diff --git a/internal/sync/debris_test.go b/internal/sync/debris_test.go
new file mode 100644
index 0000000..77d6d1f
--- /dev/null
+++ b/internal/sync/debris_test.go
@@ -0,0 +1,86 @@
+package sync
+
+import (
+	"context"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/tawanorg/claude-sync/internal/config"
+)
+
+func TestIsExcludedSkipsPerMachineDebris(t *testing.T) {
+	s := &Syncer{cfg: &config.Config{}}
+
+	excluded := []string{
+		"tasks/6bf39315-3333-4394-86bb-2d57f8759b5a/.lock",
+		".lock",
+		"projects/p1/sess.jsonl.conflict.20260729-145427",
+		"tasks/x/3.json.conflict.20260101-000000",
+	}
+	for _, p := range excluded {
+		if !s.isExcluded(p) {
+			t.Errorf("expected %q to be excluded as per-machine debris", p)
+		}
+	}
+
+	// Real data must never be caught. The .conflict rule in particular is
+	// anchored on the exact timestamp format this tool writes, because
+	// excluding an already-tracked file prunes it from the bucket on the
+	// next push — a loose pattern would delete user data remotely.
+	kept := []string{
+		"projects/p1/sess.jsonl",
+		"tasks/x/3.json",
+		"CLAUDE.md",
+		"projects/notes.conflict.md",              // ".conflict." but not our artifact
+		"projects/p1/my.conflict.2026.txt",        // wrong timestamp shape
+		"projects/p1/sess.jsonl.conflict.2026072", // truncated timestamp
+		"projects/lockfiles/package.lock",         // ends in .lock but is not a bare .lock
+		"projects/p1/.lockfile",
+	}
+	for _, p := range kept {
+		if s.isExcluded(p) {
+			t.Errorf("expected %q NOT to be excluded", p)
+		}
+	}
+}
+
+func TestIsExcludedStillHonorsUserPatterns(t *testing.T) {
+	s := &Syncer{cfg: &config.Config{Exclude: []string{"**/*.tmp"}}}
+	if !s.isExcluded("projects/p1/scratch.tmp") {
+		t.Error("user exclude patterns must still apply")
+	}
+	if s.isExcluded("projects/p1/keep.txt") {
+		t.Error("unrelated file must not be excluded")
+	}
+}
+
+func TestPushSkipsDebrisFiles(t *testing.T) {
+	env := setupTestEnv(t)
+	ctx := context.Background()
+
+	writeFile(t, env.claudeDir, "projects/p1/sess.jsonl", "{\"uuid\":\"u1\"}\n")
+	writeFile(t, env.claudeDir, "tasks/t1/.lock", "")
+	writeFile(t, env.claudeDir, "projects/p1/sess.jsonl.conflict.20260729-145427", "stale remote copy\n")
+
+	result, err := env.syncer.Push(ctx)
+	if err != nil {
+		t.Fatalf("Push failed: %v", err)
+	}
+	if len(result.Uploaded) != 1 || result.Uploaded[0] != "projects/p1/sess.jsonl" {
+		t.Errorf("only the transcript should upload, got %v", result.Uploaded)
+	}
+
+	objs, err := env.store.ListUserObjects(ctx)
+	if err != nil {
+		t.Fatalf("list: %v", err)
+	}
+	if len(objs) != 1 {
+		t.Errorf("expected 1 remote object, got %d: %v", len(objs), objs)
+	}
+
+	// The debris stays on disk; it is simply never published.
+	if _, err := os.Stat(filepath.Join(env.claudeDir, "tasks", "t1", ".lock")); err != nil {
+		t.Errorf("local .lock must not be removed: %v", err)
+	}
+}
diff --git a/internal/sync/sync.go b/internal/sync/sync.go
index 0a45bec..7169cdd 100644
--- a/internal/sync/sync.go
+++ b/internal/sync/sync.go
@@ -9,6 +9,7 @@ import (
 	"io"
 	"os"
 	"path/filepath"
+	"regexp"
 	"strings"
 	"sync"
 	"sync/atomic"
@@ -26,6 +27,14 @@ import (
 
 const defaultWorkers = 10
 
+// conflictArtifactRe matches only this tool's own conflict artifacts
+// (<name>.conflict.<yyyymmdd>-<hhmmss>, the format handleConflict writes),
+// never user files that merely contain ".conflict." in their names. The
+// anchoring matters: excluding a path that is already tracked in state makes
+// the next push prune it from the bucket, so a loose pattern would turn into
+// remote deletion of real data.
+var conflictArtifactRe = regexp.MustCompile(`\.conflict\.\d{8}-\d{6}$`)
+
 // maxDecompressedSize is the maximum allowed size for decompressed data (500MB).
 // This prevents decompression bomb attacks from consuming excessive memory.
 const maxDecompressedSize = 500 * 1024 * 1024
@@ -146,6 +155,15 @@ func (s *Syncer) progress(event ProgressEvent) {
 }
 
 func (s *Syncer) isExcluded(relPath string) bool {
+	// Per-machine debris never syncs, regardless of user excludes. A .lock is
+	// a local process lock — on another machine it is indistinguishable from a
+	// lock genuinely held there. A .conflict.<ts> file is this tool's own local
+	// recovery artifact; uploading it replicates the artifact to every machine,
+	// where each copy can itself be re-detected and spawn further ones.
+	base := filepath.Base(relPath)
+	if base == ".lock" || conflictArtifactRe.MatchString(base) {
+		return true
+	}
 	return s.cfg.IsExcluded(relPath)
 }