diff --git a/cmd/server/tests/cross_library_move_test.go b/cmd/server/tests/cross_library_move_test.go
new file mode 100644
index 0000000..c8e562f
--- /dev/null
+++ b/cmd/server/tests/cross_library_move_test.go
@@ -0,0 +1,639 @@
+package main
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"net/http"
+	"os"
+	"path/filepath"
+	"testing"
+	"time"
+
+	"bookhoard/internal/database"
+	"bookhoard/internal/services"
+
+	"github.com/google/uuid"
+	"github.com/jackc/pgx/v5/pgtype"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// Cross-library SHA move detection + archived-twin merge integration tests.
+// These cover the scenario where identical book content moves between
+// libraries of the same type: the row must be repointed (history follows)
+// instead of duplicated, deliberate copies stay independent, type rules are
+// enforced, and archived twins can be merged into their active copy.
+
+const (
+	moveTestAdminEmail = "testuser@tests.bookhoard.internal"
+	moveTestFileName   = "Test Book.epub"
+)
+
+// createEbooksTestLibrary creates a library of the given type via the API and
+// swaps its default folder for a fresh temp directory the test controls.
+func createTypedTestLibrary(t *testing.T, setup *TestServerSetup, name, libType string) (pgtype.UUID, string) {
+	t.Helper()
+	ctx := context.Background()
+
+	libBody, _ := json.Marshal(map[string]interface{}{"name": name, "type": libType})
+	req, _ := http.NewRequest("POST", setup.Server.URL+"/api/libraries", bytes.NewBuffer(libBody))
+	req.Header.Set("Content-Type", "application/json")
+	req.Header.Set("Authorization", "Bearer "+setup.Token)
+	resp, err := http.DefaultClient.Do(req)
+	require.NoError(t, err)
+	defer func() { _ = resp.Body.Close() }()
+	require.Equal(t, http.StatusCreated, resp.StatusCode, "library creation should succeed")
+
+	var libResp map[string]interface{}
+	require.NoError(t, json.NewDecoder(resp.Body).Decode(&libResp))
+	libID, err := uuid.Parse(libResp["id"].(string))
+	require.NoError(t, err)
+	pgLib := pgtype.UUID{Bytes: libID, Valid: true}
+
+	folders, err := setup.DB.GetLibraryFolders(ctx, pgLib)
+	require.NoError(t, err)
+	for _, f := range folders {
+		_, err = setup.DB.DeleteLibraryFolder(ctx, database.DeleteLibraryFolderParams{
+			LibraryID:  pgLib,
+			FolderPath: f.FolderPath,
+		})
+		require.NoError(t, err)
+	}
+
+	tmpDir := t.TempDir()
+	_, err = setup.DB.AddLibraryFolder(ctx, database.AddLibraryFolderParams{
+		LibraryID:  pgLib,
+		FolderPath: tmpDir,
+	})
+	require.NoError(t, err)
+	return pgLib, tmpDir
+}
+
+// deleteLibraries removes libraries (cascading to media items, progress,
+// annotations, and processing issues) so reruns never see leftovers from a
+// previous run — stale rows with identical content hashes would otherwise be
+// picked up as move candidates. Call via defer AFTER the setup.Close defer so
+// it executes while the pool is still open (defers run LIFO).
+func deleteLibraries(t *testing.T, setup *TestServerSetup, libs ...pgtype.UUID) {
+	t.Helper()
+	for _, lib := range libs {
+		_ = setup.DB.DeleteLibrary(context.Background(), lib)
+	}
+}
+
+// scanTestFolder runs a synchronous scan over one folder.
+func scanTestFolder(t *testing.T, setup *TestServerSetup, adminID pgtype.UUID, folder string) {
+	t.Helper()
+	scanner := services.NewMediaScanner(setup.DB)
+	defer func() { _ = scanner.Close() }()
+	scanner.SetAdminID(adminID)
+	require.NoError(t, scanner.SetFolders([]string{folder}, false))
+	require.NoError(t, scanner.ScanFolders(context.Background()))
+}
+
+// waitForCond polls cond until true or the timeout elapses.
+func waitForCond(t *testing.T, timeout time.Duration, msg string, cond func() bool) {
+	t.Helper()
+	deadline := time.Now().Add(timeout)
+	for time.Now().Before(deadline) {
+		if cond() {
+			return
+		}
+		time.Sleep(100 * time.Millisecond)
+	}
+	t.Fatalf("condition not met within %v: %s", timeout, msg)
+}
+
+func moveTestAdminID(t *testing.T, setup *TestServerSetup) (pgtype.UUID, pgtype.UUID) {
+	t.Helper()
+	admin, err := setup.DB.GetUserByEmail(context.Background(), moveTestAdminEmail)
+	require.NoError(t, err)
+	pgAdmin := pgtype.UUID{Bytes: admin.ID.Bytes, Valid: true}
+	return pgAdmin, pgAdmin
+}
+
+// writeTestEbook places an identical dummy epub under folder/Author/.
+func writeTestEbook(t *testing.T, folder string, content []byte) string {
+	t.Helper()
+	authorDir := filepath.Join(folder, "Author")
+	require.NoError(t, os.MkdirAll(authorDir, 0755))
+	bookPath := filepath.Join(authorDir, moveTestFileName)
+	require.NoError(t, os.WriteFile(bookPath, content, 0644))
+	return bookPath
+}
+
+// TestCrossLibraryMovePreservesHistory is the core scenario: a book moves
+// from library A to library B (same type). The existing row must be repointed
+// to library B keeping its ID so progress and annotations survive, instead of
+// creating a duplicate while the old row goes missing.
+func TestCrossLibraryMovePreservesHistory(t *testing.T) {
+	setup := setupTestServer(t)
+	defer func() { _ = setup.Close() }()
+
+	ctx := context.Background()
+	adminID, adminPgID := moveTestAdminID(t, setup)
+	libA, folderA := createTypedTestLibrary(t, setup, "Move Src Library", "ebooks")
+	libB, folderB := createTypedTestLibrary(t, setup, "Move Dst Library", "ebooks")
+	defer deleteLibraries(t, setup, libA, libB)
+
+	content := []byte("cross library move test epub content")
+	writeTestEbook(t, folderA, content)
+
+	scanTestFolder(t, setup, adminID, folderA)
+
+	var original database.ListMediaItemsByLibraryRow
+	waitForCond(t, 5*time.Second, "book should be imported into library A", func() bool {
+		books, err := setup.DB.ListMediaItemsByLibrary(ctx, libA)
+		if err != nil || len(books) != 1 {
+			return false
+		}
+		original = books[0]
+		return true
+	})
+
+	// Give the book real user data at its original location.
+	_, err := setup.DB.UpdateReadingProgress(ctx, database.UpdateReadingProgressParams{
+		MediaItemID: original.ID,
+		UserID:      adminPgID,
+		CurrentPage: pgtype.Int4{Int32: 12, Valid: true},
+		TotalPages:  pgtype.Int4{Int32: 300, Valid: true},
+	})
+	require.NoError(t, err)
+	_, err = setup.DB.CreateMediaHighlight(ctx, database.CreateMediaHighlightParams{
+		MediaItemID:   original.ID,
+		UserID:        adminPgID,
+		SelectionText: "a line worth keeping",
+		Color:         pgtype.Text{String: "#ffff00", Valid: true},
+	})
+	require.NoError(t, err)
+
+	// Move the file: same bytes, new library folder, old copy deleted.
+	writeTestEbook(t, folderB, content)
+	require.NoError(t, os.Remove(filepath.Join(folderA, "Author", moveTestFileName)))
+
+	scanTestFolder(t, setup, adminID, folderB)
+
+	var moved database.ListMediaItemsByLibraryRow
+	waitForCond(t, 5*time.Second, "book should be repointed into library B", func() bool {
+		books, err := setup.DB.ListMediaItemsByLibrary(ctx, libB)
+		if err != nil || len(books) != 1 {
+			return false
+		}
+		moved = books[0]
+		return true
+	})
+
+	assert.Equal(t, original.ID, moved.ID, "row ID must be preserved so history follows the book")
+	assert.Equal(t, libB, moved.LibraryID, "row must now belong to library B")
+	assert.Equal(t, filepath.ToSlash(filepath.Join("Author", moveTestFileName)), moved.FilePath, "path must be repointed relative to library B")
+	assert.False(t, moved.ArchivedAt.Valid, "moved row must not be archived")
+	assert.Equal(t, int32(0), moved.MissingScanCount, "moved row must not be marked missing")
+	assert.True(t, moved.LibraryTypeName.Valid && moved.LibraryTypeName.String == "ebooks", "library_type_name must stay truthful after library change")
+
+	// History followed the row.
+	progress, err := setup.DB.GetReadingProgress(ctx, database.GetReadingProgressParams{MediaItemID: original.ID, UserID: adminPgID})
+	require.NoError(t, err)
+	assert.Equal(t, int32(12), progress.CurrentPage.Int32, "reading progress must survive the move")
+
+	highlights, err := setup.DB.GetMediaHighlights(ctx, database.GetMediaHighlightsParams{MediaItemID: original.ID, UserID: adminPgID})
+	require.NoError(t, err)
+	assert.Len(t, highlights, 1, "annotation must survive the move")
+
+	// Library A no longer holds the row: no duplicate, no archived ghost.
+	booksA, err := setup.DB.ListMediaItemsByLibrary(ctx, libA)
+	require.NoError(t, err)
+	assert.Empty(t, booksA, "library A must not keep a duplicate row after the move")
+}
+
+// TestCrossLibraryCopyStaysIndependent verifies that placing identical
+// content in two libraries on purpose creates two independent rows: the
+// second import is NOT blocked, and neither row is repointed or archived.
+func TestCrossLibraryCopyStaysIndependent(t *testing.T) {
+	setup := setupTestServer(t)
+	defer func() { _ = setup.Close() }()
+
+	ctx := context.Background()
+	adminID, adminPgID := moveTestAdminID(t, setup)
+	libA, folderA := createTypedTestLibrary(t, setup, "Copy Lib A", "ebooks")
+	libB, folderB := createTypedTestLibrary(t, setup, "Copy Lib B", "ebooks")
+	defer deleteLibraries(t, setup, libA, libB)
+
+	content := []byte("deliberate duplicate content")
+	writeTestEbook(t, folderA, content)
+	writeTestEbook(t, folderB, content)
+
+	scanTestFolder(t, setup, adminID, folderA)
+	scanTestFolder(t, setup, adminID, folderB)
+
+	var itemA, itemB database.ListMediaItemsByLibraryRow
+	waitForCond(t, 5*time.Second, "both libraries should hold their own copy", func() bool {
+		booksA, errA := setup.DB.ListMediaItemsByLibrary(ctx, libA)
+		booksB, errB := setup.DB.ListMediaItemsByLibrary(ctx, libB)
+		if errA != nil || errB != nil || len(booksA) != 1 || len(booksB) != 1 {
+			return false
+		}
+		itemA, itemB = booksA[0], booksB[0]
+		return true
+	})
+
+	assert.NotEqual(t, itemA.ID, itemB.ID, "deliberate copies must stay independent rows")
+	assert.False(t, itemA.ArchivedAt.Valid && itemB.ArchivedAt.Valid, "no copy should be archived")
+
+	// History on one copy does not leak into the other.
+	_, err := setup.DB.UpdateReadingProgress(ctx, database.UpdateReadingProgressParams{
+		MediaItemID: itemA.ID,
+		UserID:      adminPgID,
+		CurrentPage: pgtype.Int4{Int32: 7, Valid: true},
+		TotalPages:  pgtype.Int4{Int32: 100, Valid: true},
+	})
+	require.NoError(t, err)
+
+	_, err = setup.DB.GetReadingProgress(ctx, database.GetReadingProgressParams{MediaItemID: itemB.ID, UserID: adminPgID})
+	assert.Error(t, err, "copy B must not inherit copy A's progress")
+}
+
+// TestCrossLibraryMoveRejectedForTypeMismatch verifies the type guard: the
+// cross-library lookup only matches same-type libraries, so a reflowable EPUB
+// registered in a manga library whose file moves to ANOTHER manga library is
+// detected as a move but must be rejected by the type rules (manga libraries
+// only accept fixed-layout/image content). The move is rejected, a processing
+// issue is recorded, and the file stays untracked rather than being
+// force-imported.
+func TestCrossLibraryMoveRejectedForTypeMismatch(t *testing.T) {
+	setup := setupTestServer(t)
+	defer func() { _ = setup.Close() }()
+
+	ctx := context.Background()
+	adminID, _ := moveTestAdminID(t, setup)
+	libA, folderA := createTypedTestLibrary(t, setup, "Type Src Manga", "manga")
+	libM, folderM := createTypedTestLibrary(t, setup, "Type Dst Manga", "manga")
+	defer deleteLibraries(t, setup, libA, libM)
+
+	content := []byte("reflowable epub that manga libraries must not adopt via move")
+	writeTestEbook(t, folderA, content)
+	scanTestFolder(t, setup, adminID, folderA)
+
+	var original database.ListMediaItemsByLibraryRow
+	waitForCond(t, 5*time.Second, "book should be imported into the first manga library", func() bool {
+		books, err := setup.DB.ListMediaItemsByLibrary(ctx, libA)
+		if err != nil || len(books) != 1 {
+			return false
+		}
+		original = books[0]
+		return true
+	})
+	require.Equal(t, "reflowable", original.FormatGroup, "dummy epub must scan as reflowable for this test")
+
+	// Move into the second manga library: same hash, same type, but the
+	// reflowable format violates manga library rules.
+	writeTestEbook(t, folderM, content)
+	require.NoError(t, os.Remove(filepath.Join(folderA, "Author", moveTestFileName)))
+
+	scanTestFolder(t, setup, adminID, folderM)
+
+	// The second manga library must not adopt the item.
+	booksM, err := setup.DB.ListMediaItemsByLibrary(ctx, libM)
+	require.NoError(t, err)
+	assert.Empty(t, booksM, "reflowable epub must not be repointed between manga libraries")
+
+	// The original row is untouched in the first manga library.
+	after, err := setup.DB.GetMediaItem(ctx, original.ID)
+	require.NoError(t, err)
+	assert.Equal(t, libA, after.LibraryID, "original row must stay in its library")
+
+	// A processing issue explains the rejection.
+	waitForCond(t, 5*time.Second, "processing issue should be recorded", func() bool {
+		issues, err := setup.DB.ListProcessingIssuesByLibrary(ctx, libM)
+		if err != nil {
+			return false
+		}
+		for _, issue := range issues {
+			if issue.IssueType == "library_type_mismatch" {
+				return true
+			}
+		}
+		return false
+	})
+}
+
+// TestListHiddenMediaItemsMatchesActiveTwin verifies the archived-page query:
+// an archived row with an active same-hash twin reports it in match_* columns;
+// an archived row without a twin reports no match.
+func TestListHiddenMediaItemsMatchesActiveTwin(t *testing.T) {
+	setup := setupTestServer(t)
+	defer func() { _ = setup.Close() }()
+
+	ctx := context.Background()
+	adminID, _ := moveTestAdminID(t, setup)
+	libA, folderA := createTypedTestLibrary(t, setup, "Twin Lib A", "ebooks")
+	libB, folderB := createTypedTestLibrary(t, setup, "Twin Lib B", "ebooks")
+	libC, folderC := createTypedTestLibrary(t, setup, "Twin Lib C", "ebooks")
+	defer deleteLibraries(t, setup, libA, libB, libC)
+
+	twinContent := []byte("twin content shared by two libraries")
+	writeTestEbook(t, folderA, twinContent)
+	writeTestEbook(t, folderB, twinContent)
+	lonelyContent := []byte("unique content with no twin anywhere")
+	writeTestEbook(t, folderC, lonelyContent)
+
+	scanTestFolder(t, setup, adminID, folderA)
+	scanTestFolder(t, setup, adminID, folderB)
+	scanTestFolder(t, setup, adminID, folderC)
+
+	booksA, err := setup.DB.ListMediaItemsByLibrary(ctx, libA)
+	require.NoError(t, err)
+	require.Len(t, booksA, 1)
+	twin := booksA[0]
+
+	booksC, err := setup.DB.ListMediaItemsByLibrary(ctx, libC)
+	require.NoError(t, err)
+	require.Len(t, booksC, 1)
+	lonely := booksC[0]
+
+	// Archive both: the A row now has an active twin in B, the C row has none.
+	require.NoError(t, setup.DB.ArchiveMediaItem(ctx, twin.ID))
+	require.NoError(t, setup.DB.ArchiveMediaItem(ctx, lonely.ID))
+
+	rows, err := setup.DB.ListHiddenMediaItems(ctx)
+	require.NoError(t, err)
+
+	foundTwin, foundLonely := false, false
+	for _, row := range rows {
+		if row.ID == twin.ID {
+			foundTwin = true
+			assert.True(t, row.MatchID.Valid, "twin row must report its active match")
+			if row.MatchID.Valid {
+				assert.Equal(t, findTwinTargetID(t, setup, libB), uuid.UUID(row.MatchID.Bytes).String())
+			}
+			assert.Equal(t, "Test Book", row.MatchTitle)
+		}
+		if row.ID == lonely.ID {
+			foundLonely = true
+			assert.False(t, row.MatchID.Valid, "row without a twin must report no match")
+		}
+	}
+	require.True(t, foundTwin, "twin row should be listed")
+	require.True(t, foundLonely, "lonely row should be listed")
+}
+
+// findTwinTargetID returns the single active item in the given library.
+func findTwinTargetID(t *testing.T, setup *TestServerSetup, libID pgtype.UUID) string {
+	t.Helper()
+	books, err := setup.DB.ListMediaItemsByLibrary(context.Background(), libID)
+	require.NoError(t, err)
+	require.Len(t, books, 1)
+	return uuid.UUID(books[0].ID.Bytes).String()
+}
+
+// mergeRequest issues POST /api/media-items/{source}/merge.
+func mergeRequest(t *testing.T, setup *TestServerSetup, token, sourceID string, body map[string]interface{}) *http.Response {
+	t.Helper()
+	payload, _ := json.Marshal(body)
+	req, _ := http.NewRequest("POST", setup.Server.URL+"/api/media-items/"+sourceID+"/merge", bytes.NewBuffer(payload))
+	req.Header.Set("Content-Type", "application/json")
+	req.Header.Set("Authorization", "Bearer "+token)
+	resp, err := http.DefaultClient.Do(req)
+	require.NoError(t, err)
+	return resp
+}
+
+// archiveAndIsolate imports one book into the library and archives it,
+// simulating a file that was copied elsewhere then deleted from here.
+func archiveAndIsolate(t *testing.T, setup *TestServerSetup, adminID, adminPgID pgtype.UUID, libID pgtype.UUID, folder, content string) database.ListMediaItemsByLibraryRow {
+	t.Helper()
+	ctx := context.Background()
+	writeTestEbook(t, folder, []byte(content))
+	scanTestFolder(t, setup, adminID, folder)
+
+	books, err := setup.DB.ListMediaItemsByLibrary(ctx, libID)
+	require.NoError(t, err)
+	require.Len(t, books, 1)
+	item := books[0]
+
+	// Reading history that must survive the merge.
+	_, err = setup.DB.UpdateReadingProgress(ctx, database.UpdateReadingProgressParams{
+		MediaItemID: item.ID,
+		UserID:      adminPgID,
+		CurrentPage: pgtype.Int4{Int32: 42, Valid: true},
+		TotalPages:  pgtype.Int4{Int32: 200, Valid: true},
+	})
+	require.NoError(t, err)
+	_, err = setup.DB.CreateMediaHighlight(ctx, database.CreateMediaHighlightParams{
+		MediaItemID:   item.ID,
+		UserID:        adminPgID,
+		SelectionText: "annotation on the old copy",
+		Color:         pgtype.Text{String: "#00ff00", Valid: true},
+	})
+	require.NoError(t, err)
+
+	require.NoError(t, setup.DB.ArchiveMediaItem(ctx, item.ID))
+	return item
+}
+
+// TestMergeArchivedItemIntoActiveTwin covers the copy-then-delete-later
+// workflow: the active copy exists in another library, the archived twin
+// holds the real reading history. Merging moves progress + annotations onto
+// the active row and removes the archived row.
+func TestMergeArchivedItemIntoActiveTwin(t *testing.T) {
+	setup := setupTestServer(t)
+	defer func() { _ = setup.Close() }()
+
+	ctx := context.Background()
+	adminID, adminPgID := moveTestAdminID(t, setup)
+	libOld, folderOld := createTypedTestLibrary(t, setup, "Merge Old Lib", "ebooks")
+	libNew, folderNew := createTypedTestLibrary(t, setup, "Merge New Lib", "ebooks")
+	defer deleteLibraries(t, setup, libOld, libNew)
+
+	content := "merge workflow epub content"
+
+	// Old library: book + history, then archived (file "deleted").
+	oldItem := archiveAndIsolate(t, setup, adminID, adminPgID, libOld, folderOld, content)
+
+	// New library: the surviving active copy, no history of its own.
+	writeTestEbook(t, folderNew, []byte(content))
+	scanTestFolder(t, setup, adminID, folderNew)
+	booksNew, err := setup.DB.ListMediaItemsByLibrary(ctx, libNew)
+	require.NoError(t, err)
+	require.Len(t, booksNew, 1)
+	active := booksNew[0]
+
+	oldID := uuid.UUID(oldItem.ID.Bytes).String()
+	activeID := uuid.UUID(active.ID.Bytes).String()
+	resp := mergeRequest(t, setup, setup.Token, oldID, map[string]interface{}{"target_id": activeID})
+	defer func() { _ = resp.Body.Close() }()
+	require.Equal(t, http.StatusOK, resp.StatusCode, "merge should succeed")
+
+	var mergeResp map[string]interface{}
+	require.NoError(t, json.NewDecoder(resp.Body).Decode(&mergeResp))
+
+	// Archived row is gone.
+	_, err = setup.DB.GetMediaItem(ctx, oldItem.ID)
+	assert.Error(t, err, "archived row must be removed after merge")
+
+	// History landed on the active row.
+	progress, err := setup.DB.GetReadingProgress(ctx, database.GetReadingProgressParams{MediaItemID: active.ID, UserID: adminPgID})
+	require.NoError(t, err)
+	assert.Equal(t, int32(42), progress.CurrentPage.Int32, "progress must merge into the active copy")
+
+	highlights, err := setup.DB.GetMediaHighlights(ctx, database.GetMediaHighlightsParams{MediaItemID: active.ID, UserID: adminPgID})
+	require.NoError(t, err)
+	assert.Len(t, highlights, 1, "annotations must merge into the active copy")
+
+	booksOld, err := setup.DB.ListMediaItemsByLibrary(ctx, libOld)
+	require.NoError(t, err)
+	assert.Empty(t, booksOld, "old library must be empty after merge")
+}
+
+func TestMergeArchivedItemRejections(t *testing.T) {
+	setup := setupTestServer(t)
+	defer func() { _ = setup.Close() }()
+
+	ctx := context.Background()
+	adminID, adminPgID := moveTestAdminID(t, setup)
+	libA, folderA := createTypedTestLibrary(t, setup, "Reject Lib A", "ebooks")
+	libB, folderB := createTypedTestLibrary(t, setup, "Reject Lib B", "ebooks")
+	defer deleteLibraries(t, setup, libA, libB)
+
+	t.Run("requires admin", func(t *testing.T) {
+		writeTestEbook(t, folderA, []byte("admin check content"))
+		scanTestFolder(t, setup, adminID, folderA)
+		booksA, err := setup.DB.ListMediaItemsByLibrary(ctx, libA)
+		require.NoError(t, err)
+		require.Len(t, booksA, 1)
+		itemID := uuid.UUID(booksA[0].ID.Bytes).String()
+
+		resp := mergeRequest(t, setup, setup.RegularToken, itemID, map[string]interface{}{"target_id": itemID})
+		defer func() { _ = resp.Body.Close() }()
+		assert.Equal(t, http.StatusForbidden, resp.StatusCode)
+	})
+
+	t.Run("rejects active source", func(t *testing.T) {
+		writeTestEbook(t, folderA, []byte("active source content"))
+		scanTestFolder(t, setup, adminID, folderA)
+		booksA, err := setup.DB.ListMediaItemsByLibrary(ctx, libA)
+		require.NoError(t, err)
+		require.Len(t, booksA, 1)
+		sourceID := uuid.UUID(booksA[0].ID.Bytes).String()
+
+		writeTestEbook(t, folderB, []byte("active source content"))
+		scanTestFolder(t, setup, adminID, folderB)
+		targetID := findTwinTargetID(t, setup, libB)
+
+		resp := mergeRequest(t, setup, setup.Token, sourceID, map[string]interface{}{"target_id": targetID})
+		defer func() { _ = resp.Body.Close() }()
+		assert.Equal(t, http.StatusBadRequest, resp.StatusCode, "source is not archived/missing")
+	})
+
+	t.Run("rejects hash mismatch", func(t *testing.T) {
+		writeTestEbook(t, folderA, []byte("hash mismatch content A"))
+		scanTestFolder(t, setup, adminID, folderA)
+		booksA, err := setup.DB.ListMediaItemsByLibrary(ctx, libA)
+		require.NoError(t, err)
+		require.Len(t, booksA, 1)
+		source := booksA[0]
+		require.NoError(t, setup.DB.ArchiveMediaItem(ctx, source.ID))
+		sourceID := uuid.UUID(source.ID.Bytes).String()
+
+		writeTestEbook(t, folderB, []byte("hash mismatch content B - different bytes"))
+		scanTestFolder(t, setup, adminID, folderB)
+		targetID := findTwinTargetID(t, setup, libB)
+
+		resp := mergeRequest(t, setup, setup.Token, sourceID, map[string]interface{}{"target_id": targetID})
+		defer func() { _ = resp.Body.Close() }()
+		assert.Equal(t, http.StatusBadRequest, resp.StatusCode, "different content must not merge")
+
+		// Both rows still exist.
+		_, err = setup.DB.GetMediaItem(ctx, source.ID)
+		assert.NoError(t, err, "source must survive a rejected merge")
+	})
+
+	t.Run("requires target_id", func(t *testing.T) {
+		writeTestEbook(t, folderA, []byte("missing target content"))
+		scanTestFolder(t, setup, adminID, folderA)
+		booksA, err := setup.DB.ListMediaItemsByLibrary(ctx, libA)
+		require.NoError(t, err)
+		require.Len(t, booksA, 1)
+		require.NoError(t, setup.DB.ArchiveMediaItem(ctx, booksA[0].ID))
+		sourceID := uuid.UUID(booksA[0].ID.Bytes).String()
+
+		resp := mergeRequest(t, setup, setup.Token, sourceID, map[string]interface{}{})
+		defer func() { _ = resp.Body.Close() }()
+		assert.Equal(t, http.StatusBadRequest, resp.StatusCode)
+	})
+
+	t.Run("rejects self merge", func(t *testing.T) {
+		writeTestEbook(t, folderA, []byte("self merge content"))
+		scanTestFolder(t, setup, adminID, folderA)
+		booksA, err := setup.DB.ListMediaItemsByLibrary(ctx, libA)
+		require.NoError(t, err)
+		require.Len(t, booksA, 1)
+		require.NoError(t, setup.DB.ArchiveMediaItem(ctx, booksA[0].ID))
+		sourceID := uuid.UUID(booksA[0].ID.Bytes).String()
+
+		resp := mergeRequest(t, setup, setup.Token, sourceID, map[string]interface{}{"target_id": sourceID})
+		defer func() { _ = resp.Body.Close() }()
+		assert.Equal(t, http.StatusBadRequest, resp.StatusCode)
+	})
+
+	t.Run("rejects archived target", func(t *testing.T) {
+		content := []byte("archived target content")
+		writeTestEbook(t, folderA, content)
+		scanTestFolder(t, setup, adminID, folderA)
+		booksA, err := setup.DB.ListMediaItemsByLibrary(ctx, libA)
+		require.NoError(t, err)
+		require.Len(t, booksA, 1)
+		require.NoError(t, setup.DB.ArchiveMediaItem(ctx, booksA[0].ID))
+		sourceID := uuid.UUID(booksA[0].ID.Bytes).String()
+
+		writeTestEbook(t, folderB, content)
+		scanTestFolder(t, setup, adminID, folderB)
+		booksB, err := setup.DB.ListMediaItemsByLibrary(ctx, libB)
+		require.NoError(t, err)
+		require.Len(t, booksB, 1)
+		require.NoError(t, setup.DB.ArchiveMediaItem(ctx, booksB[0].ID))
+		targetID := uuid.UUID(booksB[0].ID.Bytes).String()
+
+		resp := mergeRequest(t, setup, setup.Token, sourceID, map[string]interface{}{"target_id": targetID})
+		defer func() { _ = resp.Body.Close() }()
+		assert.Equal(t, http.StatusBadRequest, resp.StatusCode, "target must be active")
+	})
+
+	_ = adminPgID
+}
+
+// TestLibraryTypeNameTriggerOnLibraryChange proves the schema trigger keeps
+// media_items.library_type_name in sync when a row is repointed to a library
+// of a different type (the INSERT-only trigger previously left it stale).
+func TestLibraryTypeNameTriggerOnLibraryChange(t *testing.T) {
+	setup := setupTestServer(t)
+	defer func() { _ = setup.Close() }()
+
+	ctx := context.Background()
+	adminID, _ := moveTestAdminID(t, setup)
+	libE, folderE := createTypedTestLibrary(t, setup, "Trigger Ebooks", "ebooks")
+	libC, _ := createTypedTestLibrary(t, setup, "Trigger Comics", "comics")
+	defer deleteLibraries(t, setup, libE, libC)
+
+	writeTestEbook(t, folderE, []byte("trigger check content"))
+	scanTestFolder(t, setup, adminID, folderE)
+
+	books, err := setup.DB.ListMediaItemsByLibrary(ctx, libE)
+	require.NoError(t, err)
+	require.Len(t, books, 1)
+	item := books[0]
+	require.True(t, item.LibraryTypeName.Valid && item.LibraryTypeName.String == "ebooks")
+
+	require.NoError(t, setup.DB.MoveMediaItemToLibrary(ctx, database.MoveMediaItemToLibraryParams{
+		ID:        item.ID,
+		LibraryID: libC,
+		FilePath:  "Author/Test Book.epub",
+		FileSize:  pgtype.Int8{Int64: 27, Valid: true},
+	}))
+
+	after, err := setup.DB.GetMediaItem(ctx, item.ID)
+	require.NoError(t, err)
+	assert.Equal(t, libC, after.LibraryID)
+	assert.True(t, after.LibraryTypeName.Valid && after.LibraryTypeName.String == "comics",
+		"trigger must refresh library_type_name when library_id changes")
+}
diff --git a/cmd/server/tests/test_helpers_test.go b/cmd/server/tests/test_helpers_test.go
index 9a4b593..9492226 100644
--- a/cmd/server/tests/test_helpers_test.go
+++ b/cmd/server/tests/test_helpers_test.go
@@ -601,6 +601,16 @@ func setupTestServer(t *testing.T) *TestServerSetup {
 	if existing, gerr := queries.GetUserByEmail(ctx, "testuser@tests.bookhoard.internal"); gerr == nil {
 		adminUUID, err = uuid.FromBytes(existing.ID.Bytes[:])
 		require.NoError(t, err, "Failed to parse admin user UUID")
+		// Reset the password to the known test constant: other tests (e.g.
+		// password-change tests) mutate the shared admin's password, which
+		// would otherwise lock every later test out with login 401s until
+		// the database is wiped.
+		if updErr := queries.UpdatePassword(ctx, database.UpdatePasswordParams{
+			ID:           existing.ID,
+			PasswordHash: passwordHash,
+		}); updErr != nil {
+			require.NoError(t, updErr, "Failed to reset shared admin test password")
+		}
 	} else {
 		adminUser, cerr := queries.CreateUser(ctx, database.CreateUserParams{
 			Email:        "testuser@tests.bookhoard.internal",