diff --git a/docs/internal/artifacts.md b/docs/internal/artifacts.md
index 409ea58..d424f01 100644
--- a/docs/internal/artifacts.md
+++ b/docs/internal/artifacts.md
@@ -220,6 +220,21 @@ Caller policy is intentionally outside artifacts helpers:
 - some callers downgrade missing state to status/findings;
 - some callers skip optional behavior when state is missing.
 
+## Remote Lock Objects
+
+The artifacts package defines the independent remote-lock payload and identity
+contract. Each canonical publish source maps to one object below the validated
+session `locks/` prefix; its filename is the lowercase SHA-256 digest of the
+exact source followed by `.lock`.
+
+Lock payloads are strict versioned JSON containing the canonical source, an
+optional reason, and a UTC creation time. Reads are bounded to
+`MaxRemoteLockObjectBytes` (64 KiB), reject unknown fields and trailing values,
+and verify that the payload source matches the key digest. Exact lookup reports
+presence explicitly, prefix listing validates every object and sorts by source,
+creation uses an absence precondition, forced replacement uses one observed
+generation, and removal is idempotent.
+
 ## Key Path Helpers
 
 `internal/artifacts/paths.go` and S3-key helpers define canonical helpers for:
diff --git a/docs/roadmap/implementation.md b/docs/roadmap/implementation.md
index 33c6bc2..f303cce 100644
--- a/docs/roadmap/implementation.md
+++ b/docs/roadmap/implementation.md
@@ -110,7 +110,7 @@ Run `go test ./internal/adapters/storage`,
 
 ## Stage 2 — Define Independent Lock Objects
 
-**Status:** Pending
+**Status:** Completed
 
 ### Goal
 
diff --git a/internal/artifacts/remote_lock.go b/internal/artifacts/remote_lock.go
new file mode 100644
index 0000000..621a267
--- /dev/null
+++ b/internal/artifacts/remote_lock.go
@@ -0,0 +1,328 @@
+package artifacts
+
+import (
+	"bytes"
+	"context"
+	"crypto/sha256"
+	"encoding/hex"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"os"
+	"path"
+	"sort"
+	"strings"
+	"time"
+
+	"gitea.maximumdirect.net/eric/narratio/internal/adapters/storage"
+	"gitea.maximumdirect.net/eric/narratio/internal/artifactpolicy"
+	"gitea.maximumdirect.net/eric/narratio/internal/config"
+)
+
+const (
+	RemoteLockFormatVersion  = 1
+	MaxRemoteLockObjectBytes = 64 << 10
+)
+
+const remoteLocksSegment = "locks"
+
+// RemoteLock records an operator lock on one canonical publish source.
+type RemoteLock struct {
+	FormatVersion int
+	Source        string
+	Reason        string
+	CreatedAt     time.Time
+}
+
+// RemoteLockLookup distinguishes an absent lock from a present validated lock.
+type RemoteLockLookup struct {
+	Lock       RemoteLock
+	Present    bool
+	Generation string
+}
+
+type remoteLockDocument struct {
+	FormatVersion int    `json:"format_version"`
+	Source        string `json:"source"`
+	Reason        string `json:"reason,omitempty"`
+	CreatedAt     string `json:"created_at"`
+}
+
+// NewRemoteLock constructs and validates one lock payload.
+func NewRemoteLock(source, reason string, createdAt time.Time) (RemoteLock, error) {
+	lock := RemoteLock{
+		FormatVersion: RemoteLockFormatVersion,
+		Source:        source,
+		Reason:        reason,
+		CreatedAt:     createdAt,
+	}
+	if err := lock.Validate(); err != nil {
+		return RemoteLock{}, err
+	}
+	return lock, nil
+}
+
+// Validate checks the complete canonical lock payload contract.
+func (l RemoteLock) Validate() error {
+	if l.FormatVersion != RemoteLockFormatVersion {
+		return fmt.Errorf("unsupported remote lock format version %d", l.FormatVersion)
+	}
+	if err := validateRemoteLockSource(l.Source); err != nil {
+		return fmt.Errorf("remote lock source: %w", err)
+	}
+	if strings.TrimSpace(l.Reason) != l.Reason {
+		return fmt.Errorf("remote lock reason must not have surrounding whitespace")
+	}
+	if l.CreatedAt.IsZero() {
+		return fmt.Errorf("remote lock created_at is required")
+	}
+	_, offset := l.CreatedAt.Zone()
+	if offset != 0 {
+		return fmt.Errorf("remote lock created_at must be UTC")
+	}
+	return nil
+}
+
+// EncodeRemoteLock returns the canonical bounded JSON representation of a lock.
+func EncodeRemoteLock(lock RemoteLock) ([]byte, error) {
+	if err := lock.Validate(); err != nil {
+		return nil, err
+	}
+	document := remoteLockDocument{
+		FormatVersion: lock.FormatVersion,
+		Source:        lock.Source,
+		Reason:        lock.Reason,
+		CreatedAt:     lock.CreatedAt.UTC().Format(time.RFC3339Nano),
+	}
+	data, err := json.Marshal(document)
+	if err != nil {
+		return nil, fmt.Errorf("encode remote lock: %w", err)
+	}
+	if len(data) > MaxRemoteLockObjectBytes {
+		return nil, fmt.Errorf("encode remote lock: payload exceeds %d-byte limit", MaxRemoteLockObjectBytes)
+	}
+	return data, nil
+}
+
+// DecodeRemoteLock strictly decodes and validates one bounded lock payload.
+func DecodeRemoteLock(data []byte) (RemoteLock, error) {
+	if len(data) > MaxRemoteLockObjectBytes {
+		return RemoteLock{}, fmt.Errorf("decode remote lock: payload exceeds %d-byte limit", MaxRemoteLockObjectBytes)
+	}
+	var document remoteLockDocument
+	if err := decodeStrictJSON(data, &document); err != nil {
+		return RemoteLock{}, fmt.Errorf("decode remote lock: %w", err)
+	}
+	createdAt, err := time.Parse(time.RFC3339Nano, document.CreatedAt)
+	if err != nil {
+		return RemoteLock{}, fmt.Errorf("decode remote lock created_at: %w", err)
+	}
+	if !strings.HasSuffix(document.CreatedAt, "Z") || createdAt.UTC().Format(time.RFC3339Nano) != document.CreatedAt {
+		return RemoteLock{}, fmt.Errorf("decode remote lock created_at: must be canonical RFC 3339 UTC")
+	}
+	lock := RemoteLock{
+		FormatVersion: document.FormatVersion,
+		Source:        document.Source,
+		Reason:        document.Reason,
+		CreatedAt:     createdAt,
+	}
+	if err := lock.Validate(); err != nil {
+		return RemoteLock{}, fmt.Errorf("decode remote lock: %w", err)
+	}
+	return lock, nil
+}
+
+// RemoteLockKey returns the independent lock key for one canonical source.
+func RemoteLockKey(sessionPrefix, source string) (string, error) {
+	locksPrefix, err := remoteLocksPrefix(sessionPrefix)
+	if err != nil {
+		return "", err
+	}
+	if err := validateRemoteLockSource(source); err != nil {
+		return "", err
+	}
+	return locksPrefix + remoteLockDigest(source) + ".lock", nil
+}
+
+// LookupRemoteLock opens and validates the lock for one source when present.
+func LookupRemoteLock(ctx context.Context, store storage.ObjectStore, sessionPrefix, source string) (RemoteLockLookup, error) {
+	if store == nil {
+		return RemoteLockLookup{}, fmt.Errorf("lookup remote lock: object store is required")
+	}
+	key, err := RemoteLockKey(sessionPrefix, source)
+	if err != nil {
+		return RemoteLockLookup{}, fmt.Errorf("lookup remote lock: %w", err)
+	}
+	return lookupRemoteLockAtKey(ctx, store, key, source)
+}
+
+// ListRemoteLocks validates every object in the reserved lock prefix and sorts
+// the resulting locks by source.
+func ListRemoteLocks(ctx context.Context, store storage.ObjectStore, sessionPrefix string) ([]RemoteLock, error) {
+	if store == nil {
+		return nil, fmt.Errorf("list remote locks: object store is required")
+	}
+	locksPrefix, err := remoteLocksPrefix(sessionPrefix)
+	if err != nil {
+		return nil, fmt.Errorf("list remote locks: %w", err)
+	}
+	objects, err := store.List(ctx, locksPrefix)
+	if err != nil {
+		return nil, fmt.Errorf("list remote locks under %q: %w", locksPrefix, err)
+	}
+	locks := make([]RemoteLock, 0, len(objects))
+	seenSources := make(map[string]struct{}, len(objects))
+	for _, object := range objects {
+		digest, err := remoteLockDigestFromKey(locksPrefix, object.Key)
+		if err != nil {
+			return nil, fmt.Errorf("list remote locks: %w", err)
+		}
+		info, data, err := storage.ReadObjectBounded(ctx, store, object.Key, MaxRemoteLockObjectBytes)
+		if err != nil {
+			return nil, fmt.Errorf("read remote lock %q: %w", object.Key, err)
+		}
+		if info.Key != "" && info.Key != object.Key {
+			return nil, fmt.Errorf("read remote lock %q: store returned key %q", object.Key, info.Key)
+		}
+		lock, err := DecodeRemoteLock(data)
+		if err != nil {
+			return nil, fmt.Errorf("read remote lock %q: %w", object.Key, err)
+		}
+		if actual := remoteLockDigest(lock.Source); actual != digest {
+			return nil, fmt.Errorf("remote lock %q source hash mismatch", object.Key)
+		}
+		if _, duplicate := seenSources[lock.Source]; duplicate {
+			return nil, fmt.Errorf("remote lock source %q is duplicated", lock.Source)
+		}
+		seenSources[lock.Source] = struct{}{}
+		locks = append(locks, lock)
+	}
+	sort.Slice(locks, func(i, j int) bool { return locks[i].Source < locks[j].Source })
+	return locks, nil
+}
+
+// AddRemoteLock conditionally creates one lock. With force, one observed lock
+// generation may be replaced; conflicts are returned for operator retry.
+func AddRemoteLock(ctx context.Context, store storage.ObjectStore, sessionPrefix string, lock RemoteLock, force bool) error {
+	if store == nil {
+		return fmt.Errorf("add remote lock: object store is required")
+	}
+	data, err := EncodeRemoteLock(lock)
+	if err != nil {
+		return fmt.Errorf("add remote lock: %w", err)
+	}
+	key, err := RemoteLockKey(sessionPrefix, lock.Source)
+	if err != nil {
+		return fmt.Errorf("add remote lock: %w", err)
+	}
+	condition := storage.WriteCondition{RequireAbsent: true}
+	if force {
+		current, err := lookupRemoteLockAtKey(ctx, store, key, lock.Source)
+		if err != nil {
+			return fmt.Errorf("add remote lock %q: %w", lock.Source, err)
+		}
+		if current.Present {
+			if strings.TrimSpace(current.Generation) == "" {
+				return fmt.Errorf("add remote lock %q: current generation is required for replacement", lock.Source)
+			}
+			condition = storage.WriteCondition{MatchETag: current.Generation}
+		}
+	}
+	_, err = store.UploadConditional(ctx, bytes.NewReader(data), key, storage.UploadOptions{ContentType: "application/json"}, condition)
+	if err != nil {
+		return fmt.Errorf("add remote lock %q: %w", lock.Source, err)
+	}
+	return nil
+}
+
+// RemoveRemoteLock idempotently removes the lock for one canonical source.
+func RemoveRemoteLock(ctx context.Context, store storage.ObjectStore, sessionPrefix, source string) error {
+	if store == nil {
+		return fmt.Errorf("remove remote lock: object store is required")
+	}
+	key, err := RemoteLockKey(sessionPrefix, source)
+	if err != nil {
+		return fmt.Errorf("remove remote lock: %w", err)
+	}
+	if err := store.Delete(ctx, key); err != nil {
+		return fmt.Errorf("remove remote lock %q: %w", source, err)
+	}
+	return nil
+}
+
+func lookupRemoteLockAtKey(ctx context.Context, store storage.ObjectStore, key, source string) (RemoteLockLookup, error) {
+	info, data, err := storage.ReadObjectBounded(ctx, store, key, MaxRemoteLockObjectBytes)
+	if errors.Is(err, os.ErrNotExist) {
+		return RemoteLockLookup{}, nil
+	}
+	if err != nil {
+		return RemoteLockLookup{}, fmt.Errorf("read object %q: %w", key, err)
+	}
+	lock, err := DecodeRemoteLock(data)
+	if err != nil {
+		return RemoteLockLookup{}, fmt.Errorf("read object %q: %w", key, err)
+	}
+	if lock.Source != source || remoteLockDigest(lock.Source) != strings.TrimSuffix(path.Base(key), ".lock") {
+		return RemoteLockLookup{}, fmt.Errorf("remote lock %q source hash mismatch", key)
+	}
+	return RemoteLockLookup{Lock: lock, Present: true, Generation: info.ETag}, nil
+}
+
+func validateRemoteLockSource(source string) error {
+	classified, err := artifactpolicy.ClassifySource(source)
+	if err != nil || classified.Kind == artifactpolicy.SourceKindPreviousArtifact {
+		return fmt.Errorf("source %q is not a supported publish source", source)
+	}
+	if classified.ID != source {
+		return fmt.Errorf("source %q is not canonical", source)
+	}
+	return nil
+}
+
+func remoteLocksPrefix(sessionPrefix string) (string, error) {
+	if err := validateRemoteLockSessionPrefix(sessionPrefix); err != nil {
+		return "", err
+	}
+	return sessionPrefix + remoteLocksSegment + "/", nil
+}
+
+func validateRemoteLockSessionPrefix(sessionPrefix string) error {
+	if sessionPrefix == "" || strings.TrimSpace(sessionPrefix) != sessionPrefix || strings.Contains(sessionPrefix, `\`) || !strings.HasSuffix(sessionPrefix, "/") {
+		return fmt.Errorf("session prefix %q must be a canonical relative object prefix", sessionPrefix)
+	}
+	withoutSlash := strings.TrimSuffix(sessionPrefix, "/")
+	if err := validateRemoteObjectKey(withoutSlash); err != nil {
+		return fmt.Errorf("session prefix %q: %w", sessionPrefix, err)
+	}
+	parts := strings.Split(withoutSlash, "/")
+	if len(parts) < 4 || parts[len(parts)-4] != config.S3CampaignsSegment || parts[len(parts)-2] != config.S3SessionsSegment {
+		return fmt.Errorf("session prefix %q does not identify a campaign session", sessionPrefix)
+	}
+	if err := ValidateSessionIdentity(parts[len(parts)-3], parts[len(parts)-1]); err != nil {
+		return fmt.Errorf("session prefix %q: %w", sessionPrefix, err)
+	}
+	return nil
+}
+
+func remoteLockDigest(source string) string {
+	sum := sha256.Sum256([]byte(source))
+	return hex.EncodeToString(sum[:])
+}
+
+func remoteLockDigestFromKey(prefix, key string) (string, error) {
+	if err := validateRemoteObjectKey(key); err != nil {
+		return "", fmt.Errorf("invalid remote lock key %q: %w", key, err)
+	}
+	if !strings.HasPrefix(key, prefix) {
+		return "", fmt.Errorf("unexpected object %q outside remote lock prefix %q", key, prefix)
+	}
+	name := strings.TrimPrefix(key, prefix)
+	if strings.Contains(name, "/") || !strings.HasSuffix(name, ".lock") {
+		return "", fmt.Errorf("unexpected object %q in remote lock prefix", key)
+	}
+	digest := strings.TrimSuffix(name, ".lock")
+	if err := validateSHA256(digest); err != nil {
+		return "", fmt.Errorf("unexpected remote lock object %q: %w", key, err)
+	}
+	return digest, nil
+}
diff --git a/internal/artifacts/remote_lock_test.go b/internal/artifacts/remote_lock_test.go
new file mode 100644
index 0000000..f12f882
--- /dev/null
+++ b/internal/artifacts/remote_lock_test.go
@@ -0,0 +1,281 @@
+package artifacts
+
+import (
+	"context"
+	"errors"
+	"strings"
+	"testing"
+	"time"
+
+	"gitea.maximumdirect.net/eric/narratio/internal/adapters/storage"
+)
+
+const testRemoteLockSessionPrefix = "dnd/campaigns/forsaken/sessions/2026-09-05/"
+
+func TestRemoteLockKeyUsesCanonicalSourceDigest(t *testing.T) {
+	tests := []struct {
+		source string
+		digest string
+	}{
+		{source: "narratio.artifact.session_recap", digest: "cf21d18edf63a8c5b5e63a7e205510dcaf328fc6d7ab4e00ee264683e1b62655"},
+		{source: "narratio.transcript.final_trimmed", digest: "8896c4f6e4fb1f677b6757d232d2ca72c285a4471123bb0437c9830d9cf5fe94"},
+		{source: "narratio.extraction.encounters", digest: "c14d3512feaf44f651ff4039ce27914c619816de87121a739bb9bb75a84ab35d"},
+	}
+	for _, test := range tests {
+		t.Run(test.source, func(t *testing.T) {
+			got, err := RemoteLockKey(testRemoteLockSessionPrefix, test.source)
+			if err != nil {
+				t.Fatalf("RemoteLockKey() error = %v", err)
+			}
+			want := testRemoteLockSessionPrefix + "locks/" + test.digest + ".lock"
+			if got != want {
+				t.Fatalf("RemoteLockKey() = %q, want %q", got, want)
+			}
+		})
+	}
+}
+
+func TestRemoteLockKeyRejectsNoncanonicalInputs(t *testing.T) {
+	for _, test := range []struct {
+		name   string
+		prefix string
+		source string
+	}{
+		{name: "prefix without slash", prefix: strings.TrimSuffix(testRemoteLockSessionPrefix, "/"), source: "narratio.artifact.recap"},
+		{name: "non-session prefix", prefix: "dnd/campaigns/forsaken/", source: "narratio.artifact.recap"},
+		{name: "traversal prefix", prefix: "dnd/campaigns/forsaken/sessions/../", source: "narratio.artifact.recap"},
+		{name: "source whitespace", prefix: testRemoteLockSessionPrefix, source: " narratio.artifact.recap"},
+		{name: "previous source", prefix: testRemoteLockSessionPrefix, source: "narratio.previous_session.artifact.recap"},
+		{name: "unsupported source", prefix: testRemoteLockSessionPrefix, source: "narratio.artifact.Bad"},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			if _, err := RemoteLockKey(test.prefix, test.source); err == nil {
+				t.Fatal("RemoteLockKey() error = nil, want validation failure")
+			}
+		})
+	}
+}
+
+func TestRemoteLockEncodingIsStrictAndCanonical(t *testing.T) {
+	createdAt := time.Date(2026, 9, 5, 12, 34, 56, 123000000, time.UTC)
+	lock, err := NewRemoteLock("narratio.artifact.session_recap", "manual edits", createdAt)
+	if err != nil {
+		t.Fatalf("NewRemoteLock() error = %v", err)
+	}
+	data, err := EncodeRemoteLock(lock)
+	if err != nil {
+		t.Fatalf("EncodeRemoteLock() error = %v", err)
+	}
+	want := `{"format_version":1,"source":"narratio.artifact.session_recap","reason":"manual edits","created_at":"2026-09-05T12:34:56.123Z"}`
+	if string(data) != want {
+		t.Fatalf("EncodeRemoteLock() = %s, want %s", data, want)
+	}
+	decoded, err := DecodeRemoteLock(data)
+	if err != nil {
+		t.Fatalf("DecodeRemoteLock() error = %v", err)
+	}
+	if decoded.FormatVersion != lock.FormatVersion || decoded.Source != lock.Source || decoded.Reason != lock.Reason || !decoded.CreatedAt.Equal(lock.CreatedAt) {
+		t.Fatalf("DecodeRemoteLock() = %#v, want %#v", decoded, lock)
+	}
+}
+
+func TestDecodeRemoteLockRejectsMalformedContracts(t *testing.T) {
+	valid := `{"format_version":1,"source":"narratio.artifact.session_recap","created_at":"2026-09-05T12:34:56Z"}`
+	tests := []struct {
+		name string
+		data string
+	}{
+		{name: "unknown field", data: strings.Replace(valid, `}`, `,"extra":true}`, 1)},
+		{name: "trailing value", data: valid + `{}`},
+		{name: "unsupported version", data: strings.Replace(valid, `"format_version":1`, `"format_version":2`, 1)},
+		{name: "noncanonical source", data: strings.Replace(valid, `narratio.artifact.session_recap`, ` narratio.artifact.session_recap`, 1)},
+		{name: "previous source", data: strings.Replace(valid, `narratio.artifact.session_recap`, `narratio.previous_session.artifact.session_recap`, 1)},
+		{name: "non-UTC timestamp", data: strings.Replace(valid, `2026-09-05T12:34:56Z`, `2026-09-05T12:34:56+00:00`, 1)},
+		{name: "invalid timestamp", data: strings.Replace(valid, `2026-09-05T12:34:56Z`, `yesterday`, 1)},
+		{name: "reason whitespace", data: strings.Replace(valid, `,"created_at"`, `,"reason":" padded ","created_at"`, 1)},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			if _, err := DecodeRemoteLock([]byte(test.data)); err == nil {
+				t.Fatal("DecodeRemoteLock() error = nil, want validation failure")
+			}
+		})
+	}
+}
+
+func TestRemoteLockEncodingAndDecodingEnforceSizeLimit(t *testing.T) {
+	lock, err := NewRemoteLock("narratio.artifact.session_recap", strings.Repeat("x", MaxRemoteLockObjectBytes), time.Now().UTC())
+	if err != nil {
+		t.Fatalf("NewRemoteLock() error = %v", err)
+	}
+	if _, err := EncodeRemoteLock(lock); err == nil {
+		t.Fatal("EncodeRemoteLock() error = nil, want size failure")
+	}
+	if _, err := DecodeRemoteLock(make([]byte, MaxRemoteLockObjectBytes+1)); err == nil {
+		t.Fatal("DecodeRemoteLock() error = nil, want size failure")
+	}
+}
+
+func TestLookupRemoteLockReportsPresenceAndAbsence(t *testing.T) {
+	store := &storage.FakeBackend{}
+	lookup, err := LookupRemoteLock(context.Background(), store, testRemoteLockSessionPrefix, "narratio.artifact.session_recap")
+	if err != nil || lookup.Present {
+		t.Fatalf("LookupRemoteLock() absent = %#v, error = %v", lookup, err)
+	}
+
+	lock := mustTestRemoteLock(t, "narratio.artifact.session_recap", "manual edits")
+	seedRemoteLock(t, store, testRemoteLockSessionPrefix, lock)
+	lookup, err = LookupRemoteLock(context.Background(), store, testRemoteLockSessionPrefix, lock.Source)
+	if err != nil {
+		t.Fatalf("LookupRemoteLock() error = %v", err)
+	}
+	if !lookup.Present || lookup.Lock.Source != lock.Source || lookup.Generation == "" {
+		t.Fatalf("LookupRemoteLock() = %#v, want present validated lock", lookup)
+	}
+}
+
+func TestListRemoteLocksSortsAndValidatesObjects(t *testing.T) {
+	store := &storage.FakeBackend{}
+	seedRemoteLock(t, store, testRemoteLockSessionPrefix, mustTestRemoteLock(t, "narratio.transcript.final_trimmed", ""))
+	seedRemoteLock(t, store, testRemoteLockSessionPrefix, mustTestRemoteLock(t, "narratio.artifact.session_recap", ""))
+
+	locks, err := ListRemoteLocks(context.Background(), store, testRemoteLockSessionPrefix)
+	if err != nil {
+		t.Fatalf("ListRemoteLocks() error = %v", err)
+	}
+	if len(locks) != 2 || locks[0].Source != "narratio.artifact.session_recap" || locks[1].Source != "narratio.transcript.final_trimmed" {
+		t.Fatalf("ListRemoteLocks() = %#v, want source-sorted locks", locks)
+	}
+}
+
+func TestListRemoteLocksRejectsInvalidPrefixContents(t *testing.T) {
+	validLock := mustTestRemoteLock(t, "narratio.artifact.session_recap", "")
+	validData, err := EncodeRemoteLock(validLock)
+	if err != nil {
+		t.Fatal(err)
+	}
+	validKey, err := RemoteLockKey(testRemoteLockSessionPrefix, validLock.Source)
+	if err != nil {
+		t.Fatal(err)
+	}
+	tests := []struct {
+		name string
+		key  string
+		data []byte
+	}{
+		{name: "unexpected name", key: testRemoteLockSessionPrefix + "locks/index.json", data: validData},
+		{name: "nested object", key: testRemoteLockSessionPrefix + "locks/nested/" + strings.TrimPrefix(validKey, testRemoteLockSessionPrefix+"locks/"), data: validData},
+		{name: "hash mismatch", key: testRemoteLockSessionPrefix + "locks/8896c4f6e4fb1f677b6757d232d2ca72c285a4471123bb0437c9830d9cf5fe94.lock", data: validData},
+		{name: "oversize payload", key: validKey, data: make([]byte, MaxRemoteLockObjectBytes+1)},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			store := &storage.FakeBackend{}
+			store.SeedObject(storage.FakeObject{Key: test.key, Data: test.data})
+			if _, err := ListRemoteLocks(context.Background(), store, testRemoteLockSessionPrefix); err == nil {
+				t.Fatal("ListRemoteLocks() error = nil, want validation failure")
+			}
+		})
+	}
+}
+
+func TestListRemoteLocksRejectsDuplicateSources(t *testing.T) {
+	store := &storage.FakeBackend{}
+	lock := mustTestRemoteLock(t, "narratio.artifact.session_recap", "")
+	seedRemoteLock(t, store, testRemoteLockSessionPrefix, lock)
+	key, _ := RemoteLockKey(testRemoteLockSessionPrefix, lock.Source)
+	duplicating := &duplicateRemoteLockListing{FakeBackend: store, key: key}
+	if _, err := ListRemoteLocks(context.Background(), duplicating, testRemoteLockSessionPrefix); err == nil || !strings.Contains(err.Error(), "duplicated") {
+		t.Fatalf("ListRemoteLocks() error = %v, want duplicate-source failure", err)
+	}
+}
+
+func TestAddRemoteLockConditionalCreateAndForcedReplacement(t *testing.T) {
+	ctx := context.Background()
+	store := &storage.FakeBackend{}
+	original := mustTestRemoteLock(t, "narratio.artifact.session_recap", "first")
+	if err := AddRemoteLock(ctx, store, testRemoteLockSessionPrefix, original, false); err != nil {
+		t.Fatalf("AddRemoteLock() create error = %v", err)
+	}
+	if err := AddRemoteLock(ctx, store, testRemoteLockSessionPrefix, original, false); !errors.Is(err, storage.ErrConditionNotMet) {
+		t.Fatalf("AddRemoteLock() duplicate error = %v, want ErrConditionNotMet", err)
+	}
+
+	replacement := mustTestRemoteLock(t, original.Source, "replacement")
+	if err := AddRemoteLock(ctx, store, testRemoteLockSessionPrefix, replacement, true); err != nil {
+		t.Fatalf("AddRemoteLock() force error = %v", err)
+	}
+	lookup, err := LookupRemoteLock(ctx, store, testRemoteLockSessionPrefix, original.Source)
+	if err != nil || !lookup.Present || lookup.Lock.Reason != "replacement" {
+		t.Fatalf("LookupRemoteLock() after replacement = %#v, error = %v", lookup, err)
+	}
+}
+
+func TestAddRemoteLockForcedReplacementReportsGenerationConflict(t *testing.T) {
+	store := &storage.FakeBackend{}
+	original := mustTestRemoteLock(t, "narratio.artifact.session_recap", "first")
+	seedRemoteLock(t, store, testRemoteLockSessionPrefix, original)
+	store.UploadHook = func(storage.FakeUploadCall) error {
+		changed := mustTestRemoteLock(t, original.Source, "concurrent")
+		seedRemoteLock(t, store, testRemoteLockSessionPrefix, changed)
+		store.UploadHook = nil
+		return nil
+	}
+	err := AddRemoteLock(context.Background(), store, testRemoteLockSessionPrefix, mustTestRemoteLock(t, original.Source, "replacement"), true)
+	if !errors.Is(err, storage.ErrConditionNotMet) {
+		t.Fatalf("AddRemoteLock() error = %v, want ErrConditionNotMet", err)
+	}
+}
+
+func TestRemoveRemoteLockIsIdempotent(t *testing.T) {
+	store := &storage.FakeBackend{}
+	lock := mustTestRemoteLock(t, "narratio.artifact.session_recap", "")
+	seedRemoteLock(t, store, testRemoteLockSessionPrefix, lock)
+	for range 2 {
+		if err := RemoveRemoteLock(context.Background(), store, testRemoteLockSessionPrefix, lock.Source); err != nil {
+			t.Fatalf("RemoveRemoteLock() error = %v", err)
+		}
+	}
+	lookup, err := LookupRemoteLock(context.Background(), store, testRemoteLockSessionPrefix, lock.Source)
+	if err != nil || lookup.Present {
+		t.Fatalf("LookupRemoteLock() after remove = %#v, error = %v", lookup, err)
+	}
+}
+
+type duplicateRemoteLockListing struct {
+	*storage.FakeBackend
+	key string
+}
+
+func (s *duplicateRemoteLockListing) List(ctx context.Context, prefix string) ([]storage.ObjectInfo, error) {
+	objects, err := s.FakeBackend.List(ctx, prefix)
+	if err != nil {
+		return nil, err
+	}
+	if len(objects) == 1 && objects[0].Key == s.key {
+		objects = append(objects, objects[0])
+	}
+	return objects, nil
+}
+
+func mustTestRemoteLock(t *testing.T, source, reason string) RemoteLock {
+	t.Helper()
+	lock, err := NewRemoteLock(source, reason, time.Date(2026, 9, 5, 12, 34, 56, 0, time.UTC))
+	if err != nil {
+		t.Fatalf("NewRemoteLock() error = %v", err)
+	}
+	return lock
+}
+
+func seedRemoteLock(t *testing.T, store *storage.FakeBackend, sessionPrefix string, lock RemoteLock) {
+	t.Helper()
+	key, err := RemoteLockKey(sessionPrefix, lock.Source)
+	if err != nil {
+		t.Fatalf("RemoteLockKey() error = %v", err)
+	}
+	data, err := EncodeRemoteLock(lock)
+	if err != nil {
+		t.Fatalf("EncodeRemoteLock() error = %v", err)
+	}
+	store.SeedObject(storage.FakeObject{Key: key, Data: data})
+}