChanged is Reconsider after a change of the nodes: one came online or went, a build module became healthy or
+// stopped being, or a toolchain moved. A done key built on one node whose second-node build failed is tried again only
+// after such a change, not at every modules value (issue 1664).
+func (b *Builder) NodesChanged(ctx context.Context) {
+	b.init()
+	b.mu.Lock()
+	b.epoch++
+	b.mu.Unlock()
+	b.Reconsider(ctx)
+}
+
+// Sweep closes, at start, every key left building for a fingerprint its module no longer has (issue 1660, ADR 0478
+// clause 6): the key can never be built, since nothing asks for it, and would stay building for ever. It fails the key
+// naming who left it. A key of the fingerprint the module has now is not touched: the build takes it over. Call it
+// once the modules bucket has been read, so every module is known.
+func (b *Builder) Sweep(ctx context.Context) error {
+	b.init()
+	keys, err := b.Claimer.Store.Keys(ctx)
+	if err != nil {
+		return err
+	}
+	b.mu.Lock()
+	current := map[string]bool{}
+	for name, m := range b.known {
+		if m.FP2 != "" {
+			if k, err := delivery.BuilderBuildKey(name, m.FP2); err == nil {
+				current[k] = true
+			}
+		}
+	}
+	b.mu.Unlock()
+	for _, key := range keys {
+		if current[key] {
+			continue
+		}
+		value, rev, found, err := b.Claimer.Store.Get(ctx, key)
+		if err != nil || !found {
+			continue
+		}
+		cur, err := delivery.DecodeBuilderBuild(value)
+		if err != nil || cur.State != delivery.BuildBuilding {
+			continue
+		}
+		closed := delivery.BuilderBuild{State: delivery.BuildFailed, By: b.Claimer.By, Manifest: cur.Manifest, Step: cur.Step,
+			Error: fmt.Sprintf("closed at start: left building by %s, and the module no longer has this fingerprint, so nothing will build it", cur.By)}
+		data, err := delivery.EncodeBuilderBuild(closed)
+		if err != nil {
+			return err
+		}
+		if _, err := b.Claimer.Store.Update(ctx, key, data, rev); err != nil && !errors.Is(err, ErrConflict) {
+			return fmt.Errorf("%s: %w", key, err)
+		}
+		b.logf("%s: closed, left building by %s for a fingerprint the module no longer has", key, cur.By)
+	}
+	return nil
+}
+
 // Forget takes a module out (its key was deleted).
 func (b *Builder) Forget(key string) {
 	b.init()
@@ -206,8 +269,14 @@ func (b *Builder) try(ctx context.Context, module string, m delivery.Module) {
 	b.mu.Lock()
 	first := !b.tried[key]
 	b.tried[key] = true
+	epoch := b.epoch
+	failedAt, failedBefore := b.secondAt[key]
 	b.mu.Unlock()
-	cl, err := b.Claimer.Claim(ctx, module, m.FP2, m.Manifest, ClaimOptions{TakeOver: first, MoreNodes: func(modules []string) bool { return b.moreNodes(modules) }})
+	moreNodes := func(modules []string) bool {
+		// A second build that failed is not tried again until the nodes change (issue 1664).
+		return (!failedBefore || epoch > failedAt) && b.moreNodes(modules)
+	}
+	cl, err := b.Claimer.Claim(ctx, module, m.FP2, m.Manifest, ClaimOptions{TakeOver: first, MoreNodes: moreNodes})
 	if err != nil {
 		b.logf("%s: %v", module, err)
 		return
@@ -228,6 +297,9 @@ func (b *Builder) try(ctx context.Context, module string, m delivery.Module) {
 		// A done key built again on a second node keeps its done value when the new build fails for another reason than
 		// the digests: the first build stands, and a later change of the nodes looks again.
 		b.logf("%s: built again on a second node and failed, the done build stands: %s", cl.Key, result.Error)
+		b.mu.Lock()
+		b.secondAt[cl.Key] = epoch
+		b.mu.Unlock()
 		return
 	}
 	if err := b.Claimer.Finish(ctx, cl, result); err != nil {
@@ -383,7 +455,7 @@ func (b *Builder) online(node string) bool { return b.Presence == nil || b.Prese
 
 // qualifies is whether the node can build with the build module now: it is online and the build module on it is healthy.
 func (b *Builder) qualifies(node, bm string) bool {
-	return b.online(node) && (b.Health == nil || b.Health.Healthy(node, bm))
+	return b.Places.CapacityOf(node) > 0 && b.online(node) && (b.Health == nil || b.Health.Healthy(node, bm))
 }
 
 // moreNodes is whether any of these build modules has two nodes that qualify now.
diff --git a/modules/mesh-delivery-builder/manifest_test.go b/modules/mesh-delivery-builder/manifest_test.go
index 374764b..66ac3cf 100644
--- a/modules/mesh-delivery-builder/manifest_test.go
+++ b/modules/mesh-delivery-builder/manifest_test.go
@@ -32,13 +32,19 @@ type artifact struct {
 }
 
 type manifest struct {
-	Module       string            `json:"module"`
-	Version      string            `json:"version"`
-	Slug         string            `json:"slug"`
-	Consumes     []string          `json:"consumes"`
-	State        []state           `json:"state"`
-	Reads        []string          `json:"reads"`
-	Invokes      []string          `json:"invokes"`
+	Module   string   `json:"module"`
+	Version  string   `json:"version"`
+	Slug     string   `json:"slug"`
+	Consumes []string `json:"consumes"`
+	State    []state  `json:"state"`
+	Reads    []string `json:"reads"`
+	Invokes  []string `json:"invokes"`
+	Config   map[string]struct {
+		Type        string `json:"type"`
+		Description string `json:"description"`
+		Pattern     string `json:"pattern"`
+		Default     string `json:"default"`
+	} `json:"config"`
 	OwnSecrets   map[string]string `json:"own-secrets"`
 	SecretsOwner string            `json:"secrets-owner"`
 	Resources    []map[string]any  `json:"resources"`
@@ -123,16 +129,35 @@ func TestManifestProcess(t *testing.T) { // 1607/AC1
 		t.Errorf("process = %v", p)
 	}
 	env, _ := p["env"].(map[string]any)
-	// The build capacity of each node: how many build steps the builder runs on it at once (a node not named runs one). No
-	// part of delivery reads a per-node setting yet, so the value is the manifest's own; changing it is a change of this line.
-	if len(env) != 2 || env["MESH_BROKER_FILE"] != "${dir:state}/broker" || env["MESH_BUILDER_CAPACITY"] != "shanks=3,ace=2" {
+	// The build capacity of each node is the module's setting `capacity` (issue 1661, ADR 0478 clause 3), laid into the
+	// settings file the program reads; empty is one step at a time everywhere. The environment holds no capacity.
+	if len(env) != 2 || env["MESH_BROKER_FILE"] != "${dir:state}/broker" || env["MESH_BUILDER_SETTINGS"] != "${dir:state}/settings.json" {
 		t.Errorf("env = %v", p["env"])
 	}
+	if !reflect.DeepEqual(p["restart-on"], []any{"settings"}) {
+		t.Errorf("restart-on = %v, want the settings file, so a changed capacity reaches the running program", p["restart-on"])
+	}
+	c, ok := m.Config["capacity"]
+	if !ok || len(m.Config) != 1 || c.Type != "string" || c.Default != "shanks=3,ace=2,g14=0,novox=0" || c.Description == "" {
+		t.Errorf("config = %+v, want one string setting, capacity, shanks=3,ace=2,g14=0,novox=0 to start", m.Config)
+	}
+	var settings map[string]any
+	for _, r := range m.Resources {
+		if r["id"] == "settings" {
+			settings = r
+		}
+	}
+	if settings == nil || settings["type"] != "file" || settings["merge"] != "json" || !reflect.DeepEqual(settings["takes-config"], []any{"capacity"}) || settings["path"] != "${dir:state}/settings.json" {
+		t.Errorf("settings resource = %v", settings)
+	}
+	if raw, err := os.ReadFile("files/settings.json"); err != nil || !strings.Contains(string(raw), `"capacity": "shanks=3,ace=2,g14=0,novox=0"`) {
+		t.Errorf("files/settings.json = %q, %v, want the setting's starting value", raw, err)
+	}
 	if _, bus := p["bus"]; bus {
 		t.Errorf("the process declares a bus: it runs on its own broker credential")
 	}
-	if len(m.Build.Artifacts) != 1 {
-		t.Fatalf("artifacts = %+v", m.Build.Artifacts)
+	if len(m.Build.Artifacts) != 2 || m.Build.Artifacts[1].Name != "files" || m.Build.Artifacts[1].Kind != "archive" || m.Build.Artifacts[1].From != "files" {
+		t.Fatalf("artifacts = %+v, want the program and the files", m.Build.Artifacts)
 	}
 	a := m.Build.Artifacts[0]
 	if st, err := os.Stat(a.From); err != nil || !st.IsDir() || a.Binary != "mesh-delivery-builder" || !strings.HasSuffix(a.From, a.Binary) || a.Kind != "bundle" || a.Language != "go" {
diff --git a/modules/mesh-delivery-builder/module.json b/modules/mesh-delivery-builder/module.json
index 115de62..292c9ae 100644
--- a/modules/mesh-delivery-builder/module.json
+++ b/modules/mesh-delivery-builder/module.json
@@ -25,6 +25,14 @@
     "mesh-build-node.build_node_build",
     "mesh-build-node.build_node_fetch"
   ],
+  "config": {
+    "capacity": {
+      "type": "string",
+      "description": "How many build steps the builder runs at once on each node, as node=steps pairs separated by commas, such as shanks=3,ace=2 (ADR 0478 clause 3); a node set to 0 never builds, a node not named runs one at a time, and empty is one everywhere. The builder builds on shanks and ace only (the operator, 2026-10-21).",
+      "pattern": "|[a-z0-9_-]+=[0-9]+(,[a-z0-9_-]+=[0-9]+)*",
+      "default": "shanks=3,ace=2,g14=0,novox=0"
+    }
+  },
   "own-secrets": {
     "broker": "${dir:state}/broker"
   },
@@ -45,6 +53,17 @@
       "place": ".",
       "unknown": "program"
     },
+    {
+      "id": "settings",
+      "trusted": false,
+      "type": "file",
+      "path": "${dir:state}/settings.json",
+      "mode": "0600",
+      "owner": "mesh-delivery-builder",
+      "takes-config": ["capacity"],
+      "merge": "json",
+      "template": "files/settings.json"
+    },
     {
       "id": "builder",
       "type": "process",
@@ -55,8 +74,11 @@
       ],
       "env": {
         "MESH_BROKER_FILE": "${dir:state}/broker",
-        "MESH_BUILDER_CAPACITY": "shanks=3,ace=2"
+        "MESH_BUILDER_SETTINGS": "${dir:state}/settings.json"
       },
+      "restart-on": [
+        "settings"
+      ],
       "user": "mesh-delivery-builder",
       "health": {
         "kind": "unit"
@@ -72,6 +94,11 @@
         "system": "arch",
         "from": "cmd/mesh-delivery-builder",
         "binary": "mesh-delivery-builder"
+      },
+      {
+        "name": "files",
+        "kind": "archive",
+        "from": "files"
       }
     ]
   }