diff --git a/docs/architecture.md b/docs/architecture.md
index af8de07..03d458f 100644
--- a/docs/architecture.md
+++ b/docs/architecture.md
@@ -11,6 +11,7 @@
 | gcp provider | `internal/provider/gcp` | library | — | Compute Engine VMs, four API calls, fire-and-forget ops |
 | kvm provider | `internal/provider/kvm` | library | — | VMs on libvirt hosts over SSH-forwarded sockets; image cache, overlay disks, NoCloud seeds |
 | kvm2 provider | `internal/provider/kvm2` | library | — | the same backend, built clean-room from the provider contract as a second type: per-RPC child spans, pool-space quota, per-call budgets, its own metadata namespace, credentials re-read on every dial |
+| Shared provider helpers | `internal/provider/nocloud`, `internal/provider/keyedlock` | library | — | the NoCloud seed ISO and vendor-data builder, and one-slot semaphores by key; used by kvm2 and proxmox |
 | Health engine | `internal/health` | Runnable | yes | through-the-proxy probes, thresholds, transition events |
 | Lease store | `internal/lease` | Runnable (expiry sweep) | no | in-memory leases + cooldowns, single mutex |
 | Discovery API | `internal/discovery` | Runnable | no | HTTP list/lease/release/report on `:8090` |
diff --git a/docs/kvm2-provider-spec.md b/docs/kvm2-provider-spec.md
index d8821b8..f07b75e 100644
--- a/docs/kvm2-provider-spec.md
+++ b/docs/kvm2-provider-spec.md
@@ -511,7 +511,7 @@ every 15 s) marks the connection dead on failure.
 per host, dialled lazily, under a mutex with a per-host dial mutex so
 concurrent callers dial once and share the result), the per-`(host, volume)`
 download locks, and a lock per instance name. Both lock sets are one instance
-each of `keyedLocks`: a one-slot semaphore per key, so a waiter gives up when
+each of `keyedlock.Locks` (`internal/provider/keyedlock`): a one-slot semaphore per key, so a waiter gives up when
 its ctx ends, reference-counted so an entry is removed from the map once no
 caller holds or waits for it and the map does not grow with every name ever
 seen, while two waiters on one key always share one semaphore. Create and
diff --git a/docs/kvm2-provider.md b/docs/kvm2-provider.md
index 287297f..b8e8140 100644
--- a/docs/kvm2-provider.md
+++ b/docs/kvm2-provider.md
@@ -252,7 +252,8 @@ UIDs, and a marked domain with a matching or empty `uid` is adopted. The
 namespace is kvm2's own, so the `kvm` provider's domains are invisible to
 it (see *Migrating from kvm*).
 
-The seed (`buildSeedISO` in `seed.go`) is an ISO 9660 image labelled
+The seed (`nocloud.BuildSeedISO`, shared with the proxmox provider in
+`internal/provider/nocloud`) is an ISO 9660 image labelled
 `cidata` with three files. `user-data` is the Proxy's `cloudInit`, **byte
 for byte**: any format cloud-init accepts works, including a Jinja
 template, a MIME archive, a shell script or a gzipped payload; the
@@ -267,7 +268,7 @@ previous attempt may have been cut off mid-upload and a truncated seed
 would boot a VM without cloud-init.
 
 The guest agent rides along in `vendor-data` as a **shell script**
-(`guestAgentScript` in `seed.go`), not a cloud-config, on purpose:
+(`nocloud.GuestAgentScript`), not a cloud-config, on purpose:
 cloud-init merges a vendor-data cloud-config under the user-data with
 mergers that keep the user-data's value for every top-level key both
 define and never append lists, so a Proxy carrying its own `packages:`
diff --git a/docs/plans-executions/2026-09-18-1502-proxmox-provider.md b/docs/plans-executions/2026-09-18-1502-proxmox-provider.md
index 24d7fcb..3c5e57e 100644
--- a/docs/plans-executions/2026-09-18-1502-proxmox-provider.md
+++ b/docs/plans-executions/2026-09-18-1502-proxmox-provider.md
@@ -7,7 +7,7 @@ Pairs with [docs/plans/2026-09-18-1502-proxmox-provider.md](../plans/2026-09-18-
 - [x] Phase 0 — Plan file, branch, host recon, fixtures
 - [x] Phase 1 — The spec (`docs/proxmox-provider-spec.md`)
 - [x] Phase 2 — Config block, registration, package skeleton
-- [ ] Phase 3 — Extract `nocloud` and `keyedlock` from kvm2
+- [x] Phase 3 — Extract `nocloud` and `keyedlock` from kvm2
 - [ ] Phase 4 — Seam, HTTP client, decorator, fake
 - [ ] Phase 5 — Get, ListByTag, task helpers
 - [ ] Phase 6 — Create
@@ -381,3 +381,69 @@ go test -count=1 -v ./internal/provider/proxmox/ | grep -c '^=== RUN' # 152
 Worth noting: the `validProxmoxYAML` constant is the shape Phase 9's commented
 sample entry should take, with the user's values (`192.168.0.8`, `pv1-1`,
 `proxy-seeds`, `vlanTag: 4`, template 9001).
+
+## Phase 3 — Extract `nocloud` and `keyedlock` from kvm2
+
+kvm2's `seed.go` and `seed_test.go` moved to `internal/provider/nocloud`
+with `git mv`, so history follows them. The package exports `BuildSeedISO`,
+`BuildVendorData`, `GuestAgentScript`, `SeedLabel` and a new `ErrEmptyName`
+sentinel in place of kvm2's `permanentf`. kvm2's `keyedLocks` became
+`internal/provider/keyedlock` (`Locks`, `New`, `Acquire`), and its three
+`TestKeyedLocks_*` tests moved with it under their old names. kvm2 now
+imports both packages, and its `create.go` still wraps every seed-build error
+with `permanentf`, so kvm2's behaviour is unchanged. proxmox gained the
+`nameLocks` field and `lockName`.
+
+The test counts match the plan's Verify clause: kvm2 dropped from 465 to 447
+`=== RUN` entries, which is the 12 moved tests plus the 6 subtests of
+`TestSeedISO_userDataByteForByte`. `nocloud` (15) and `keyedlock` (3) carry
+exactly those 18.
+
+```bash
+git mv internal/provider/kvm2/seed.go internal/provider/nocloud/seed.go
+git mv internal/provider/kvm2/seed_test.go internal/provider/nocloud/seed_test.go
+go test -count=1 -v ./internal/provider/kvm2/ | grep -c '^=== RUN'   # 465 before, 447 after
+grep -rn 'iso9660' internal/provider/kvm2/                          # nothing
+go build ./... && go vet ./... && go vet -tags=integration ./...
+go test -race -count=1 ./internal/provider/...                      # all ok, run three times
+go test -short -count=1 ./...                                       # all ok
+```
+
+**Pulled forward from Phase 6 so proxmox imports `nocloud` now**, as the
+"Done when" line requires: `seed.go` with `seedISOName`, `seedVolID` and
+`buildSeed`, the pure part of the seed step. `uploadSeed` still belongs to
+Phase 6, which calls `buildSeed`. The new tests are `TestSeedNames_table`,
+`TestBuildSeed_table` (4 rows),
+`TestBuildSeed_emptyNameIsPermanentWithoutPayload`,
+`TestBuildSeed_stagingFailureIsTransient` and
+`TestLockName_givesUpWithContextAsTransient`. proxmox went from 152 to 161
+test entries.
+
+**proxmox does not copy kvm2's open review finding S5.1**
+(`docs/reviews/2026-09-17-kvm2-provider-review/sweep.md`). In kvm2, a
+staging failure such as a missing `/tmp` or a full emptyDir becomes
+Permanent, and that latches the Proxy. `buildSeed` makes only
+`ErrEmptyName` Permanent and wraps any other error with `%w`, so
+`classify` reads it as Transient. The sentinel makes the same three-line
+fix possible in kvm2, but this phase leaves kvm2's behaviour unchanged.
+
+Worth noting:
+
+- The older `kvm` provider has its own `seed.go` with a different
+  signature (`buildSeedISO` returns `[]byte`). The plan scopes the
+  extraction to kvm2, so `kvm` is untouched.
+- `GuestAgentScript` now says the operator reads the address "from the
+  hypervisor" instead of "through libvirt", in both the Go comment and the
+  script's comment line inside the guest. The script's commands are
+  unchanged.
+- The docs now name the new packages: `kvm2-provider.md`,
+  `kvm2-provider-spec.md` entry 19, the kvm2 columns and section of
+  `providers-existing.md`, plus a "Shared provider helpers" row in the
+  `architecture.md` component table. `docs/reviews/` keeps the old names,
+  because it records the code as it was reviewed.
+- `TestCheck_hostsCheckedConcurrently` in kvm2 failed once under the full
+  `-race` suite (latency 199.96 ms against a 200 ms floor). It passed 20
+  runs out of 20 in isolation, with and without this phase's changes. The
+  test starts its 200 ms release timers before `Check` starts its clock, so
+  under load the measured latency can come in just under 200 ms. The flake
+  predates this phase, and the test is not touched here.
diff --git a/docs/providers-existing.md b/docs/providers-existing.md
index b5667eb..80cb104 100644
--- a/docs/providers-existing.md
+++ b/docs/providers-existing.md
@@ -58,7 +58,7 @@ against.
 |---|---|---|---|---|
 | Create verifies the adopted instance's UID | no | no | no | **yes**: a domain without the kvm2 mark or with a different non-empty UID is Permanent; a marked one with a matching or empty UID is adopted (`create`) |
 | Placement used | none | Zone, MachineType, Image | Zone (host), MachineType, Image | Zone (host), MachineType, Image |
-| CloudInit delivered | **no**: ignored | yes: `user-data` metadata | yes: NoCloud seed ISO built in-process (`buildSeedISO`) | yes: NoCloud seed ISO built in-process (`buildSeedISO`) |
+| CloudInit delivered | **no**: ignored | yes: `user-data` metadata | yes: NoCloud seed ISO built in-process (`buildSeedISO`) | yes: NoCloud seed ISO built in-process (`nocloud.BuildSeedISO`) |
 | Port used | yes: Squid config and containerPort (`squidConf`) | no: firewall is manual | no | no |
 | States produced | Provisioning, Running, Terminated | all four | Provisioning, Running, Stopped | Provisioning, Running, Stopped |
 | Deleting instance still reports its state | yes: a Pod being deleted reports its phase | n/a | n/a | n/a: Delete waits, and a concurrent Get sees the domain until it is undefined |
@@ -68,7 +68,7 @@ against.
 | Credential rotation without restart | n/a | via ADC | no: cached after first successful load | yes: both files read on every dial (`loadCredentials`) |
 | Connection handling | client-go | SDK client, never closed | one pooled SSH plus libvirt connection per host, 15s keepalive (`hostPool`) | one pooled SSH plus libvirt connection per host, 15s keepalive (`hostTable`); dedicated connections for the image upload and the check (`dialFresh`) |
 | Unit-test double | controller-runtime fake client | `fakeAPI` behind `instancesAPI`, plus `httptest` for the real SDK client in `check_test.go` | `fakeAPI` behind `libvirtAPI`, plus an in-process `x/crypto/ssh` server in `conn_test.go` | `fakeBackend` behind `backend` (`fake_test.go`), plus an in-process `x/crypto/ssh` server (`sshserver_test.go`) and go-libvirt's own mock for the adapter |
-| Proxy software | generated Squid config (`pod.go`) | from `cloudInit`; the image must run cloud-init | from `cloudInit`, plus a vendor-data script that installs the guest agent (`seed.go`) | from `cloudInit`, plus a vendor-data script that installs the guest agent (`seed.go`) |
+| Proxy software | generated Squid config (`pod.go`) | from `cloudInit`; the image must run cloud-init | from `cloudInit`, plus a vendor-data script that installs the guest agent (`seed.go`) | from `cloudInit`, plus a vendor-data script that installs the guest agent (`nocloud.GuestAgentScript`) |
 | IP reachable from the operator | Pod network, so in-cluster only (not from `make run-dev` on a laptop) | external IP, once a firewall rule allows the cluster's egress IP | LAN address on the host's bridge; the operator needs a route to that LAN | LAN address on the host's bridge; the operator needs a route to that LAN |
 | Distinct egress IP per proxy | no: the cluster's egress | yes: ephemeral external IP | whatever the host's LAN routes through; not per VM by itself | whatever the host's LAN routes through; not per VM by itself |
 
@@ -347,8 +347,10 @@ same libvirt.
   production adapter and the budgets), `ssh.go` (credentials, the forwarded
   socket, keepalive), `hosts.go` (the pooled table and the production
   dialer), `observe.go` (the span and log decorator), `image.go` (cache,
-  download, quota precheck), `seed.go`, `xml.go`, `volume.go`,
-  `metadata.go`, `id.go`, `state.go`, `errors.go`, `check.go`. The seam is
+  download, quota precheck), `xml.go`, `volume.go`, `metadata.go`,
+  `id.go`, `state.go`, `errors.go`, `check.go`; the seed ISO comes from
+  the shared `internal/provider/nocloud` and both lock sets are
+  `internal/provider/keyedlock`. The seam is
   `backend`: the twenty go-libvirt calls the provider uses, each with `ctx`
   first (go-libvirt's own methods take none and block), plus `Close`.
   `libvirtBackend` adapts `*libvirt.Libvirt` and runs every call through
diff --git a/internal/provider/keyedlock/keyedlock.go b/internal/provider/keyedlock/keyedlock.go
new file mode 100644
index 0000000..7e25ba1
--- /dev/null
+++ b/internal/provider/keyedlock/keyedlock.go
@@ -0,0 +1,60 @@
+// Package keyedlock provides one-slot semaphores by key, shared by the
+// providers that serialise work per instance name or per cached image
+// (kvm2, proxmox).
+package keyedlock
+
+import (
+	"context"
+	"sync"
+)
+
+// Locks is a set of one-slot semaphores by key: a semaphore rather than a
+// mutex so a waiter can give up with its ctx. Each entry counts the callers
+// holding or waiting for it and is dropped from the map at zero, so the map
+// does not grow with every key ever seen while two waiters on one key still
+// always share one semaphore. The zero value is not usable; call New.
+type Locks struct {
+	mu    sync.Mutex
+	locks map[string]*lock
+}
+
+type lock struct {
+	sem  chan struct{}
+	refs int
+}
+
+func New() *Locks {
+	return &Locks{locks: make(map[string]*lock)}
+}
+
+// Acquire takes the lock for key, or gives up with ctx's error. The
+// returned func releases it and must be called exactly once.
+func (k *Locks) Acquire(ctx context.Context, key string) (release func(), err error) {
+	k.mu.Lock()
+	l, ok := k.locks[key]
+	if !ok {
+		l = &lock{sem: make(chan struct{}, 1)}
+		k.locks[key] = l
+	}
+	l.refs++
+	k.mu.Unlock()
+	select {
+	case l.sem <- struct{}{}:
+		return func() {
+			<-l.sem
+			k.unref(key, l)
+		}, nil
+	case <-ctx.Done():
+		k.unref(key, l)
+		return nil, ctx.Err()
+	}
+}
+
+func (k *Locks) unref(key string, l *lock) {
+	k.mu.Lock()
+	defer k.mu.Unlock()
+	l.refs--
+	if l.refs == 0 {
+		delete(k.locks, key)
+	}
+}
diff --git a/internal/provider/keyedlock/keyedlock_test.go b/internal/provider/keyedlock/keyedlock_test.go
new file mode 100644
index 0000000..7a77aa4
--- /dev/null
+++ b/internal/provider/keyedlock/keyedlock_test.go
@@ -0,0 +1,90 @@
+package keyedlock
+
+import (
+	"context"
+	"errors"
+	"sync"
+	"sync/atomic"
+	"testing"
+	"time"
+)
+
+func lockCount(k *Locks) int {
+	k.mu.Lock()
+	defer k.mu.Unlock()
+	return len(k.locks)
+}
+
+func TestKeyedLocks_entryRemovedWhenUncontended(t *testing.T) {
+	t.Parallel()
+	k := New()
+	release, err := k.Acquire(context.Background(), "a")
+	if err != nil {
+		t.Fatalf("acquire error = %v", err)
+	}
+	if lockCount(k) != 1 {
+		t.Errorf("entries while held = %d, want 1", lockCount(k))
+	}
+	release()
+	if lockCount(k) != 0 {
+		t.Errorf("entries after release = %d, want 0", lockCount(k))
+	}
+	if _, err := k.Acquire(context.Background(), "a"); err != nil {
+		t.Fatalf("acquire after release error = %v", err)
+	}
+}
+
+func TestKeyedLocks_waitersShareOneLock(t *testing.T) {
+	t.Parallel()
+	k := New()
+	const n = 20
+	var inside, overlaps atomic.Int32
+	var wg sync.WaitGroup
+	for range n {
+		wg.Go(func() {
+			release, err := k.Acquire(context.Background(), "shared")
+			if err != nil {
+				t.Errorf("acquire error = %v", err)
+				return
+			}
+			if inside.Add(1) > 1 {
+				overlaps.Add(1)
+			}
+			time.Sleep(time.Millisecond)
+			inside.Add(-1)
+			release()
+		})
+	}
+	wg.Wait()
+	if overlaps.Load() != 0 {
+		t.Errorf("%d holders overlapped on one key", overlaps.Load())
+	}
+	if lockCount(k) != 0 {
+		t.Errorf("entries after every holder released = %d, want 0", lockCount(k))
+	}
+}
+
+func TestKeyedLocks_waiterGivesUpWithContext(t *testing.T) {
+	t.Parallel()
+	k := New()
+	release, err := k.Acquire(context.Background(), "a")
+	if err != nil {
+		t.Fatalf("acquire error = %v", err)
+	}
+	ctx, cancel := context.WithTimeout(context.Background(), 20*time.Millisecond)
+	defer cancel()
+	start := time.Now()
+	if _, err := k.Acquire(ctx, "a"); !errors.Is(err, context.DeadlineExceeded) {
+		t.Fatalf("waiter error = %v, want DeadlineExceeded", err)
+	}
+	if time.Since(start) > time.Second {
+		t.Errorf("waiter took %v, want to give up with its ctx", time.Since(start))
+	}
+	if lockCount(k) != 1 {
+		t.Errorf("entries with the holder still in = %d, want 1: the waiter's exit must not drop the holder's entry", lockCount(k))
+	}
+	release()
+	if lockCount(k) != 0 {
+		t.Errorf("entries after the holder released = %d, want 0", lockCount(k))
+	}
+}
diff --git a/internal/provider/kvm2/create.go b/internal/provider/kvm2/create.go
index 0de7e45..0778ba4 100644
--- a/internal/provider/kvm2/create.go
+++ b/internal/provider/kvm2/create.go
@@ -11,6 +11,7 @@ import (
 	"github.com/go-logr/logr"
 
 	"gitea.home.hrajfrisbee.cz/kacerr/egress-proxies-operator/internal/provider"
+	"gitea.home.hrajfrisbee.cz/kacerr/egress-proxies-operator/internal/provider/nocloud"
 )
 
 // The wait after DomainCreate (spec entry 18): Create returns only once the
@@ -191,12 +192,12 @@ func (p *Provider) writeSeed(ctx context.Context, log logr.Logger, be backend, p
 		return libvirt.StorageVol{}, err
 	}
 	installAgent := p.cfg.InstallGuestAgent == nil || *p.cfg.InstallGuestAgent
-	vendorData, err := buildVendorData(p.cfg.VendorData, installAgent)
+	vendorData, err := nocloud.BuildVendorData(p.cfg.VendorData, installAgent)
 	if err != nil {
 		return libvirt.StorageVol{}, permanentf("building vendor-data for %s: %v", req.Name, err)
 	}
 	var iso bytes.Buffer
-	if err := buildSeedISO(req.Name, req.CloudInit, vendorData, &iso); err != nil {
+	if err := nocloud.BuildSeedISO(req.Name, req.CloudInit, vendorData, &iso); err != nil {
 		return libvirt.StorageVol{}, permanentf("building seed ISO for %s: %v", req.Name, err)
 	}
 	xmlDoc, err := buildVolumeXML(volumeSpec{Name: name, CapacityBytes: int64(iso.Len()), Format: provider.KVMFormatRaw})
diff --git a/internal/provider/kvm2/create_test.go b/internal/provider/kvm2/create_test.go
index 66fb26c..218c0ba 100644
--- a/internal/provider/kvm2/create_test.go
+++ b/internal/provider/kvm2/create_test.go
@@ -12,6 +12,7 @@ import (
 	"github.com/digitalocean/go-libvirt"
 
 	"gitea.home.hrajfrisbee.cz/kacerr/egress-proxies-operator/internal/provider"
+	"gitea.home.hrajfrisbee.cz/kacerr/egress-proxies-operator/internal/provider/nocloud"
 )
 
 func TestCreate_repeatReturnsSameIDAndCreatesNothing(t *testing.T) {
@@ -95,7 +96,7 @@ func TestCreate_resumesAfterEachPartialStep(t *testing.T) {
 			},
 			check: func(t *testing.T, fx *fixture) {
 				got := fx.fake.vol(seed)
-				if got == nil || len(got.content) <= 3 || !bytes.Contains(got.content, []byte(seedLabel)) {
+				if got == nil || len(got.content) <= 3 || !bytes.Contains(got.content, []byte(nocloud.SeedLabel)) {
 					t.Errorf("seed volume not rebuilt")
 				}
 			},
@@ -461,7 +462,7 @@ func TestCreate_seedAlwaysRebuilt(t *testing.T) {
 	if got := fx.fake.callCount("StorageVolUpload"); got != uploads+1 {
 		t.Errorf("StorageVolUpload calls = %d, want %d", got, uploads+1)
 	}
-	if len(second.content) != len(first.content) || !bytes.Contains(second.content, []byte(seedLabel)) {
+	if len(second.content) != len(first.content) || !bytes.Contains(second.content, []byte(nocloud.SeedLabel)) {
 		t.Errorf("rebuilt seed is %d bytes, first was %d; want a complete seed of the same size", len(second.content), len(first.content))
 	}
 }
diff --git a/internal/provider/kvm2/image.go b/internal/provider/kvm2/image.go
index f0d6664..f7bcae5 100644
--- a/internal/provider/kvm2/image.go
+++ b/internal/provider/kvm2/image.go
@@ -73,7 +73,7 @@ func resolveImage(entry string, cfg provider.KVMConfig, image string) (imageSour
 // returns promptly (contract §1.5) instead of waiting out a 20-minute
 // transfer.
 func (p *Provider) lockImage(ctx context.Context, hostName, volume string) (func(), error) {
-	release, err := p.imageLocks.acquire(ctx, hostName+"/"+volume)
+	release, err := p.imageLocks.Acquire(ctx, hostName+"/"+volume)
 	if err != nil {
 		return nil, fmt.Errorf("%w: waiting for image %s on host %s: %w", provider.ErrTransient, volume, hostName, err)
 	}
diff --git a/internal/provider/kvm2/kvm2.go b/internal/provider/kvm2/kvm2.go
index 11090b8..e7a13e0 100644
--- a/internal/provider/kvm2/kvm2.go
+++ b/internal/provider/kvm2/kvm2.go
@@ -9,13 +9,13 @@ import (
 	"fmt"
 	"net/http"
 	"os"
-	"sync"
 	"time"
 
 	"github.com/go-logr/logr"
 	logf "sigs.k8s.io/controller-runtime/pkg/log"
 
 	"gitea.home.hrajfrisbee.cz/kacerr/egress-proxies-operator/internal/provider"
+	"gitea.home.hrajfrisbee.cz/kacerr/egress-proxies-operator/internal/provider/keyedlock"
 	"gitea.home.hrajfrisbee.cz/kacerr/egress-proxies-operator/internal/tracing"
 )
 
@@ -41,12 +41,12 @@ type Provider struct {
 	runningPoll time.Duration
 
 	// imageLocks serialises image fetches per (host, volume), spec entry 19.
-	imageLocks *keyedLocks
+	imageLocks *keyedlock.Locks
 
 	// nameLocks serialises Create and Delete per instance name, spec entry
 	// 19, so two Deletes never race on destroy and undefine and a Delete
 	// cannot interleave with a Create's steps.
-	nameLocks *keyedLocks
+	nameLocks *keyedlock.Locks
 }
 
 // providerOption adjusts what newProvider builds; tests inject an HTTP
@@ -111,8 +111,8 @@ func newProvider(name string, cfg provider.KVMConfig, d dialer, opts ...provider
 		http:        http.DefaultClient,
 		runningWait: defaultRunningWait,
 		runningPoll: defaultRunningPoll,
-		imageLocks:  newKeyedLocks(),
-		nameLocks:   newKeyedLocks(),
+		imageLocks:  keyedlock.New(),
+		nameLocks:   keyedlock.New(),
 	}
 	for _, opt := range opts {
 		opt(p)
@@ -187,60 +187,9 @@ func (p *Provider) wrapErr(op, id string, err error) error {
 // so a caller whose ctx ends while another caller's Create is downloading
 // an image returns promptly.
 func (p *Provider) lockName(ctx context.Context, name string) (func(), error) {
-	release, err := p.nameLocks.acquire(ctx, name)
+	release, err := p.nameLocks.Acquire(ctx, name)
 	if err != nil {
 		return nil, fmt.Errorf("%w: waiting for the lock on %s: %w", provider.ErrTransient, name, err)
 	}
 	return release, nil
 }
-
-// keyedLocks is a set of one-slot semaphores by key (spec entry 19): a
-// semaphore rather than a mutex so a waiter can give up with its ctx. Each
-// entry counts the callers holding or waiting for it and is dropped from
-// the map at zero, so the map does not grow with every name ever seen while
-// two waiters on one key still always share one semaphore.
-type keyedLocks struct {
-	mu    sync.Mutex
-	locks map[string]*keyedLock
-}
-
-type keyedLock struct {
-	sem  chan struct{}
-	refs int
-}
-
-func newKeyedLocks() *keyedLocks {
-	return &keyedLocks{locks: make(map[string]*keyedLock)}
-}
-
-// acquire takes the lock for key, or gives up with ctx's error. The
-// returned func releases it and must be called exactly once.
-func (k *keyedLocks) acquire(ctx context.Context, key string) (func(), error) {
-	k.mu.Lock()
-	l, ok := k.locks[key]
-	if !ok {
-		l = &keyedLock{sem: make(chan struct{}, 1)}
-		k.locks[key] = l
-	}
-	l.refs++
-	k.mu.Unlock()
-	select {
-	case l.sem <- struct{}{}:
-		return func() {
-			<-l.sem
-			k.unref(key, l)
-		}, nil
-	case <-ctx.Done():
-		k.unref(key, l)
-		return nil, ctx.Err()
-	}
-}
-
-func (k *keyedLocks) unref(key string, l *keyedLock) {
-	k.mu.Lock()
-	defer k.mu.Unlock()
-	l.refs--
-	if l.refs == 0 {
-		delete(k.locks, key)
-	}
-}
diff --git a/internal/provider/kvm2/kvm2_test.go b/internal/provider/kvm2/kvm2_test.go
index 909d00d..a04c9ae 100644
--- a/internal/provider/kvm2/kvm2_test.go
+++ b/internal/provider/kvm2/kvm2_test.go
@@ -5,7 +5,6 @@ import (
 	"errors"
 	"strings"
 	"sync"
-	"sync/atomic"
 	"testing"
 	"time"
 
@@ -267,86 +266,6 @@ func TestWithDefaults_doesNotAliasInput(t *testing.T) {
 	}
 }
 
-func lockCount(k *keyedLocks) int {
-	k.mu.Lock()
-	defer k.mu.Unlock()
-	return len(k.locks)
-}
-
-func TestKeyedLocks_entryRemovedWhenUncontended(t *testing.T) {
-	t.Parallel()
-	k := newKeyedLocks()
-	release, err := k.acquire(context.Background(), "a")
-	if err != nil {
-		t.Fatalf("acquire error = %v", err)
-	}
-	if lockCount(k) != 1 {
-		t.Errorf("entries while held = %d, want 1", lockCount(k))
-	}
-	release()
-	if lockCount(k) != 0 {
-		t.Errorf("entries after release = %d, want 0", lockCount(k))
-	}
-	if _, err := k.acquire(context.Background(), "a"); err != nil {
-		t.Fatalf("acquire after release error = %v", err)
-	}
-}
-
-func TestKeyedLocks_waitersShareOneLock(t *testing.T) {
-	t.Parallel()
-	k := newKeyedLocks()
-	const n = 20
-	var inside, overlaps atomic.Int32
-	var wg sync.WaitGroup
-	for range n {
-		wg.Go(func() {
-			release, err := k.acquire(context.Background(), "shared")
-			if err != nil {
-				t.Errorf("acquire error = %v", err)
-				return
-			}
-			if inside.Add(1) > 1 {
-				overlaps.Add(1)
-			}
-			time.Sleep(time.Millisecond)
-			inside.Add(-1)
-			release()
-		})
-	}
-	wg.Wait()
-	if overlaps.Load() != 0 {
-		t.Errorf("%d holders overlapped on one key", overlaps.Load())
-	}
-	if lockCount(k) != 0 {
-		t.Errorf("entries after every holder released = %d, want 0", lockCount(k))
-	}
-}
-
-func TestKeyedLocks_waiterGivesUpWithContext(t *testing.T) {
-	t.Parallel()
-	k := newKeyedLocks()
-	release, err := k.acquire(context.Background(), "a")
-	if err != nil {
-		t.Fatalf("acquire error = %v", err)
-	}
-	ctx, cancel := context.WithTimeout(context.Background(), 20*time.Millisecond)
-	defer cancel()
-	start := time.Now()
-	if _, err := k.acquire(ctx, "a"); !errors.Is(err, context.DeadlineExceeded) {
-		t.Fatalf("waiter error = %v, want DeadlineExceeded", err)
-	}
-	if time.Since(start) > time.Second {
-		t.Errorf("waiter took %v, want to give up with its ctx", time.Since(start))
-	}
-	if lockCount(k) != 1 {
-		t.Errorf("entries with the holder still in = %d, want 1: the waiter's exit must not drop the holder's entry", lockCount(k))
-	}
-	release()
-	if lockCount(k) != 0 {
-		t.Errorf("entries after the holder released = %d, want 0", lockCount(k))
-	}
-}
-
 func TestWrapErr_carriesProviderNameOpAndID(t *testing.T) {
 	t.Parallel()
 	p := newProvider("kvm2-home", twoHosts(), &stubDialer{})
diff --git a/internal/provider/kvm2/seed.go b/internal/provider/nocloud/seed.go
similarity index 77%
rename from internal/provider/kvm2/seed.go
rename to internal/provider/nocloud/seed.go
index 4c81292..77d2d56 100644
--- a/internal/provider/kvm2/seed.go
+++ b/internal/provider/nocloud/seed.go
@@ -1,4 +1,7 @@
-package kvm2
+// Package nocloud builds cloud-init NoCloud seed ISOs in process, shared by
+// the providers that attach a seed as a CD-ROM (kvm2, proxmox). It never
+// parses or logs the payload.
+package nocloud
 
 import (
 	"bytes"
@@ -12,19 +15,22 @@ import (
 	"github.com/kdomanski/iso9660"
 )
 
-// seedLabel is the volume label cloud-init's NoCloud datasource looks for.
-const seedLabel = "cidata"
+// SeedLabel is the volume label cloud-init's NoCloud datasource looks for.
+const SeedLabel = "cidata"
 
-// guestAgentScript is the vendor-data payload that gets qemu-guest-agent
-// running so the provider can read the VM's address through libvirt. It is
+// ErrEmptyName is returned by BuildSeedISO for an empty instance name.
+var ErrEmptyName = errors.New("seed ISO: instance name is empty")
+
+// GuestAgentScript is the vendor-data payload that gets qemu-guest-agent
+// running so the provider can read the VM's address from the hypervisor. It is
 // a shell script and not a cloud-config on purpose: cloud-init merges a
 // vendor-data cloud-config under the user-data with mergers that keep the
 // user-data's value for every top-level key and never append lists, so a
 // Proxy with its own packages: would silently lose the agent. Scripts
 // bypass the merge.
-const guestAgentScript = `#!/bin/sh
+const GuestAgentScript = `#!/bin/sh
 # Added by egress-proxies-operator: installs and starts qemu-guest-agent so
-# the operator can learn this VM's IP address through libvirt.
+# the operator can learn this VM's IP address from the hypervisor.
 # To leave this out, set installGuestAgent: false in the provider's config.
 set -e
 if command -v qemu-ga >/dev/null 2>&1 || [ -x /usr/sbin/qemu-ga ]; then
@@ -53,15 +59,15 @@ else
 fi
 `
 
-// buildSeedISO writes a NoCloud seed to w: user-data byte for byte,
+// BuildSeedISO writes a NoCloud seed to w: user-data byte for byte,
 // meta-data naming the instance, and vendor-data (an empty file when there
 // is none, so cloud-init does not look elsewhere). iso9660.NewWriter stages
 // the files in os.MkdirTemp("", ""), that is under os.TempDir(): $TMPDIR
 // when set, else /tmp, the manager's 64Mi emptyDir. The staging dir is
 // removed before returning. Nothing about the payload is logged here.
-func buildSeedISO(name, userData, vendorData string, w io.Writer) (err error) {
+func BuildSeedISO(name, userData, vendorData string, w io.Writer) (err error) {
 	if name == "" {
-		return permanentf("seed ISO: instance name is empty")
+		return ErrEmptyName
 	}
 	iw, err := iso9660.NewWriter()
 	if err != nil {
@@ -83,30 +89,30 @@ func buildSeedISO(name, userData, vendorData string, w io.Writer) (err error) {
 			return fmt.Errorf("adding %s to seed ISO: %w", f.name, err)
 		}
 	}
-	if err := iw.WriteTo(w, seedLabel); err != nil {
+	if err := iw.WriteTo(w, SeedLabel); err != nil {
 		return fmt.Errorf("writing seed ISO: %w", err)
 	}
 	return nil
 }
 
-// buildVendorData composes the seed's vendor-data from the configured
+// BuildVendorData composes the seed's vendor-data from the configured
 // document and the guest agent script. With both, they travel as separate
 // parts of a MIME multipart, so cloud-init handles each by its own type; the
 // MIME-Version header comes first because cloud-init detects multipart by
 // finding "mime-version:" within the first 4096 bytes.
-func buildVendorData(configured string, installAgent bool) (string, error) {
+func BuildVendorData(configured string, installAgent bool) (string, error) {
 	switch {
 	case !installAgent:
 		return configured, nil
 	case strings.TrimSpace(configured) == "":
-		return guestAgentScript, nil
+		return GuestAgentScript, nil
 	}
 	var buf bytes.Buffer
 	mw := multipart.NewWriter(&buf)
 	fmt.Fprintf(&buf, "MIME-Version: 1.0\r\nContent-Type: multipart/mixed; boundary=%q\r\n\r\n", mw.Boundary())
 	parts := []struct{ contentType, body string }{
 		{"text/plain", configured},
-		{"text/x-shellscript", guestAgentScript},
+		{"text/x-shellscript", GuestAgentScript},
 	}
 	for _, p := range parts {
 		pw, err := mw.CreatePart(textproto.MIMEHeader{"Content-Type": {p.contentType}})
diff --git a/internal/provider/kvm2/seed_test.go b/internal/provider/nocloud/seed_test.go
similarity index 91%
rename from internal/provider/kvm2/seed_test.go
rename to internal/provider/nocloud/seed_test.go
index e0bb40e..2ac2575 100644
--- a/internal/provider/kvm2/seed_test.go
+++ b/internal/provider/nocloud/seed_test.go
@@ -1,8 +1,9 @@
-package kvm2
+package nocloud
 
 import (
 	"bufio"
 	"bytes"
+	"errors"
 	"io"
 	"mime"
 	"mime/multipart"
@@ -18,8 +19,8 @@ import (
 func readSeed(t *testing.T, name, userData, vendorData string) (map[string]string, string) {
 	t.Helper()
 	var buf bytes.Buffer
-	if err := buildSeedISO(name, userData, vendorData, &buf); err != nil {
-		t.Fatalf("buildSeedISO: %v", err)
+	if err := BuildSeedISO(name, userData, vendorData, &buf); err != nil {
+		t.Fatalf("BuildSeedISO: %v", err)
 	}
 	img, err := iso9660.OpenImage(bytes.NewReader(buf.Bytes()))
 	if err != nil {
@@ -57,8 +58,8 @@ func TestSeedISO_filesAndLabel(t *testing.T) {
 	userData := "#cloud-config\npackages: [squid]\n"
 	vendorData := "#!/bin/sh\necho hi\n"
 	files, label := readSeed(t, "proxy-abc", userData, vendorData)
-	if label != seedLabel {
-		t.Errorf("label = %q, want %q", label, seedLabel)
+	if label != SeedLabel {
+		t.Errorf("label = %q, want %q", label, SeedLabel)
 	}
 	want := map[string]string{
 		"user-data":   userData,
@@ -117,8 +118,8 @@ func TestSeedISO_tempDirRemoved(t *testing.T) {
 		t.Fatalf("os.TempDir() = %q, want %q", got, tmp)
 	}
 	var buf bytes.Buffer
-	if err := buildSeedISO("proxy-abc", "#cloud-config\n", guestAgentScript, &buf); err != nil {
-		t.Fatalf("buildSeedISO: %v", err)
+	if err := BuildSeedISO("proxy-abc", "#cloud-config\n", GuestAgentScript, &buf); err != nil {
+		t.Fatalf("BuildSeedISO: %v", err)
 	}
 	if buf.Len() == 0 {
 		t.Fatal("seed ISO is empty")
@@ -151,8 +152,8 @@ func TestSeedISO_emptyVendorDataStillHasFile(t *testing.T) {
 func TestSeedISO_rejectsEmptyName(t *testing.T) {
 	t.Parallel()
 	var buf bytes.Buffer
-	if err := buildSeedISO("", "#cloud-config\n", "", &buf); err == nil {
-		t.Fatal("buildSeedISO with empty name succeeded")
+	if err := BuildSeedISO("", "#cloud-config\n", "", &buf); !errors.Is(err, ErrEmptyName) {
+		t.Fatalf("BuildSeedISO with empty name error = %v, want ErrEmptyName", err)
 	}
 	if buf.Len() != 0 {
 		t.Errorf("wrote %d bytes alongside the error", buf.Len())
@@ -161,7 +162,7 @@ func TestSeedISO_rejectsEmptyName(t *testing.T) {
 
 func TestVendorData_isShellScriptNotCloudConfig(t *testing.T) {
 	t.Parallel()
-	got, err := buildVendorData("", true)
+	got, err := BuildVendorData("", true)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -193,7 +194,7 @@ func bufioReader(s string) *bufio.Reader {
 func TestVendorData_multipartWhenExtraProvided(t *testing.T) {
 	t.Parallel()
 	configured := "#cloud-config\nntp:\n  enabled: true\n"
-	got, err := buildVendorData(configured, true)
+	got, err := BuildVendorData(configured, true)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -229,7 +230,7 @@ func TestVendorData_multipartWhenExtraProvided(t *testing.T) {
 		}
 		parts = append(parts, part{p.Header.Get("Content-Type"), string(body)})
 	}
-	want := []part{{"text/plain", configured}, {"text/x-shellscript", guestAgentScript}}
+	want := []part{{"text/plain", configured}, {"text/x-shellscript", GuestAgentScript}}
 	if len(parts) != len(want) {
 		t.Fatalf("got %d parts, want %d", len(parts), len(want))
 	}
@@ -242,7 +243,7 @@ func TestVendorData_multipartWhenExtraProvided(t *testing.T) {
 
 func TestVendorData_offWithoutConfiguredIsEmpty(t *testing.T) {
 	t.Parallel()
-	got, err := buildVendorData("", false)
+	got, err := BuildVendorData("", false)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -254,7 +255,7 @@ func TestVendorData_offWithoutConfiguredIsEmpty(t *testing.T) {
 func TestVendorData_offWithConfiguredPassesThrough(t *testing.T) {
 	t.Parallel()
 	configured := "#cloud-config\nntp:\n  enabled: true"
-	got, err := buildVendorData(configured, false)
+	got, err := BuildVendorData(configured, false)
 	if err != nil {
 		t.Fatal(err)
 	}
diff --git a/internal/provider/proxmox/proxmox.go b/internal/provider/proxmox/proxmox.go
index d3875b5..1b19826 100644
--- a/internal/provider/proxmox/proxmox.go
+++ b/internal/provider/proxmox/proxmox.go
@@ -6,6 +6,7 @@ package proxmox
 
 import (
 	"context"
+	"fmt"
 	"os"
 	"time"
 
@@ -13,6 +14,7 @@ import (
 	logf "sigs.k8s.io/controller-runtime/pkg/log"
 
 	"gitea.home.hrajfrisbee.cz/kacerr/egress-proxies-operator/internal/provider"
+	"gitea.home.hrajfrisbee.cz/kacerr/egress-proxies-operator/internal/provider/keyedlock"
 )
 
 const (
@@ -57,6 +59,11 @@ type Provider struct {
 	api     api
 	now     func() time.Time
 	budgets budgets
+
+	// nameLocks serialises Create and Delete per instance name (spec entry
+	// 19), so the reconciler's Delete and GC's Delete never race on stop and
+	// destroy, and a Delete cannot interleave with a Create's steps.
+	nameLocks *keyedlock.Locks
 }
 
 var _ provider.Provider = (*Provider)(nil)
@@ -85,11 +92,12 @@ func New(ctx context.Context, pc provider.ProviderConfig) (provider.Provider, er
 // newProvider is the constructor tests use, with a in-memory api.
 func newProvider(name string, cfg provider.ProxmoxConfig, a api) *Provider {
 	return &Provider{
-		name:    name,
-		cfg:     withDefaults(cfg),
-		api:     a,
-		now:     time.Now,
-		budgets: defaultBudgets(),
+		name:      name,
+		cfg:       withDefaults(cfg),
+		api:       a,
+		now:       time.Now,
+		budgets:   defaultBudgets(),
+		nameLocks: keyedlock.New(),
 	}
 }
 
@@ -152,6 +160,17 @@ func (p *Provider) wrapErr(op, id string, err error) error {
 	return provider.Wrap(classify(op, err), op, p.name, id, err)
 }
 
+// lockName takes the per-name lock of spec entry 19, or gives up with ctx,
+// so a caller whose ctx ends while another caller's Create waits on a clone
+// returns promptly.
+func (p *Provider) lockName(ctx context.Context, name string) (func(), error) {
+	release, err := p.nameLocks.Acquire(ctx, name)
+	if err != nil {
+		return nil, fmt.Errorf("%w: waiting for the lock on %s: %w", provider.ErrTransient, name, err)
+	}
+	return release, nil
+}
+
 // The four contract methods are stubs until the phases that implement them
 // (Get and ListByTag in Phase 5, Create in 6, Delete in 7). Each answers
 // Transient, so a Proxy on this provider retries rather than latching.
diff --git a/internal/provider/proxmox/proxmox_test.go b/internal/provider/proxmox/proxmox_test.go
index 1b25f70..1197571 100644
--- a/internal/provider/proxmox/proxmox_test.go
+++ b/internal/provider/proxmox/proxmox_test.go
@@ -206,3 +206,24 @@ func TestStubs_answerTransient(t *testing.T) {
 		}
 	}
 }
+
+func TestLockName_givesUpWithContextAsTransient(t *testing.T) {
+	t.Parallel()
+	p := newProvider("proxmox-home", testConfig(), nil)
+	release, err := p.lockName(context.Background(), "proxy-abc")
+	if err != nil {
+		t.Fatalf("lockName error = %v", err)
+	}
+	defer release()
+	other, err := p.lockName(context.Background(), "proxy-def")
+	if err != nil {
+		t.Fatalf("lockName on another name error = %v, want it free", err)
+	}
+	other()
+	ctx, cancel := context.WithTimeout(context.Background(), 20*time.Millisecond)
+	defer cancel()
+	_, err = p.lockName(ctx, "proxy-abc")
+	if !errors.Is(err, provider.ErrTransient) || !errors.Is(err, context.DeadlineExceeded) {
+		t.Errorf("lockName on a held name error = %v, want ErrTransient wrapping DeadlineExceeded", err)
+	}
+}
diff --git a/internal/provider/proxmox/seed.go b/internal/provider/proxmox/seed.go
new file mode 100644
index 0000000..cb7dc20
--- /dev/null
+++ b/internal/provider/proxmox/seed.go
@@ -0,0 +1,43 @@
+package proxmox
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+
+	"gitea.home.hrajfrisbee.cz/kacerr/egress-proxies-operator/internal/provider"
+	"gitea.home.hrajfrisbee.cz/kacerr/egress-proxies-operator/internal/provider/nocloud"
+)
+
+// seedISOName is the seed's file name on the ISO storage. Upload refuses a
+// name without the .iso suffix (F12), and GC's blind spot is documented by
+// this pattern (spec entry 11).
+func seedISOName(name string) string {
+	return name + "-seed.iso"
+}
+
+// seedVolID is the seed's volume id, as ide2 references it and as the
+// storage content delete takes it.
+func seedVolID(storage, name string) string {
+	return storage + ":iso/" + seedISOName(name)
+}
+
+// buildSeed renders the NoCloud seed for req in memory (spec entry 5): a
+// seed is tens of KiB. Nothing about its content is logged or put in an
+// error. Only an empty name is Permanent; a staging failure (no /tmp, a
+// full emptyDir) is left unclassified, so classify makes it Transient and
+// the reconciler retries instead of latching the Proxy.
+func (p *Provider) buildSeed(req provider.CreateRequest) (*bytes.Buffer, error) {
+	vendorData, err := nocloud.BuildVendorData(p.cfg.VendorData, *p.cfg.InstallGuestAgent)
+	if err != nil {
+		return nil, fmt.Errorf("building vendor-data for %s: %w", req.Name, err)
+	}
+	var iso bytes.Buffer
+	if err := nocloud.BuildSeedISO(req.Name, req.CloudInit, vendorData, &iso); err != nil {
+		if errors.Is(err, nocloud.ErrEmptyName) {
+			return nil, permanentf("building seed ISO: %v", err)
+		}
+		return nil, fmt.Errorf("building seed ISO for %s: %w", req.Name, err)
+	}
+	return &iso, nil
+}
diff --git a/internal/provider/proxmox/seed_test.go b/internal/provider/proxmox/seed_test.go
new file mode 100644
index 0000000..1ccc611
--- /dev/null
+++ b/internal/provider/proxmox/seed_test.go
@@ -0,0 +1,139 @@
+package proxmox
+
+import (
+	"bytes"
+	"errors"
+	"io"
+	"strings"
+	"testing"
+
+	"github.com/kdomanski/iso9660"
+
+	"gitea.home.hrajfrisbee.cz/kacerr/egress-proxies-operator/internal/provider"
+	"gitea.home.hrajfrisbee.cz/kacerr/egress-proxies-operator/internal/provider/nocloud"
+)
+
+// seedFiles opens a built seed and returns its label and files by name.
+func seedFiles(t *testing.T, iso *bytes.Buffer) (string, map[string]string) {
+	t.Helper()
+	img, err := iso9660.OpenImage(bytes.NewReader(iso.Bytes()))
+	if err != nil {
+		t.Fatalf("OpenImage: %v", err)
+	}
+	label, err := img.Label()
+	if err != nil {
+		t.Fatalf("Label: %v", err)
+	}
+	root, err := img.RootDir()
+	if err != nil {
+		t.Fatalf("RootDir: %v", err)
+	}
+	children, err := root.GetChildren()
+	if err != nil {
+		t.Fatalf("GetChildren: %v", err)
+	}
+	files := make(map[string]string, len(children))
+	for _, c := range children {
+		data, err := io.ReadAll(c.Reader())
+		if err != nil {
+			t.Fatalf("reading %s: %v", c.Name(), err)
+		}
+		files[c.Name()] = string(data)
+	}
+	return label, files
+}
+
+func TestSeedNames_table(t *testing.T) {
+	t.Parallel()
+	if got, want := seedISOName("proxy-abc"), "proxy-abc-seed.iso"; got != want {
+		t.Errorf("seedISOName = %q, want %q", got, want)
+	}
+	if got, want := seedVolID("proxy-seeds", "proxy-abc"), "proxy-seeds:iso/proxy-abc-seed.iso"; got != want {
+		t.Errorf("seedVolID = %q, want %q", got, want)
+	}
+}
+
+func TestBuildSeed_table(t *testing.T) {
+	t.Parallel()
+	off := false
+	userData := "#cloud-config\r\npackages: [squid]\r\nwrite_files:\n  - content: \"caf\xc3\xa9\"\n"
+	configured := "#cloud-config\nntp:\n  enabled: true\n"
+	tests := []struct {
+		name       string
+		vendorData string
+		install    *bool
+		want       func(t *testing.T, got string)
+	}{
+		{name: "agent script by default", want: func(t *testing.T, got string) {
+			if got != nocloud.GuestAgentScript {
+				t.Errorf("vendor-data is not the guest agent script: %q", got)
+			}
+		}},
+		{name: "configured joined with the agent script", vendorData: configured, want: func(t *testing.T, got string) {
+			if !strings.HasPrefix(got, "MIME-Version: 1.0\r\n") || !strings.Contains(got, configured) || !strings.Contains(got, nocloud.GuestAgentScript) {
+				t.Errorf("vendor-data is not a multipart of both documents: %q", got)
+			}
+		}},
+		{name: "agent off passes configured through", vendorData: configured, install: &off, want: func(t *testing.T, got string) {
+			if got != configured {
+				t.Errorf("vendor-data = %q, want the configured document", got)
+			}
+		}},
+		{name: "agent off with nothing configured is empty", install: &off, want: func(t *testing.T, got string) {
+			if got != "" {
+				t.Errorf("vendor-data = %q, want empty", got)
+			}
+		}},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+			cfg := testConfig()
+			cfg.VendorData = tc.vendorData
+			cfg.InstallGuestAgent = tc.install
+			p := newProvider("proxmox-home", cfg, nil)
+			iso, err := p.buildSeed(provider.CreateRequest{Name: "proxy-abc", CloudInit: userData})
+			if err != nil {
+				t.Fatalf("buildSeed error = %v", err)
+			}
+			label, files := seedFiles(t, iso)
+			if label != nocloud.SeedLabel {
+				t.Errorf("label = %q, want %q", label, nocloud.SeedLabel)
+			}
+			if files["user-data"] != userData {
+				t.Errorf("user-data changed:\n got %q\nwant %q", files["user-data"], userData)
+			}
+			if want := "instance-id: proxy-abc\nlocal-hostname: proxy-abc\n"; files["meta-data"] != want {
+				t.Errorf("meta-data = %q, want %q", files["meta-data"], want)
+			}
+			tc.want(t, files["vendor-data"])
+		})
+	}
+}
+
+func TestBuildSeed_emptyNameIsPermanentWithoutPayload(t *testing.T) {
+	t.Parallel()
+	const sentinel = "SENTINEL-cloud-init-payload"
+	p := newProvider("proxmox-home", testConfig(), nil)
+	_, err := p.buildSeed(provider.CreateRequest{CloudInit: sentinel})
+	if !errors.Is(err, provider.ErrPermanent) {
+		t.Fatalf("buildSeed error = %v, want ErrPermanent", err)
+	}
+	if strings.Contains(err.Error(), sentinel) {
+		t.Errorf("error carries the cloud-init payload: %v", err)
+	}
+}
+
+// Not parallel: it points TMPDIR at a path that cannot exist for the
+// process.
+func TestBuildSeed_stagingFailureIsTransient(t *testing.T) {
+	t.Setenv("TMPDIR", t.TempDir()+"/missing")
+	p := newProvider("proxmox-home", testConfig(), nil)
+	_, err := p.buildSeed(provider.CreateRequest{Name: "proxy-abc", CloudInit: "#cloud-config\n"})
+	if err == nil {
+		t.Fatal("buildSeed with no staging dir succeeded")
+	}
+	if got := classify("create", err); got != provider.ErrTransient {
+		t.Errorf("classify(buildSeed error) = %v, want ErrTransient: %v", got, err)
+	}
+}