diff --git a/go/nix/bnix/bnix.go b/go/nix/bnix/bnix.go
index 7542daa5a5..4a33ddfe86 100644
--- a/go/nix/bnix/bnix.go
+++ b/go/nix/bnix/bnix.go
@@ -2,26 +2,356 @@ package main
 
 import (
 	"context"
+	"errors"
 	"flag"
 	"fmt"
 	"io"
 	"log"
+	"math/rand"
 	"net/url"
 	"os"
+	"sync"
+	"time"
 
 	"golang.org/x/crypto/ssh"
+	"hg.lukegb.com/lukegb/depot/go/nix/nar/narinfo"
 	"hg.lukegb.com/lukegb/depot/go/nix/nixdrv"
 	"hg.lukegb.com/lukegb/depot/go/nix/nixstore"
 )
 
 var (
-	remoteFlag = flag.String("remote", "unix://", "Remote.")
+	remoteFlag   = flag.String("remote", "unix://", "Remote.")
+	remote2Flag  = flag.String("remote2", "unix://", "Remote. But the second one. The destination, for copy.")
+	maxConnsFlag = flag.Int("max_conns", 10, "Max connections.")
+
+	verboseFlag = flag.Bool("verbose", false, "Verbose.")
 )
 
 func ensure(ctx context.Context, d *nixstore.Daemon, at *nixstore.ActivityTracker, path string) error {
 	return d.EnsurePath(at, path)
 }
 
+func narInfo(ctx context.Context, d *nixstore.Daemon, at *nixstore.ActivityTracker, path string) error {
+	ni, err := d.NARInfo(path)
+	if err != nil {
+		return err
+	}
+	fmt.Printf("%s\n", ni)
+	return nil
+}
+
+func tryEnsure(ctx context.Context, d *nixstore.Daemon, at *nixstore.ActivityTracker, path string) (bool, error) {
+	err := d.EnsurePath(at, path)
+	if err == nil {
+		return true, nil
+	}
+	var ne nixstore.NixError
+	if errors.As(err, &ne) {
+		if ne.Code == 1 {
+			return false, nil
+		}
+	}
+	return false, err
+}
+
+type connectionPool struct {
+	factory func() (*nixstore.Daemon, error)
+
+	cond      *sync.Cond
+	limit     int
+	generated int
+	available []*nixstore.Daemon
+}
+
+func newConnectionPool(ctx context.Context, dst string, limit int) *connectionPool {
+	return &connectionPool{
+		factory: func() (*nixstore.Daemon, error) {
+			var d *nixstore.Daemon
+			var err error
+			retryInterval := 10 * time.Millisecond
+			maxRetry := 10 * time.Second
+			for n := 0; n < 20; n++ {
+				d, _, err = connectTo(ctx, dst)
+				if err == nil {
+					break
+				}
+				time.Sleep(retryInterval + time.Duration((rand.Float64()-0.5)*float64(retryInterval)))
+				retryInterval = retryInterval * 2
+				if retryInterval > maxRetry {
+					retryInterval = maxRetry
+				}
+			}
+			return d, err
+		},
+
+		cond:  sync.NewCond(new(sync.Mutex)),
+		limit: limit,
+	}
+}
+
+func (p *connectionPool) getFromAvailableLocked() *nixstore.Daemon {
+	end := len(p.available) - 1
+	d := p.available[end]
+	p.available = p.available[:end]
+	return d
+}
+
+func (p *connectionPool) Get() (*nixstore.Daemon, error) {
+	p.cond.L.Lock()
+	if len(p.available) > 0 {
+		d := p.getFromAvailableLocked()
+		p.cond.L.Unlock()
+		return d, nil
+	}
+
+	if p.generated < p.limit {
+		p.generated++
+		p.cond.L.Unlock()
+		d, err := p.factory()
+		if err != nil {
+			p.cond.L.Lock()
+			p.generated--
+			p.cond.L.Unlock()
+			return nil, err
+		}
+		return d, nil
+	}
+
+	for len(p.available) == 0 {
+		p.cond.Wait()
+	}
+	d := p.getFromAvailableLocked()
+	p.cond.L.Unlock()
+	return d, nil
+}
+
+func (p *connectionPool) Put(d *nixstore.Daemon) {
+	p.cond.L.Lock()
+	p.available = append(p.available, d)
+	p.cond.Signal()
+	p.cond.L.Unlock()
+}
+
+type workState int
+
+const (
+	StateCheckingAlreadyExists workState = iota
+	StateFetchingPathInfo
+	StateEnqueueingReferences
+	StateAwaitingReferences
+	StateCopying
+	StateDone
+	StateFailed
+)
+
+func (s workState) String() string {
+	return map[workState]string{
+		StateCheckingAlreadyExists: "checking already exists",
+		StateFetchingPathInfo:      "fetching path info",
+		StateEnqueueingReferences:  "enqueueing references",
+		StateAwaitingReferences:    "awaiting references",
+		StateCopying:               "copying",
+		StateDone:                  "done!",
+		StateFailed:                "failed :(",
+	}[s]
+}
+
+func (s workState) Terminal() bool { return s == StateDone || s == StateFailed }
+
+type workItem struct {
+	doneCh chan struct{}
+	state  workState
+	path   string
+
+	err       error
+	narInfo   *narinfo.NarInfo
+	childWork []*workItem
+
+	srcPool *connectionPool
+	srcAT   *nixstore.ActivityTracker
+	dstPool *connectionPool
+	dstAT   *nixstore.ActivityTracker
+
+	wh *workHolder
+}
+
+func (i *workItem) checkAlreadyExists(ctx context.Context) (workState, error) {
+	// i.state == StateCheckingAlreadyExists
+	dst, err := i.dstPool.Get()
+	if err != nil {
+		return StateFailed, err
+	}
+	defer i.dstPool.Put(dst)
+
+	ok, err := tryEnsure(ctx, dst, i.dstAT, i.path)
+	if err != nil {
+		return StateFailed, err
+	}
+
+	if ok {
+		return StateDone, nil
+	}
+
+	return StateFetchingPathInfo, nil
+}
+
+func (i *workItem) fetchPathInfo(ctx context.Context) (workState, error) {
+	// i.state == StateFetchingPathInfo
+	src, err := i.srcPool.Get()
+	if err != nil {
+		return StateFailed, err
+	}
+	defer i.srcPool.Put(src)
+
+	ni, err := src.NARInfo(i.path)
+	if err != nil {
+		return StateFailed, err
+	}
+
+	i.narInfo = ni
+	return StateEnqueueingReferences, nil
+}
+
+func (i *workItem) enqueueReferences(ctx context.Context) (workState, error) {
+	// i.state == StateEnqueueingReferences
+	const nixStorePrefix = "/nix/store/"
+	var childWork []*workItem
+	for _, ref := range i.narInfo.References {
+		refPath := nixStorePrefix + ref
+		if i.path == refPath {
+			continue
+		}
+
+		childWork = append(childWork, i.wh.addWork(ctx, refPath))
+	}
+	i.childWork = childWork
+	if len(i.childWork) == 0 {
+		return StateCopying, nil
+	}
+	return StateAwaitingReferences, nil
+}
+
+func (i *workItem) awaitReferences(ctx context.Context) (workState, error) {
+	// i.state == StateAwaitingReferences
+	for _, ci := range i.childWork {
+		select {
+		case <-ctx.Done():
+			return StateFailed, ctx.Err()
+		case <-ci.doneCh:
+			if ci.state == StateFailed {
+				return StateFailed, fmt.Errorf("reference %v failed", ci.path)
+			}
+			if !ci.state.Terminal() {
+				panic(fmt.Sprintf("%v: reference %v declared done in non-terminal state %v", i.path, ci.path, ci.state))
+			}
+		}
+	}
+	return StateCopying, nil
+}
+
+func (i *workItem) performCopy(ctx context.Context) (workState, error) {
+	// i.state == StateCopying
+	src, err := i.srcPool.Get()
+	if err != nil {
+		return StateFailed, err
+	}
+	defer i.srcPool.Put(src)
+
+	dst, err := i.dstPool.Get()
+	if err != nil {
+		return StateFailed, err
+	}
+	defer i.dstPool.Put(dst)
+
+	// TODO: the rest of the owl
+
+	return StateDone, nil
+}
+
+func (i *workItem) run(ctx context.Context) error {
+	defer close(i.doneCh)
+	i.state = StateCheckingAlreadyExists
+	for !i.state.Terminal() {
+		if ctx.Err() != nil {
+			return ctx.Err()
+		}
+
+		log.Printf("%s: running worker for %s", i.path, i.state)
+		var nextState workState
+		var err error
+		switch i.state {
+		case StateCheckingAlreadyExists:
+			nextState, err = i.checkAlreadyExists(ctx)
+		case StateFetchingPathInfo:
+			nextState, err = i.fetchPathInfo(ctx)
+		case StateEnqueueingReferences:
+			nextState, err = i.enqueueReferences(ctx)
+		case StateAwaitingReferences:
+			nextState, err = i.awaitReferences(ctx)
+		case StateCopying:
+			nextState, err = i.performCopy(ctx)
+		default:
+			log.Printf("%s: ended up in unimplemented state %s", i.path, i.state)
+			<-make(chan struct{})
+		}
+		i.state = nextState
+		if err != nil {
+			log.Printf("%s: transitioning to %s: %s", i.path, nextState, err)
+			i.err = err
+			return fmt.Errorf("%s: %w", i.state, err)
+		}
+		log.Printf("%s: transitioning to %s", i.path, nextState)
+	}
+	return nil
+}
+
+type workHolder struct {
+	mu   sync.Mutex
+	work map[string]*workItem
+
+	srcPool *connectionPool
+	srcAT   *nixstore.ActivityTracker
+	dstPool *connectionPool
+	dstAT   *nixstore.ActivityTracker
+}
+
+func (wh *workHolder) addWork(ctx context.Context, path string) *workItem {
+	wh.mu.Lock()
+	defer wh.mu.Unlock()
+
+	if i := wh.work[path]; i != nil {
+		return i
+	}
+
+	i := &workItem{
+		doneCh: make(chan struct{}),
+		path:   path,
+
+		srcPool: wh.srcPool,
+		srcAT:   wh.srcAT,
+		dstPool: wh.dstPool,
+		dstAT:   wh.dstAT,
+
+		wh: wh,
+	}
+	wh.work[path] = i
+	go i.run(ctx)
+	return i
+}
+
+func copyPath(ctx context.Context, poolFrom *connectionPool, atFrom *nixstore.ActivityTracker, poolTo *connectionPool, atTo *nixstore.ActivityTracker, path string) error {
+	wh := workHolder{
+		work:    make(map[string]*workItem),
+		srcPool: poolFrom,
+		srcAT:   atFrom,
+		dstPool: poolTo,
+		dstAT:   atTo,
+	}
+	i := wh.addWork(ctx, path)
+	<-i.doneCh
+	return i.err
+}
+
 var rs nixdrv.Resolver
 
 func loadDerivation(ctx context.Context, path string) (*nixdrv.Derivation, error) {
@@ -159,6 +489,8 @@ func connectTo(ctx context.Context, remote string) (*nixstore.Daemon, nixdrv.Res
 func main() {
 	flag.Parse()
 
+	nixstore.VerboseActivities = *verboseFlag
+
 	badCall := func(f string, xs ...interface{}) {
 		fmt.Fprintf(os.Stderr, f+"\n", xs...)
 		flag.Usage()
@@ -169,6 +501,7 @@ func main() {
 		badCall("need a subcommand")
 	}
 
+	ctx := context.Background()
 	var cmd func(context.Context, *nixstore.Daemon, *nixstore.ActivityTracker) error
 	switch flag.Arg(0) {
 	case "ensure":
@@ -197,13 +530,32 @@ func main() {
 		cmd = func(ctx context.Context, d *nixstore.Daemon, at *nixstore.ActivityTracker) error {
 			return buildDerivation(ctx, d, at, flag.Arg(1))
 		}
+	case "nar-info":
+		if flag.NArg() != 2 {
+			badCall("`nar-info` needs a store path")
+		}
+		cmd = func(ctx context.Context, d *nixstore.Daemon, at *nixstore.ActivityTracker) error {
+			return narInfo(ctx, d, at, flag.Arg(1))
+		}
+	case "copy":
+		if flag.NArg() != 2 {
+			badCall("`copy` needs a store path")
+		}
+		poolSrc := newConnectionPool(ctx, *remoteFlag, *maxConnsFlag)
+		atSrc := nixstore.NewActivityTracker()
+		poolDst := newConnectionPool(ctx, *remote2Flag, *maxConnsFlag)
+		atDst := nixstore.NewActivityTracker()
+
+		if err := copyPath(ctx, poolSrc, atSrc, poolDst, atDst, flag.Arg(1)); err != nil {
+			log.Fatalf("copy: %v", err)
+		}
+		return
 	default:
 		badCall("bad subcommand %s", flag.Arg(0))
 	}
 
 	at := nixstore.NewActivityTracker()
 
-	ctx := context.Background()
 	d, rss, err := connectTo(ctx, *remoteFlag)
 	if err != nil {
 		log.Fatalf("connectTo(%q): %v", *remoteFlag, err)
diff --git a/go/nix/nixstore/activities.go b/go/nix/nixstore/activities.go
index 9f1b35991a..efac722758 100644
--- a/go/nix/nixstore/activities.go
+++ b/go/nix/nixstore/activities.go
@@ -10,6 +10,8 @@ const (
 	progressLogRateLimit = 1 * time.Second
 )
 
+var VerboseActivities = false
+
 type ActivityType uint64
 
 const (
@@ -194,6 +196,9 @@ func (am *ActivityMeta) RecordResult(result ResultType, fields []any) {
 		} else {
 			llt = LogLinePostBuild
 		}
+		if VerboseActivities {
+			log.Printf("%s:%d> %s", am.ActivityID, fields[0].(string))
+		}
 		am.logs = append(am.logs, &ActivityLog{
 			Timestamp:   now,
 			LogLineType: llt,
@@ -206,6 +211,10 @@ func (am *ActivityMeta) RecordResult(result ResultType, fields []any) {
 			NewPhase:  phase,
 		})
 		am.phase = phase
+
+		if VerboseActivities {
+			log.Printf("%d [NEW PHASE] %v", am.ActivityID, phase)
+		}
 	case ResProgress:
 		am.progress = fields
 
@@ -231,6 +240,9 @@ func (am *ActivityMeta) RecordResult(result ResultType, fields []any) {
 			am.lastProgressLog.Progress = fields
 			if time.Since(am.lastProgressLogCreated) > progressLogRateLimit {
 				// If our last progress log was more than rate limit ago, bump the log entry forwards and mark that we should create a new one.
+				if VerboseActivities {
+					log.Printf("%d [PROGRESS] %v", am.ActivityID, fields)
+				}
 				am.lastProgressLog = nil
 			}
 		}
@@ -240,6 +252,9 @@ func (am *ActivityMeta) RecordResult(result ResultType, fields []any) {
 			Timestamp: now,
 			Expected:  fields,
 		})
+		if VerboseActivities {
+			log.Printf("%d [EXPECTED] %v", am.ActivityID, fields)
+		}
 	}
 	am.mu.Unlock()
 }
@@ -401,6 +416,10 @@ func (al *ActivityLogger) StartActivity(at ActivityType, am ActivityMeta) Activi
 	al.activities[am.ActivityID] = a
 	al.mu.Unlock()
 
+	if VerboseActivities {
+		log.Printf("%d [START] %s", am.ActivityID, at)
+	}
+
 	return a
 }
 
@@ -429,6 +448,10 @@ func (al *ActivityLogger) EndActivity(a Activity) {
 		return
 	}
 
+	if VerboseActivities {
+		log.Printf("%d [END] %s", a.Meta().ActivityID, a.ActivityType())
+	}
+
 	al.mu.Lock()
 	al.logs = append(al.logs, ActivityMetaLog{
 		Timestamp:        time.Now(),
diff --git a/go/nix/nixstore/remotestore.go b/go/nix/nixstore/remotestore.go
index 46e42c406a..3eb6e74ae8 100644
--- a/go/nix/nixstore/remotestore.go
+++ b/go/nix/nixstore/remotestore.go
@@ -36,6 +36,12 @@ type Daemon struct {
 	err  error
 }
 
+type PathNotValidError struct{ Path string }
+
+func (err PathNotValidError) Error() string {
+	return fmt.Sprintf("path %q is not valid", err.Path)
+}
+
 func (d *Daemon) NARInfo(storePath string) (*narinfo.NarInfo, error) {
 	d.mu.Lock()
 	defer d.mu.Unlock()
@@ -57,7 +63,7 @@ func (d *Daemon) NARInfo(storePath string) (*narinfo.NarInfo, error) {
 	}
 	valid := validInt == uint64(1)
 	if !valid {
-		return nil, fmt.Errorf("path %q is not valid", storePath)
+		return nil, PathNotValidError{storePath}
 	}
 
 	ni := &narinfo.NarInfo{
@@ -156,6 +162,15 @@ func (d *Daemon) readFields() ([]any, error) {
 	return fields, nil
 }
 
+type NixError struct {
+	Code    uint64
+	Message string
+}
+
+func (ne NixError) Error() string {
+	return fmt.Sprintf("nix error %d: %v", ne.Code, ne.Message)
+}
+
 func (d *Daemon) processStderr(al *ActivityLogger, stdout io.Writer, stdin io.Reader) error {
 	for {
 		msg, err := d.r.ReadUint64()
@@ -205,7 +220,7 @@ func (d *Daemon) processStderr(al *ActivityLogger, stdout io.Writer, stdin io.Re
 				return fmt.Errorf("STDERR_ERROR reading uint64: %w", err)
 			}
 			al.AddError(status, errStr)
-			return fmt.Errorf("error code %d: %v", status, errStr)
+			return NixError{status, errStr}
 		case 0x6f6c6d67: // STDERR_NEXT
 			msg, err := d.r.ReadString()
 			if err != nil {