diff --git a/cmd/client/color.go b/cmd/client/color.go
index 71384a1..0bbdd07 100644
--- a/cmd/client/color.go
+++ b/cmd/client/color.go
@@ -3,6 +3,7 @@
 package main
 
 import (
+	"encoding/json"
 	"strings"
 
 	cli "git.ipng.ch/ipng/golang-cli"
@@ -10,10 +11,10 @@ import (
 
 // formatError returns a user-friendly error string. gRPC status errors are
 // unwrapped to show only the server's message (no "rpc error: code = ..."
-// boilerplate). The result is wrapped in red when color is enabled. The label()
-// helper and the ANSI palette now live in the golang-cli library (cli.Label,
-// cli.Red, ...); only this gRPC-specific unwrap stays here, wired in as
-// App.FormatError.
+// boilerplate). In JSON mode it returns the message as a {"error": "..."}
+// document (so failures are machine-readable, matching the data/`{}` a success
+// prints); otherwise it wraps the message in red when color is enabled.
+// App.FormatError prints the result.
 func formatError(err error) string {
 	msg := err.Error()
 	// google.golang.org/grpc/status errors format as:
@@ -21,6 +22,10 @@ func formatError(err error) string {
 	if i := strings.Index(msg, " desc = "); i >= 0 {
 		msg = msg[i+len(" desc = "):]
 	}
+	if cli.IsJSON() {
+		b, _ := json.Marshal(map[string]string{"error": msg})
+		return string(b)
+	}
 	if cli.ColorEnabled() {
 		return cli.Red + msg + cli.Reset
 	}
diff --git a/cmd/client/commands.go b/cmd/client/commands.go
index 2e55ced..9ee12e2 100644
--- a/cmd/client/commands.go
+++ b/cmd/client/commands.go
@@ -230,6 +230,9 @@ func buildTree() *node {
 	}
 
 	root.Children = []*node{show, set, watch, configNode, syncNode, quit, exit}
+	// In -json mode, make every silent successful command print "{}" (see
+	// wrapJSON); show/return-data commands emit their own JSON first.
+	wrapJSON(root)
 	return root
 }
 
@@ -272,6 +275,9 @@ func runShowVPPInfo(ctx context.Context, client grpcapi.MaglevClient, _ []string
 	if err != nil {
 		return err
 	}
+	if cli.IsJSON() {
+		return emitProto(info)
+	}
 	w := tabwriter.NewWriter(os.Stdout, 0, 0, 2, ' ', 0)
 	_, _ = fmt.Fprintf(w, "%s\t%s\n", cli.Label("version"), info.Version)
 	_, _ = fmt.Fprintf(w, "%s\t%s\n", cli.Label("build-date"), info.BuildDate)
@@ -297,6 +303,9 @@ func runShowVPPLBState(ctx context.Context, client grpcapi.MaglevClient, _ []str
 	if err != nil {
 		return err
 	}
+	if cli.IsJSON() {
+		return emitProto(state)
+	}
 
 	// ---- global config ----
 	w := tabwriter.NewWriter(os.Stdout, 0, 0, 2, ' ', 0)
@@ -354,6 +363,9 @@ func runShowVPPLBCounters(ctx context.Context, client grpcapi.MaglevClient, _ []
 	if err != nil {
 		return err
 	}
+	if cli.IsJSON() {
+		return emitProto(resp)
+	}
 
 	if len(resp.Vips) == 0 {
 		fmt.Println("(no counters — VPP disconnected or scrape pending)")
@@ -417,18 +429,18 @@ func runSyncVPPLBState(ctx context.Context, client grpcapi.MaglevClient, args []
 		name := args[0]
 		req.FrontendName = &name
 	}
-	if _, err := client.SyncVPPLBState(ctx, req); err != nil {
-		return err
-	}
-	if req.FrontendName != nil {
-		fmt.Printf("synced frontend %q to VPP\n", *req.FrontendName)
-	} else {
-		fmt.Println("synced full LB state to VPP")
-	}
-	return nil
+	_, err := client.SyncVPPLBState(ctx, req)
+	return err
 }
 
 func runShowVersion(_ context.Context, _ grpcapi.MaglevClient, _ []string) error {
+	if cli.IsJSON() {
+		return emitJSON(map[string]string{
+			"version": buildinfo.Version(),
+			"commit":  buildinfo.Commit(),
+			"built":   buildinfo.Date(),
+		})
+	}
 	fmt.Printf("maglevc %s (commit %s, built %s)\n",
 		buildinfo.Version(), buildinfo.Commit(), buildinfo.Date())
 	return nil
@@ -445,6 +457,9 @@ func runShowFrontends(ctx context.Context, client grpcapi.MaglevClient, _ []stri
 	if err != nil {
 		return err
 	}
+	if cli.IsJSON() {
+		return emitProto(resp)
+	}
 	for _, name := range resp.FrontendNames {
 		fmt.Println(name)
 	}
@@ -461,6 +476,9 @@ func runShowFrontend(ctx context.Context, client grpcapi.MaglevClient, args []st
 	if err != nil {
 		return err
 	}
+	if cli.IsJSON() {
+		return emitProto(info)
+	}
 
 	w := tabwriter.NewWriter(os.Stdout, 0, 0, 2, ' ', 0)
 	_, _ = fmt.Fprintf(w, "%s\t%s\n", cli.Label("name"), info.Name)
@@ -522,6 +540,9 @@ func runShowBackends(ctx context.Context, client grpcapi.MaglevClient, _ []strin
 	if err != nil {
 		return err
 	}
+	if cli.IsJSON() {
+		return emitProto(resp)
+	}
 	for _, name := range resp.BackendNames {
 		fmt.Println(name)
 	}
@@ -538,6 +559,9 @@ func runShowBackend(ctx context.Context, client grpcapi.MaglevClient, args []str
 	if err != nil {
 		return err
 	}
+	if cli.IsJSON() {
+		return emitProto(info)
+	}
 	w := tabwriter.NewWriter(os.Stdout, 0, 0, 2, ' ', 0)
 	_, _ = fmt.Fprintf(w, "%s\t%s\n", cli.Label("name"), info.Name)
 	_, _ = fmt.Fprintf(w, "%s\t%s\n", cli.Label("address"), info.Address)
@@ -577,6 +601,9 @@ func runShowHealthChecks(ctx context.Context, client grpcapi.MaglevClient, _ []s
 	if err != nil {
 		return err
 	}
+	if cli.IsJSON() {
+		return emitProto(resp)
+	}
 	for _, name := range resp.Names {
 		fmt.Println(name)
 	}
@@ -593,6 +620,9 @@ func runShowHealthCheck(ctx context.Context, client grpcapi.MaglevClient, args [
 	if err != nil {
 		return err
 	}
+	if cli.IsJSON() {
+		return emitProto(info)
+	}
 	w := tabwriter.NewWriter(os.Stdout, 0, 0, 2, ' ', 0)
 	_, _ = fmt.Fprintf(w, "%s\t%s\n", cli.Label("name"), info.Name)
 	_, _ = fmt.Fprintf(w, "%s\t%s\n", cli.Label("type"), info.Type)
@@ -640,12 +670,8 @@ func runPauseBackend(ctx context.Context, client grpcapi.MaglevClient, args []st
 	}
 	ctx, cancel := context.WithTimeout(ctx, callTimeout)
 	defer cancel()
-	info, err := client.PauseBackend(ctx, &grpcapi.BackendRequest{Name: args[0]})
-	if err != nil {
-		return err
-	}
-	fmt.Printf("%s: setting state to '%s'\n", info.Name, info.State)
-	return nil
+	_, err := client.PauseBackend(ctx, &grpcapi.BackendRequest{Name: args[0]})
+	return err
 }
 
 func runResumeBackend(ctx context.Context, client grpcapi.MaglevClient, args []string) error {
@@ -654,12 +680,8 @@ func runResumeBackend(ctx context.Context, client grpcapi.MaglevClient, args []s
 	}
 	ctx, cancel := context.WithTimeout(ctx, callTimeout)
 	defer cancel()
-	info, err := client.ResumeBackend(ctx, &grpcapi.BackendRequest{Name: args[0]})
-	if err != nil {
-		return err
-	}
-	fmt.Printf("%s: setting state to '%s'\n", info.Name, info.State)
-	return nil
+	_, err := client.ResumeBackend(ctx, &grpcapi.BackendRequest{Name: args[0]})
+	return err
 }
 
 func runSetFrontendPoolBackendWeight(ctx context.Context, client grpcapi.MaglevClient, args []string) error {
@@ -681,34 +703,14 @@ func setFrontendPoolBackendWeight(ctx context.Context, client grpcapi.MaglevClie
 	}
 	ctx, cancel := context.WithTimeout(ctx, callTimeout)
 	defer cancel()
-	info, err := client.SetFrontendPoolBackendWeight(ctx, &grpcapi.SetWeightRequest{
+	_, err = client.SetFrontendPoolBackendWeight(ctx, &grpcapi.SetWeightRequest{
 		Frontend: frontendName,
 		Pool:     poolName,
 		Backend:  backendName,
 		Weight:   int32(weight),
 		Flush:    flush,
 	})
-	if err != nil {
-		return err
-	}
-	// Print the updated pool so the user can confirm the new weight.
-	for _, pool := range info.Pools {
-		if pool.Name != poolName {
-			continue
-		}
-		for _, pb := range pool.Backends {
-			if pb.Name == backendName {
-				flushNote := ""
-				if flush {
-					flushNote = " (flushed)"
-				}
-				fmt.Printf("%s pool %s backend %s: weight set to %d%s\n",
-					info.Name, pool.Name, pb.Name, pb.Weight, flushNote)
-				return nil
-			}
-		}
-	}
-	return nil
+	return err
 }
 
 func runEnableBackend(ctx context.Context, client grpcapi.MaglevClient, args []string) error {
@@ -717,12 +719,8 @@ func runEnableBackend(ctx context.Context, client grpcapi.MaglevClient, args []s
 	}
 	ctx, cancel := context.WithTimeout(ctx, callTimeout)
 	defer cancel()
-	info, err := client.EnableBackend(ctx, &grpcapi.BackendRequest{Name: args[0]})
-	if err != nil {
-		return err
-	}
-	fmt.Printf("%s: enabled, state is '%s'\n", info.Name, info.State)
-	return nil
+	_, err := client.EnableBackend(ctx, &grpcapi.BackendRequest{Name: args[0]})
+	return err
 }
 
 func runDisableBackend(ctx context.Context, client grpcapi.MaglevClient, args []string) error {
@@ -731,12 +729,8 @@ func runDisableBackend(ctx context.Context, client grpcapi.MaglevClient, args []
 	}
 	ctx, cancel := context.WithTimeout(ctx, callTimeout)
 	defer cancel()
-	info, err := client.DisableBackend(ctx, &grpcapi.BackendRequest{Name: args[0]})
-	if err != nil {
-		return err
-	}
-	fmt.Printf("%s: disabled, state is '%s'\n", info.Name, info.State)
-	return nil
+	_, err := client.DisableBackend(ctx, &grpcapi.BackendRequest{Name: args[0]})
+	return err
 }
 
 func runConfigCheck(ctx context.Context, client grpcapi.MaglevClient, _ []string) error {
@@ -746,6 +740,9 @@ func runConfigCheck(ctx context.Context, client grpcapi.MaglevClient, _ []string
 	if err != nil {
 		return err
 	}
+	if cli.IsJSON() {
+		return emitProto(resp)
+	}
 	if resp.Ok {
 		fmt.Println("config ok")
 		return nil
@@ -764,7 +761,6 @@ func runConfigReload(ctx context.Context, client grpcapi.MaglevClient, _ []strin
 		return err
 	}
 	if resp.Ok {
-		fmt.Println("config reloaded")
 		return nil
 	}
 	if resp.ParseError != "" {
diff --git a/cmd/client/json.go b/cmd/client/json.go
new file mode 100644
index 0000000..659793e
--- /dev/null
+++ b/cmd/client/json.go
@@ -0,0 +1,86 @@
+// SPDX-License-Identifier: Apache-2.0
+
+package main
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+
+	"google.golang.org/protobuf/encoding/protojson"
+	"google.golang.org/protobuf/proto"
+
+	cli "git.ipng.ch/ipng/golang-cli"
+	"git.ipng.ch/ipng/vpp-maglev/internal/grpcapi"
+)
+
+// JSON output (-json). The protobuf response is the model: in JSON mode each
+// command emits it via the shared cli.EmitJSON (protojson field names/enums); in
+// text mode it keeps its curated tabwriter painter (which robot tests parse, and
+// which carries column alignment JSON does not need). A few commands print only
+// a confirmation in text mode — those emit the returned proto in JSON, or, when
+// there is nothing to return, rely on wrapJSON to print "{}". Errors render as
+// {"error": "..."} (see color.go).
+
+// jsonEmitted records whether the current command produced JSON output of its
+// own. Commands run serially, so a package var is safe. wrapJSON resets it
+// before each command and appends "{}" for a successful command that produced
+// nothing.
+var jsonEmitted bool
+
+// emitProto emits one proto message as JSON via the shared renderer.
+func emitProto(m proto.Message) error {
+	raw, err := protoField(m)
+	if err != nil {
+		return err
+	}
+	jsonEmitted = true
+	return cli.EmitJSON(raw)
+}
+
+// emitJSON emits a composite value (maps/structs) as JSON.
+func emitJSON(v any) error {
+	jsonEmitted = true
+	return cli.EmitJSON(v)
+}
+
+// protoField marshals a proto message to compact JSON (proto field names and
+// enum spellings).
+func protoField(m proto.Message) (json.RawMessage, error) {
+	b, err := protojson.Marshal(m)
+	if err != nil {
+		return nil, fmt.Errorf("marshal json: %w", err)
+	}
+	return json.RawMessage(b), nil
+}
+
+// wrapJSON decorates every runnable node so that, in -json mode, a command that
+// succeeds without emitting output of its own (e.g. sync) still produces "{}",
+// giving every command a uniform JSON contract. Commands that show or return
+// data emit it (setting jsonEmitted); errors propagate to App.FormatError, which
+// renders them as {"error": "..."}. In text mode the wrapper is a no-op.
+func wrapJSON(root *node) {
+	seen := map[*node]bool{}
+	var walk func(*node)
+	walk = func(n *node) {
+		if n == nil || seen[n] {
+			return
+		}
+		seen[n] = true
+		if n.Run != nil {
+			orig := n.Run
+			n.Run = func(ctx context.Context, c grpcapi.MaglevClient, a []string) error {
+				jsonEmitted = false
+				err := orig(ctx, c, a)
+				if err == nil && cli.IsJSON() && !jsonEmitted {
+					return emitJSON(struct{}{})
+				}
+				return err
+			}
+		}
+		for _, ch := range n.Children {
+			walk(ch)
+		}
+	}
+	walk(root)
+}
diff --git a/cmd/client/main.go b/cmd/client/main.go
index 1894e8b..8930159 100644
--- a/cmd/client/main.go
+++ b/cmd/client/main.go
@@ -33,6 +33,7 @@ func main() {
 		Date:          buildinfo.Date(),
 		Prompt:        "maglev> ",
 		Root:          buildTree(),
+		JSON:          true, // commands render via cli.IsJSON() (see json.go)
 		DefaultServer: "localhost:9090",
 		ServerEnv:     "MAGLEV_SERVER",
 		Connect:       connect,
diff --git a/cmd/client/watch.go b/cmd/client/watch.go
index b0055ad..17b45e5 100644
--- a/cmd/client/watch.go
+++ b/cmd/client/watch.go
@@ -26,7 +26,8 @@ func dynWatchEventOpts(_ context.Context, _ grpcapi.MaglevClient, _ []string) []
 // All tokens after 'events' are captured as args by the circular slot node in the tree.
 // If none of log/backend/frontend are mentioned, all three default to true.
 func runWatchEvents(ctx context.Context, client grpcapi.MaglevClient, args []string) error {
-	var maxEvents int // 0 = unlimited
+	jsonEmitted = true // watch streams its own JSON event lines; never append "{}"
+	var maxEvents int  // 0 = unlimited
 	var wantLog, wantBackend, wantFrontend bool
 	logLevel := ""
 	anyExplicit := false
diff --git a/go.mod b/go.mod
index 70e0841..81d5796 100644
--- a/go.mod
+++ b/go.mod
@@ -3,7 +3,7 @@ module git.ipng.ch/ipng/vpp-maglev
 go 1.25.0
 
 require (
-	git.ipng.ch/ipng/golang-cli v1.3.0
+	git.ipng.ch/ipng/golang-cli v1.4.0
 	github.com/charmbracelet/bubbletea v1.3.10
 	github.com/charmbracelet/lipgloss v1.1.0
 	github.com/grpc-ecosystem/go-grpc-middleware/providers/prometheus v1.1.0
diff --git a/go.sum b/go.sum
index d9fd393..b8054fe 100644
--- a/go.sum
+++ b/go.sum
@@ -1,5 +1,7 @@
 git.ipng.ch/ipng/golang-cli v1.3.0 h1:E26W55czJSl+kmSXpwFWxYHsnaT84unsNBtfdM3iT4M=
 git.ipng.ch/ipng/golang-cli v1.3.0/go.mod h1:8yeV4X7MF5hKQnxnYYJKyqby4P58EtH82zd36BrNbqY=
+git.ipng.ch/ipng/golang-cli v1.4.0 h1:KpvVqkieYoH3en7ay0QGmagiYcXJoUAiU49OLx3eZGw=
+git.ipng.ch/ipng/golang-cli v1.4.0/go.mod h1:8yeV4X7MF5hKQnxnYYJKyqby4P58EtH82zd36BrNbqY=
 github.com/aymanbagabas/go-osc52/v2 v2.0.1 h1:HwpRHbFMcZLEVr42D4p7XBqjyuxQH5SMiErDT4WkJ2k=
 github.com/aymanbagabas/go-osc52/v2 v2.0.1/go.mod h1:uYgXzlJ7ZpABp8OJ+exZzJJhRNQ2ASbcXHWsFqH8hp8=
 github.com/beorn7/perks v1.0.1 h1:VlbKKnNfV8bJzeqoa4cOKqO6bYr3WgKZxO8Z16+hsOM=