diff --git a/cmd/mesh-builder/main.go b/cmd/mesh-builder/main.go
index f7d6946..0eb8215 100644
--- a/cmd/mesh-builder/main.go
+++ b/cmd/mesh-builder/main.go
@@ -137,7 +137,12 @@ func takeWorkFrom(credential Credential, on string) (link.BuildMachine, error) {
 	if err != nil {
 		return nil, err
 	}
-	return link.MachineOverNATS(js, on), nil
+	// **The seat this machine serves is the one its credential claims** (novox/hq ADR 0190, the
+	// handover): the mesh issues a build machine's credential naming the seat its module claims,
+	// and one binary serves the old role as `builder` and the new as `build-agent` from that alone.
+	seat := link.BuildSeatClaimed(credential.seatsClaimed())
+	fmt.Fprintf(os.Stderr, "taking build work as a holder of %s\n", seat)
+	return link.MachineOverNATSOn(js, on, seat), nil
 }
 
 // answer does one build and says what happened, whichever way it went.
@@ -453,6 +458,21 @@ type Credential struct {
 	// as two fields and this machine joins them once, here, to dial.
 	User     string `json:"user,omitempty"`
 	Password string `json:"password,omitempty"`
+	// Claims are the seats the module this credential was issued for claims, as the mesh writes
+	// them beside the credential (novox/hq ADR 0159). The first is the build role this machine
+	// serves; a credential naming none is from before claims travelled in it.
+	Claims []struct {
+		Seat string `json:"seat"`
+	} `json:"claims,omitempty"`
+}
+
+// seatsClaimed is the seats the credential names, in order.
+func (c Credential) seatsClaimed() []string {
+	out := make([]string, 0, len(c.Claims))
+	for _, claim := range c.Claims {
+		out = append(out, claim.Seat)
+	}
+	return out
 }
 
 // onTheNewBus is whether a credential is for the bus being built: its address says so, and the
diff --git a/cmd/mesh-builder/seat_test.go b/cmd/mesh-builder/seat_test.go
new file mode 100644
index 0000000..8518646
--- /dev/null
+++ b/cmd/mesh-builder/seat_test.go
@@ -0,0 +1,27 @@
+package main
+
+import (
+	"encoding/json"
+	"testing"
+
+	"github.com/novox/mesh-controller/internal/link"
+)
+
+// The seat a build machine serves comes from its credential (novox/hq ADR 0190 handover).
+func TestTheCredentialSaysWhichBuildRoleThisMachineServes(t *testing.T) {
+	var held Credential
+	if err := json.Unmarshal([]byte(`{"url":"nats://bus:4222","user":"anchor.builder","password":"x",
+		"claims":[{"seat":"mesh-build-machine","scope":"mesh","serves":[]}]}`), &held); err != nil {
+		t.Fatal(err)
+	}
+	if got := link.BuildSeatClaimed(held.seatsClaimed()); got != "mesh-build-machine" {
+		t.Errorf("the old builder's credential serves %q", got)
+	}
+	var bare Credential
+	if err := json.Unmarshal([]byte(`{"url":"nats://bus:4222","user":"anchor.build-agent","password":"x"}`), &bare); err != nil {
+		t.Fatal(err)
+	}
+	if got := link.BuildSeatClaimed(bare.seatsClaimed()); got != link.TheBuildMachine {
+		t.Errorf("a credential without claims serves %q, want %s", got, link.TheBuildMachine)
+	}
+}
diff --git a/internal/link/build_seat_test.go b/internal/link/build_seat_test.go
new file mode 100644
index 0000000..4969798
--- /dev/null
+++ b/internal/link/build_seat_test.go
@@ -0,0 +1,32 @@
+package link
+
+import "testing"
+
+// A build machine serves the seat its credential claims (novox/hq ADR 0190 handover): the old
+// `builder` keeps the old role, a `build-agent` takes the new, from one binary and no flag.
+func TestABuildMachineServesTheSeatItsCredentialClaims(t *testing.T) {
+	if got := BuildSeatClaimed([]string{"mesh-build-machine"}); got != "mesh-build-machine" {
+		t.Errorf("a credential claiming the old role serves %q", got)
+	}
+	if got := BuildSeatClaimed([]string{"node-build-agent"}); got != TheBuildMachine {
+		t.Errorf("a credential claiming the new role serves %q", got)
+	}
+	if got := BuildSeatClaimed(nil); got != TheBuildMachine {
+		t.Errorf("a credential claiming nothing serves %q, want the current role", got)
+	}
+	if got := BuildSeatClaimed([]string{"", "node-build-agent"}); got != TheBuildMachine {
+		t.Errorf("an empty claim is skipped; got %q", got)
+	}
+}
+
+// What a machine says about a build is the event of the seat it took the build from, so an outcome
+// is heard where the asker of that seat listens.
+func TestABuildsEventsAreItsSeats(t *testing.T) {
+	if BuildOutcomeOf(TheBuildMachineBefore) != "mesh.seat.mesh-build-machine.event.built" {
+		t.Error(BuildOutcomeOf(TheBuildMachineBefore))
+	}
+	if BuildWorkOf(TheBuildMachine) != BuildWork() || BuildOutcomeOf(TheBuildMachine) != BuildOutcome() ||
+		BuildStartedOf(TheBuildMachine) != BuildStarted() || BuildLogOf(TheBuildMachine, "b1") != BuildLog("b1") {
+		t.Error("the no-argument forms must name the current role")
+	}
+}
diff --git a/internal/link/builds.go b/internal/link/builds.go
index 2f0b599..f43cf5b 100644
--- a/internal/link/builds.go
+++ b/internal/link/builds.go
@@ -31,10 +31,36 @@ import (
 // build-agent to the machines that build; unassign builder and forget it and its seat's stream.
 const TheBuildMachine = "node-build-agent"
 
+// TheBuildMachineBefore is the role a build was submitted to until ADR 0190: the mesh's one build
+// machine, mesh-scoped. Kept named while the handover runs — a machine whose credential claims it
+// still serves it, and the controller still hears its outcomes — and dropped with the retired seat
+// row once nothing claims it.
+const TheBuildMachineBefore = "mesh-build-machine"
+
+// BuildSeatClaimed is the build role a machine serves: the first seat its credential claims, or the
+// current role when the credential names none (a credential from before claims travelled in it, or
+// one written by hand). **The credential decides, not the binary** (ADR 0190 handover): one build
+// machine binary runs as the old `builder` on the old seat and as a `build-agent` on the new one,
+// each taking the work the mesh issued it a credential for, so neither drains the other's queue
+// and the switch needs no flag day.
+func BuildSeatClaimed(claimed []string) string {
+	for _, seat := range claimed {
+		if seat != "" {
+			return seat
+		}
+	}
+	return TheBuildMachine
+}
+
 // BuildWork is where a build request lands, and BuildOutcome is where its result does. Derived from
-// the seat, so both sides name the role and neither names the other.
-func BuildWork() string    { return "mesh.seat." + TheBuildMachine + ".accept.build" }
-func BuildOutcome() string { return "mesh.seat." + TheBuildMachine + ".event.built" }
+// the seat, so both sides name the role and neither names the other. The no-argument forms name the
+// current role; the `Of` forms take the seat, for the handover during which two roles exist.
+func BuildWork() string              { return BuildWorkOf(TheBuildMachine) }
+func BuildOutcome() string           { return BuildOutcomeOf(TheBuildMachine) }
+func BuildWorkOf(seat string) string { return "mesh.seat." + seat + ".accept.build" }
+func BuildOutcomeOf(seat string) string {
+	return "mesh.seat." + seat + ".event.built"
+}
 
 // BuildStarted is where a build machine says it has taken a build, and BuildLog is where it says
 // what it is doing, one line per message, under the build's own id (novox/hq ADR 0157).
@@ -44,8 +70,10 @@ func BuildOutcome() string { return "mesh.seat." + TheBuildMachine + ".event.bui
 // lived in one container's stderr on one machine. Every line is now an event of the role, retained
 // with the rest of the mesh's events, so a reader follows a build live by subscribing its subject,
 // or reads it back afterwards from the stream, and a viewer is a subscriber and nothing more.
-func BuildStarted() string      { return "mesh.seat." + TheBuildMachine + ".event.started" }
-func BuildLog(id string) string { return "mesh.seat." + TheBuildMachine + ".event.log." + id }
+func BuildStarted() string              { return BuildStartedOf(TheBuildMachine) }
+func BuildLog(id string) string         { return BuildLogOf(TheBuildMachine, id) }
+func BuildStartedOf(seat string) string { return "mesh.seat." + seat + ".event.started" }
+func BuildLogOf(seat, id string) string { return "mesh.seat." + seat + ".event.log." + id }
 
 // BuildStart is what a build machine says the moment it takes a build.
 type BuildStart struct {
diff --git a/internal/link/builds_nats.go b/internal/link/builds_nats.go
index 0355857..9e158b6 100644
--- a/internal/link/builds_nats.go
+++ b/internal/link/builds_nats.go
@@ -115,9 +115,16 @@ type natsMachine struct {
 	sub  *nats.Subscription
 }
 
-// MachineOverNATS takes build work from the role this machine holds.
+// MachineOverNATS takes build work from the current build role.
 func MachineOverNATS(js *broker.JetStream, on string) BuildMachine {
-	return &natsMachine{js: js, on: on, seat: TheBuildMachine}
+	return MachineOverNATSOn(js, on, TheBuildMachine)
+}
+
+// MachineOverNATSOn takes build work from the role named — the one this machine's credential claims
+// (ADR 0190 handover): its asks come from that seat's worker, and what it says about a build goes
+// out as that seat's events, so an outcome is heard where the asker listens.
+func MachineOverNATSOn(js *broker.JetStream, on, seat string) BuildMachine {
+	return &natsMachine{js: js, on: on, seat: seat}
 }
 
 func (m *natsMachine) Close() {
@@ -194,7 +201,7 @@ func (m *natsMachine) Take(ctx context.Context, do func(context.Context, Build))
 			// the ask to a second machine nor counts the wait against its deliveries.
 			working := make(chan struct{})
 			go stillWorking(msg, working)
-			do(ctx, &natsBuild{request: request, msg: msg, on: m.on, js: m.js})
+			do(ctx, &natsBuild{request: request, msg: msg, on: m.on, js: m.js, seat: m.seat})
 			close(working)
 		}
 	}
@@ -205,7 +212,9 @@ type natsBuild struct {
 	msg     *nats.Msg
 	on      string
 	js      *broker.JetStream
-	seq     int
+	// seat is the role this build was taken from; what the machine says about it is that role's.
+	seat string
+	seq  int
 }
 
 func (b *natsBuild) Request() BuildRequest { return b.request }
@@ -232,7 +241,7 @@ func (b *natsBuild) Announce(ctx context.Context, result BuildResult) error {
 	}
 	publish, cancel := context.WithTimeout(ctx, 30*time.Second)
 	defer cancel()
-	if _, err := b.js.Context().Publish(BuildOutcome(), body, nats.Context(publish)); err != nil {
+	if _, err := b.js.Context().Publish(BuildOutcomeOf(b.seat), body, nats.Context(publish)); err != nil {
 		return fmt.Errorf("cannot announce a build's outcome: %w", err)
 	}
 	return nil
@@ -252,7 +261,7 @@ func (b *natsBuild) Began(ctx context.Context) error {
 	}
 	publish, cancel := context.WithTimeout(ctx, 30*time.Second)
 	defer cancel()
-	if _, err := b.js.Context().Publish(BuildStarted(), body, nats.Context(publish)); err != nil {
+	if _, err := b.js.Context().Publish(BuildStartedOf(b.seat), body, nats.Context(publish)); err != nil {
 		return fmt.Errorf("cannot say a build started: %w", err)
 	}
 	return nil
@@ -270,7 +279,7 @@ func (b *natsBuild) Say(step, message string) {
 	if err != nil {
 		return
 	}
-	_ = b.js.Conn().Publish(BuildLog(b.request.ID), body)
+	_ = b.js.Conn().Publish(BuildLogOf(b.seat, b.request.ID), body)
 }
 
 func (b *natsBuild) Hold(after time.Duration) error { return b.msg.NakWithDelay(after) }