diff --git a/workspace-server/internal/handlers/chat_files.go b/workspace-server/internal/handlers/chat_files.go
index ccfa0d4c..f5e980bf 100644
--- a/workspace-server/internal/handlers/chat_files.go
+++ b/workspace-server/internal/handlers/chat_files.go
@@ -600,14 +600,21 @@ func (h *ChatFilesHandler) uploadPollMode(c *gin.Context, ctx context.Context, w
 		return
 	}
 
-	out := make([]uploadedFile, 0, len(headers))
+	// Phase 1: pre-validate + read every part BEFORE any DB write.
+	// A multi-file upload must commit all-or-nothing; a per-file
+	// failure halfway through used to leave rows 1..K-1 in the table
+	// while the client got a 500 and retried the whole batch — duplicate
+	// rows, orphan activity rows. Validating up-front + atomic PutBatch
+	// closes that gap.
+	type prepped struct {
+		Sanitized string
+		Mimetype  string
+		Content   []byte
+		Original  string // original (unsanitized) filename for error messages
+	}
+	prepReady := make([]prepped, 0, len(headers))
+	items := make([]pendinguploads.PutItem, 0, len(headers))
 	for _, fh := range headers {
-		// Read full content. Per-file cap enforced post-read so an
-		// oversized file fails with a clean 413 rather than a torn
-		// stream. The +1 byte ReadAll trick that the Python side
-		// uses isn't easy through multipart.FileHeader; instead we
-		// rely on the multipart layer's ContentLength header and
-		// short-circuit before opening the part.
 		if fh.Size > pendinguploads.MaxFileBytes {
 			log.Printf("chat_files uploadPollMode: per-file cap exceeded for %s: %s (%d bytes)",
 				workspaceID, fh.Filename, fh.Size)
@@ -621,45 +628,67 @@ func (h *ChatFilesHandler) uploadPollMode(c *gin.Context, ctx context.Context, w
 		}
 		content, err := readMultipartFile(fh)
 		if err != nil {
-			log.Printf("chat_files uploadPollMode: read part failed for %s/%s: %v", workspaceID, fh.Filename, err)
+			log.Printf("chat_files uploadPollMode: read part failed for %s/%s: %v",
+				workspaceID, fh.Filename, err)
 			c.JSON(http.StatusBadRequest, gin.H{"error": "could not read file part"})
 			return
 		}
-
-		sanitized := SanitizeFilename(fh.Filename)
-		mimetype := fh.Header.Get("Content-Type")
-
-		fileID, err := h.pendingUploads.Put(ctx, wsUUID, content, sanitized, mimetype)
-		if err != nil {
-			if errors.Is(err, pendinguploads.ErrTooLarge) {
-				// Belt + suspenders: the size check above already
-				// caught this, but Storage.Put re-validates so a
-				// malformed FileHeader can't slip through. 413 with
-				// the same shape so the client sees one error class.
-				c.JSON(http.StatusRequestEntityTooLarge, gin.H{
-					"error":    "file exceeds per-file cap",
-					"filename": fh.Filename,
-					"size":     len(content),
-					"max":      pendinguploads.MaxFileBytes,
-				})
-				return
-			}
-			log.Printf("chat_files uploadPollMode: storage.Put failed for %s/%s: %v",
-				workspaceID, sanitized, err)
-			c.JSON(http.StatusInternalServerError, gin.H{"error": "could not stage file"})
+		// Belt-and-braces post-read cap (multipart.FileHeader.Size can lie
+		// on some clients that don't set Content-Length per part).
+		if len(content) > pendinguploads.MaxFileBytes {
+			log.Printf("chat_files uploadPollMode: per-file cap exceeded post-read for %s: %s (%d bytes)",
+				workspaceID, fh.Filename, len(content))
+			c.JSON(http.StatusRequestEntityTooLarge, gin.H{
+				"error":    "file exceeds per-file cap",
+				"filename": fh.Filename,
+				"size":     len(content),
+				"max":      pendinguploads.MaxFileBytes,
+			})
 			return
 		}
+		sanitized := SanitizeFilename(fh.Filename)
+		mimetype := safeMimetype(fh.Header.Get("Content-Type"))
+		prepReady = append(prepReady, prepped{
+			Sanitized: sanitized, Mimetype: mimetype, Content: content, Original: fh.Filename,
+		})
+		items = append(items, pendinguploads.PutItem{
+			Content: content, Filename: sanitized, Mimetype: mimetype,
+		})
+	}
 
-		// Activity row so the workspace's inbox poller picks this up
-		// on its next cycle. activity_type=a2a_receive (NOT a new
-		// type) so the existing poll filter
-		// `?type=a2a_receive` catches it without poll-side changes;
-		// method=chat_upload_receive is the discriminator the
-		// workspace's adapter (Phase 2) uses to route to the upload
-		// fetcher instead of the agent's message handler. Same
-		// shape as A2A's tasks/send vs message/send method split.
+	// Phase 2: atomic batch insert. On failure no rows commit.
+	fileIDs, err := h.pendingUploads.PutBatch(ctx, wsUUID, items)
+	if err != nil {
+		if errors.Is(err, pendinguploads.ErrTooLarge) {
+			// Belt + suspenders: pre-validation above already caught
+			// this; surface a clean 413 if a malformed FileHeader
+			// somehow slipped through.
+			c.JSON(http.StatusRequestEntityTooLarge, gin.H{
+				"error": "one or more files exceed per-file cap",
+				"max":   pendinguploads.MaxFileBytes,
+			})
+			return
+		}
+		log.Printf("chat_files uploadPollMode: storage.PutBatch failed for %s: %v",
+			workspaceID, err)
+		c.JSON(http.StatusInternalServerError, gin.H{"error": "could not stage files"})
+		return
+	}
+
+	// Phase 3: write per-file activity rows and build the response. Activity
+	// rows are written individually (not part of the same Tx as PutBatch)
+	// because LogActivity is shared across many handlers and threading the
+	// Tx through would be a bigger refactor. The trade-off: if an activity
+	// write fails after the PutBatch commits, the pending_uploads rows
+	// orphan until the 24h TTL — significantly better than the previous
+	// "every multi-file upload could orphan" behavior, and the workspace's
+	// fetcher handles soft-404 cleanly when activity rows reference a row
+	// the platform later expired.
+	out := make([]uploadedFile, 0, len(prepReady))
+	for i, p := range prepReady {
+		fileID := fileIDs[i]
 		uri := fmt.Sprintf("platform-pending:%s/%s", workspaceID, fileID)
-		summary := "chat_upload_receive: " + sanitized
+		summary := "chat_upload_receive: " + p.Sanitized
 		method := "chat_upload_receive"
 		LogActivity(ctx, h.broadcaster, ActivityParams{
 			WorkspaceID:  workspaceID,
@@ -669,28 +698,65 @@ func (h *ChatFilesHandler) uploadPollMode(c *gin.Context, ctx context.Context, w
 			Summary:      &summary,
 			RequestBody: map[string]interface{}{
 				"file_id":  fileID.String(),
-				"name":     sanitized,
-				"mimeType": mimetype,
-				"size":     len(content),
+				"name":     p.Sanitized,
+				"mimeType": p.Mimetype,
+				"size":     len(p.Content),
 				"uri":      uri,
 			},
 			Status: "ok",
 		})
 
 		log.Printf("chat_files uploadPollMode: staged %s/%s (file_id=%s size=%d mimetype=%q)",
-			workspaceID, sanitized, fileID, len(content), mimetype)
+			workspaceID, p.Sanitized, fileID, len(p.Content), p.Mimetype)
 
 		out = append(out, uploadedFile{
 			URI:      uri,
-			Name:     sanitized,
-			Mimetype: mimetype,
-			Size:     int64(len(content)),
+			Name:     p.Sanitized,
+			Mimetype: p.Mimetype,
+			Size:     int64(len(p.Content)),
 		})
 	}
 
 	c.JSON(http.StatusOK, gin.H{"files": out})
 }
 
+// safeMimetype validates a multipart-supplied Content-Type header and
+// returns a sanitized value safe to store + serve back unmodified.
+//
+// The platform's GET /content handler reflects the stored mimetype as
+// the response Content-Type. An attacker-controlled header that
+// embedded CR/LF could split the response (header injection); a value
+// containing semicolons could carry an unexpected charset parameter
+// that confuses a downstream renderer. Strip CR/LF/control chars +
+// keep only the type/subtype prefix; reject anything that doesn't
+// match a basic `type/subtype` regex by falling back to the safe
+// default (application/octet-stream — the workspace-side handler does
+// the same fallback).
+func safeMimetype(raw string) string {
+	const fallback = "application/octet-stream"
+	// Trim parameters (`text/html; charset=utf-8` → `text/html`).
+	if i := strings.IndexByte(raw, ';'); i >= 0 {
+		raw = raw[:i]
+	}
+	raw = strings.TrimSpace(raw)
+	if raw == "" {
+		return ""
+	}
+	// Reject if any control char or whitespace is present (header
+	// injection defense). RFC 7231 mimetype grammar forbids whitespace.
+	for _, r := range raw {
+		if r < 0x21 || r > 0x7e {
+			return fallback
+		}
+	}
+	// Require exactly one slash separating type and subtype.
+	parts := strings.Split(raw, "/")
+	if len(parts) != 2 || parts[0] == "" || parts[1] == "" {
+		return fallback
+	}
+	return raw
+}
+
 // readMultipartFile reads a multipart part fully into memory. Wraps
 // the open + io.ReadAll + close idiom so the call site stays clean,
 // and so a future change (chunked reads / hashing) has one place to
diff --git a/workspace-server/internal/handlers/chat_files_poll_test.go b/workspace-server/internal/handlers/chat_files_poll_test.go
index b9aeb5d6..aa5bab34 100644
--- a/workspace-server/internal/handlers/chat_files_poll_test.go
+++ b/workspace-server/internal/handlers/chat_files_poll_test.go
@@ -67,6 +67,46 @@ func (s *inMemStorage) Put(_ context.Context, ws uuid.UUID, content []byte, file
 	return id, nil
 }
 
+// PutBatch mirrors the production atomic-batch contract: any per-item
+// failure leaves the in-memory state unchanged, simulating Tx rollback.
+// Pre-validation matches PostgresStorage.PutBatch; oversized items
+// return ErrTooLarge before any row is added.
+func (s *inMemStorage) PutBatch(_ context.Context, ws uuid.UUID, items []pendinguploads.PutItem) ([]uuid.UUID, error) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	if s.putErr != nil {
+		return nil, s.putErr
+	}
+	// Pre-validate so an oversized item rejects the whole batch before
+	// any state mutation — matches the Tx-rollback semantics.
+	for _, it := range items {
+		if len(it.Content) > pendinguploads.MaxFileBytes {
+			return nil, pendinguploads.ErrTooLarge
+		}
+	}
+	ids := make([]uuid.UUID, 0, len(items))
+	stagedRows := make(map[uuid.UUID]pendinguploads.Record, len(items))
+	stagedPuts := make([]putCall, 0, len(items))
+	for _, it := range items {
+		id := uuid.New()
+		stagedRows[id] = pendinguploads.Record{
+			FileID: id, WorkspaceID: ws, Content: it.Content,
+			Filename: it.Filename, Mimetype: it.Mimetype,
+			SizeBytes: int64(len(it.Content)), CreatedAt: time.Now(),
+			ExpiresAt: time.Now().Add(24 * time.Hour),
+		}
+		stagedPuts = append(stagedPuts, putCall{
+			WorkspaceID: ws, Filename: it.Filename, Mimetype: it.Mimetype, Size: len(it.Content),
+		})
+		ids = append(ids, id)
+	}
+	for id, r := range stagedRows {
+		s.rows[id] = r
+	}
+	s.puts = append(s.puts, stagedPuts...)
+	return ids, nil
+}
+
 func (s *inMemStorage) Get(context.Context, uuid.UUID) (pendinguploads.Record, error) {
 	return pendinguploads.Record{}, pendinguploads.ErrNotFound
 }
@@ -557,6 +597,120 @@ func TestPollUpload_SanitizesFilenameInResponse(t *testing.T) {
 	}
 }
 
+// TestPollUpload_AtomicRollbackOnSecondFileTooLarge pins the
+// transactional contract introduced in phase 5: when one file in a
+// multi-file batch fails pre-validation (oversize), NONE of the files
+// in the batch land in storage. Previously a per-file Put loop would
+// stage rows 1..K-1 before failing on row K, leaving orphan
+// pending_uploads + activity rows the client would re-create on retry.
+//
+// Pinned via inMemStorage's PutBatch (which mirrors PostgresStorage's
+// Tx-rollback behavior on a per-item validation failure) — but the
+// real atomicity guarantee is the integration test in
+// pending_uploads_integration_test.go.
+func TestPollUpload_AtomicRollbackOnSecondFileTooLarge(t *testing.T) {
+	mock := setupTestDB(t)
+	setupTestRedis(t)
+
+	wsID := "aaaaaaaa-3333-3333-4444-555555555555"
+	expectPollDeliveryMode(mock, wsID, "poll")
+
+	store := newInMemStorage()
+	h := NewChatFilesHandler(NewTemplatesHandler(t.TempDir(), nil)).
+		WithPendingUploads(store, nil)
+
+	// Two files: first OK, second over the per-file cap. Pre-validation
+	// in uploadPollMode catches it BEFORE any Put — store.puts must
+	// stay empty. (If the test ever sees len=1, the regression is
+	// "first file slipped through into storage on a partial-failure
+	// batch.")
+	tooBig := bytes.Repeat([]byte{0x42}, pendinguploads.MaxFileBytes+1)
+	body, ct := pollUploadFixture(t, map[string][]byte{
+		"ok.txt":   []byte("small"),
+		"huge.bin": tooBig,
+	})
+	c, w := makeUploadRequest(t, wsID, body, ct)
+	h.Upload(c)
+
+	if w.Code != http.StatusRequestEntityTooLarge {
+		t.Errorf("status=%d body=%s, want 413", w.Code, w.Body.String())
+	}
+	if len(store.puts) != 0 {
+		t.Errorf("expected zero Puts on rollback, got %d: %+v", len(store.puts), store.puts)
+	}
+}
+
+// TestPollUpload_AtomicRollbackOnPutBatchError validates that an in-
+// flight PutBatch failure (e.g. simulated DB error) leaves zero rows
+// — same guarantee as the pre-validation path, but exercises the
+// "Tx-Rollback after BEGIN" branch via the fake.
+func TestPollUpload_AtomicRollbackOnPutBatchError(t *testing.T) {
+	mock := setupTestDB(t)
+	setupTestRedis(t)
+
+	wsID := "bbbbbbbb-3333-3333-4444-555555555555"
+	expectPollDeliveryMode(mock, wsID, "poll")
+
+	store := newInMemStorage()
+	store.putErr = errors.New("db down mid-batch")
+	h := NewChatFilesHandler(NewTemplatesHandler(t.TempDir(), nil)).
+		WithPendingUploads(store, nil)
+
+	body, ct := pollUploadFixture(t, map[string][]byte{
+		"a.txt": []byte("aaa"),
+		"b.txt": []byte("bbb"),
+		"c.txt": []byte("ccc"),
+	})
+	c, w := makeUploadRequest(t, wsID, body, ct)
+	h.Upload(c)
+
+	if w.Code != http.StatusInternalServerError {
+		t.Errorf("status=%d, want 500", w.Code)
+	}
+	if len(store.puts) != 0 {
+		t.Errorf("expected zero Puts after PutBatch error, got %d", len(store.puts))
+	}
+}
+
+// TestPollUpload_MimetypeWithCRLFInjectionStripped pins the safeMimetype
+// hardening: a multipart-supplied Content-Type header with CR/LF is
+// rewritten to application/octet-stream so the eventual /content
+// response can't be header-split on the wire.
+func TestPollUpload_MimetypeWithCRLFInjectionStripped(t *testing.T) {
+	got := safeMimetype("text/html\r\nX-Injected: pwn")
+	if got != "application/octet-stream" {
+		t.Errorf("CRLF mimetype not stripped, got %q", got)
+	}
+	got = safeMimetype("image/png\x00")
+	if got != "application/octet-stream" {
+		t.Errorf("NUL byte mimetype not stripped, got %q", got)
+	}
+	got = safeMimetype("text/plain; charset=utf-8")
+	if got != "text/plain" {
+		t.Errorf("parameter not stripped, got %q", got)
+	}
+	got = safeMimetype("application/pdf")
+	if got != "application/pdf" {
+		t.Errorf("clean mime modified, got %q", got)
+	}
+	got = safeMimetype("")
+	if got != "" {
+		t.Errorf("empty input should pass through, got %q", got)
+	}
+	got = safeMimetype("notamime")
+	if got != "application/octet-stream" {
+		t.Errorf("non-type/subtype not coerced, got %q", got)
+	}
+	got = safeMimetype("/empty-type")
+	if got != "application/octet-stream" {
+		t.Errorf("missing type half not coerced, got %q", got)
+	}
+	got = safeMimetype("type/")
+	if got != "application/octet-stream" {
+		t.Errorf("missing subtype half not coerced, got %q", got)
+	}
+}
+
 // TestPollUpload_ActivityRowDiscriminator pins the
 // activity_type / method shape that the workspace inbox poller depends
 // on. The poller filters `GET /workspaces/:id/activity?type=a2a_receive`
diff --git a/workspace-server/internal/handlers/pending_uploads_integration_test.go b/workspace-server/internal/handlers/pending_uploads_integration_test.go
index bec9011c..61c64f86 100644
--- a/workspace-server/internal/handlers/pending_uploads_integration_test.go
+++ b/workspace-server/internal/handlers/pending_uploads_integration_test.go
@@ -44,6 +44,7 @@ import (
 	"context"
 	"database/sql"
 	"os"
+	"strings"
 	"testing"
 	"time"
 
@@ -273,6 +274,183 @@ func TestIntegration_PendingUploads_PutEnforcesSizeCap(t *testing.T) {
 	}
 }
 
+// TestIntegration_PendingUploads_PutBatch_HappyPath_AllRowsCommit pins the
+// "all rows commit" leg of the PutBatch atomicity contract against a real
+// Postgres. sqlmock can't catch a regression where the Go-side Tx machinery
+// silently no-ops the inserts (e.g., wrong driver options on BeginTx); only
+// COUNT(*) on the real table can.
+func TestIntegration_PendingUploads_PutBatch_HappyPath_AllRowsCommit(t *testing.T) {
+	conn := integrationDB_PendingUploads(t)
+	store := pendinguploads.NewPostgres(conn)
+	ctx := context.Background()
+
+	wsID := uuid.New()
+
+	// Pre-existing row so the COUNT(*) baseline is non-zero — proves
+	// PutBatch adds rows incrementally rather than overwriting.
+	if _, err := store.Put(ctx, wsID, []byte("seed"), "seed.txt", "text/plain"); err != nil {
+		t.Fatalf("seed Put: %v", err)
+	}
+
+	items := []pendinguploads.PutItem{
+		{Content: []byte("alpha"), Filename: "alpha.txt", Mimetype: "text/plain"},
+		{Content: []byte("beta"), Filename: "beta.bin", Mimetype: "application/octet-stream"},
+		{Content: []byte("gamma"), Filename: "gamma.pdf", Mimetype: "application/pdf"},
+	}
+	ids, err := store.PutBatch(ctx, wsID, items)
+	if err != nil {
+		t.Fatalf("PutBatch: %v", err)
+	}
+	if len(ids) != len(items) {
+		t.Fatalf("ids length %d, want %d", len(ids), len(items))
+	}
+
+	// Each returned id round-trips through Get with the right content.
+	for i, id := range ids {
+		rec, err := store.Get(ctx, id)
+		if err != nil {
+			t.Fatalf("Get item %d (%s): %v", i, id, err)
+		}
+		if string(rec.Content) != string(items[i].Content) {
+			t.Errorf("item %d content = %q, want %q", i, rec.Content, items[i].Content)
+		}
+		if rec.Filename != items[i].Filename {
+			t.Errorf("item %d filename = %q, want %q", i, rec.Filename, items[i].Filename)
+		}
+	}
+
+	var n int
+	if err := conn.QueryRowContext(ctx, `SELECT COUNT(*) FROM pending_uploads WHERE workspace_id = $1`, wsID).Scan(&n); err != nil {
+		t.Fatalf("count: %v", err)
+	}
+	if n != 4 {
+		t.Errorf("workspace row count = %d, want 4 (1 seed + 3 batch)", n)
+	}
+}
+
+// TestIntegration_PendingUploads_PutBatch_AtomicRollback_NoLeakOnFailure
+// proves the all-or-nothing contract end-to-end against real Postgres MVCC.
+//
+// Strategy: build a 3-item batch where item index 1 carries a filename with
+// an embedded NUL byte. lib/pq rejects NULs in TEXT columns at the protocol
+// layer (`pq: invalid byte sequence for encoding "UTF8": 0x00`), which
+// triggers the per-row INSERT error path in PutBatch. The first item's
+// INSERT…RETURNING already wrote a row to the Tx's snapshot, so a buggy
+// rollback would leave that row visible after PutBatch returns.
+//
+// Postgrest semantics: ROLLBACK is the only way a real DB can guarantee the
+// "no leak" contract; a unit test with sqlmock can prove the Go function
+// CALLED Rollback, but only this integration test proves Postgres actually
+// HONORED it.
+func TestIntegration_PendingUploads_PutBatch_AtomicRollback_NoLeakOnFailure(t *testing.T) {
+	conn := integrationDB_PendingUploads(t)
+	store := pendinguploads.NewPostgres(conn)
+	ctx := context.Background()
+
+	wsID := uuid.New()
+
+	// Baseline COUNT(*) for this workspace — must remain 0 after a failed batch.
+	var before int
+	if err := conn.QueryRowContext(ctx, `SELECT COUNT(*) FROM pending_uploads WHERE workspace_id = $1`, wsID).Scan(&before); err != nil {
+		t.Fatalf("baseline count: %v", err)
+	}
+	if before != 0 {
+		t.Fatalf("workspace not isolated: baseline = %d, want 0", before)
+	}
+
+	// Item 1 has a NUL byte in the filename — Go-side pre-validation
+	// (which only checks empty/length) lets it through, so the INSERT
+	// reaches lib/pq, which rejects it at the protocol level. That's the
+	// canonical "DB-side error mid-batch" we want to exercise.
+	items := []pendinguploads.PutItem{
+		{Content: []byte("ok"), Filename: "ok.txt", Mimetype: "text/plain"},
+		{Content: []byte("bad"), Filename: "bad\x00name.txt", Mimetype: "text/plain"},
+		{Content: []byte("never"), Filename: "never.txt", Mimetype: "text/plain"},
+	}
+	_, err := store.PutBatch(ctx, wsID, items)
+	if err == nil {
+		t.Fatalf("expected error from NUL-byte filename, got nil")
+	}
+
+	// THE assertion this whole test exists for: even though item 0's
+	// INSERT…RETURNING succeeded inside the Tx, the rollback unwound
+	// it — zero rows for this workspace, not one (let alone three).
+	var after int
+	if err := conn.QueryRowContext(ctx, `SELECT COUNT(*) FROM pending_uploads WHERE workspace_id = $1`, wsID).Scan(&after); err != nil {
+		t.Fatalf("post-failure count: %v", err)
+	}
+	if after != 0 {
+		t.Errorf("Tx rollback leaked rows: workspace count = %d, want 0", after)
+	}
+}
+
+// TestIntegration_PendingUploads_PutBatch_Oversize_NoTxOpened verifies the
+// pre-validation short-circuit: an oversized item rejects with ErrTooLarge
+// BEFORE any Tx opens, so the table is untouched. The unit test (sqlmock
+// with zero expectations) catches the Go-side path; this test sanity-checks
+// no real DB I/O happens by confirming COUNT(*) doesn't move.
+func TestIntegration_PendingUploads_PutBatch_Oversize_NoTxOpened(t *testing.T) {
+	conn := integrationDB_PendingUploads(t)
+	store := pendinguploads.NewPostgres(conn)
+	ctx := context.Background()
+
+	wsID := uuid.New()
+	tooBig := make([]byte, pendinguploads.MaxFileBytes+1)
+	_, err := store.PutBatch(ctx, wsID, []pendinguploads.PutItem{
+		{Content: []byte("ok"), Filename: "ok.txt"},
+		{Content: tooBig, Filename: "too-big.bin"},
+	})
+	if err != pendinguploads.ErrTooLarge {
+		t.Fatalf("expected ErrTooLarge, got %v", err)
+	}
+	var n int
+	if err := conn.QueryRowContext(ctx, `SELECT COUNT(*) FROM pending_uploads WHERE workspace_id = $1`, wsID).Scan(&n); err != nil {
+		t.Fatalf("count: %v", err)
+	}
+	if n != 0 {
+		t.Errorf("pre-validation did NOT short-circuit: count = %d, want 0", n)
+	}
+}
+
+// TestIntegration_PendingUploads_AckedIndexExists verifies the Phase 5a
+// migration (20260505200000_pending_uploads_acked_index.up.sql) actually
+// created idx_pending_uploads_acked with the right partial-index predicate.
+//
+// Why pg_indexes and not EXPLAIN: the planner prefers Seq Scan on tiny
+// tables regardless of available indexes — a plan-shape check would be
+// flaky under real test loads. The contract we care about is "the index
+// exists with the predicate we wrote in the migration"; pg_indexes is
+// the canonical source for that, robust to row count and planner version.
+func TestIntegration_PendingUploads_AckedIndexExists(t *testing.T) {
+	conn := integrationDB_PendingUploads(t)
+	ctx := context.Background()
+
+	var indexdef string
+	err := conn.QueryRowContext(ctx, `
+		SELECT indexdef FROM pg_indexes
+		WHERE schemaname = 'public'
+		  AND tablename = 'pending_uploads'
+		  AND indexname = 'idx_pending_uploads_acked'
+	`).Scan(&indexdef)
+	if err == sql.ErrNoRows {
+		t.Fatal("idx_pending_uploads_acked is missing — migration 20260505200000 not applied")
+	}
+	if err != nil {
+		t.Fatalf("pg_indexes query: %v", err)
+	}
+
+	// Pin the partial-index predicate. Without "WHERE acked_at IS NOT NULL"
+	// we'd be indexing the entire table (defeats the point — most rows are
+	// unacked), and the existing idx_pending_uploads_unacked already covers
+	// the inverse predicate.
+	if !strings.Contains(indexdef, "(acked_at)") {
+		t.Errorf("index missing acked_at column: %s", indexdef)
+	}
+	if !strings.Contains(indexdef, "WHERE (acked_at IS NOT NULL)") {
+		t.Errorf("index missing partial predicate: %s", indexdef)
+	}
+}
+
 func TestIntegration_PendingUploads_GetIgnoresExpiredAndAcked(t *testing.T) {
 	conn := integrationDB_PendingUploads(t)
 	store := pendinguploads.NewPostgres(conn)
diff --git a/workspace-server/internal/handlers/pending_uploads_test.go b/workspace-server/internal/handlers/pending_uploads_test.go
index e4b11a09..778e8170 100644
--- a/workspace-server/internal/handlers/pending_uploads_test.go
+++ b/workspace-server/internal/handlers/pending_uploads_test.go
@@ -77,6 +77,14 @@ func (f *fakeStorage) Sweep(_ context.Context, _ time.Duration) (pendinguploads.
 	return pendinguploads.SweepResult{}, nil
 }
 
+// PutBatch is required by the Storage interface; the upload handler
+// tests live in chat_files_poll_test.go and use a separate fake
+// (inMemStorage). Stubbed here because the Get/Ack tests don't drive
+// PutBatch, but the interface must be satisfied.
+func (f *fakeStorage) PutBatch(_ context.Context, _ uuid.UUID, _ []pendinguploads.PutItem) ([]uuid.UUID, error) {
+	return nil, nil
+}
+
 func newRouter(handler *handlers.PendingUploadsHandler) *gin.Engine {
 	gin.SetMode(gin.TestMode)
 	r := gin.New()
diff --git a/workspace-server/internal/pendinguploads/export_test.go b/workspace-server/internal/pendinguploads/export_test.go
new file mode 100644
index 00000000..c758b629
--- /dev/null
+++ b/workspace-server/internal/pendinguploads/export_test.go
@@ -0,0 +1,17 @@
+package pendinguploads
+
+import (
+	"context"
+	"time"
+)
+
+// StartSweeperWithIntervalForTest exposes startSweeperWithInterval to
+// the external test package. The production code uses StartSweeper
+// (which pins the canonical SweepInterval); tests pin a short interval
+// to exercise the ticker-driven cycle without burning real wall-clock
+// time. The Go convention `export_test.go` keeps this seam OUT of the
+// production binary — files ending in _test.go are stripped at build
+// time, so this re-export only exists during `go test`.
+func StartSweeperWithIntervalForTest(ctx context.Context, storage Storage, ackRetention, interval time.Duration) {
+	startSweeperWithInterval(ctx, storage, ackRetention, interval)
+}
diff --git a/workspace-server/internal/pendinguploads/storage.go b/workspace-server/internal/pendinguploads/storage.go
index 8bf63b1e..c4bcaf92 100644
--- a/workspace-server/internal/pendinguploads/storage.go
+++ b/workspace-server/internal/pendinguploads/storage.go
@@ -85,6 +85,15 @@ type SweepResult struct {
 // Total returns the sum of Acked + Expired — convenient for log lines.
 func (r SweepResult) Total() int { return r.Acked + r.Expired }
 
+// PutItem is one file in a PutBatch call. Same per-field rules as Put —
+// empty content, missing filename, or content > MaxFileBytes is rejected
+// up-front so a bad item in the batch doesn't poison the transaction.
+type PutItem struct {
+	Content  []byte
+	Filename string
+	Mimetype string
+}
+
 // Storage is the platform-side persistence boundary for poll-mode chat
 // uploads. The Postgres implementation backs all callers today; an S3-
 // backed implementation can drop in once RFC #2789 lands by making
@@ -99,6 +108,17 @@ type Storage interface {
 	// content > MaxFileBytes return errors before any DB write.
 	Put(ctx context.Context, workspaceID uuid.UUID, content []byte, filename, mimetype string) (uuid.UUID, error)
 
+	// PutBatch inserts N uploads atomically — either all rows commit or
+	// none do. Returns assigned file_ids in input order on success;
+	// returns an error and does NOT insert any row on failure.
+	//
+	// Use this from multi-file upload handlers so a per-row failure on
+	// row K doesn't leave rows 1..K-1 orphaned in the table (a client
+	// retry would then double-insert them on success). All-or-nothing
+	// semantics match the multipart request the canvas sends — either
+	// the whole batch succeeds or the user re-uploads.
+	PutBatch(ctx context.Context, workspaceID uuid.UUID, items []PutItem) ([]uuid.UUID, error)
+
 	// Get returns the full row including content. Returns ErrNotFound
 	// when the row is absent, acked, or past expires_at. Caller should
 	// not differentiate the three cases in the response — from the
@@ -174,6 +194,64 @@ func (p *PostgresStorage) Put(ctx context.Context, workspaceID uuid.UUID, conten
 	return fileID, nil
 }
 
+// PutBatch inserts every item atomically inside a single Tx. On any
+// per-item validation or per-row INSERT error the Tx is rolled back and
+// the caller sees the error without any rows committed — no partial
+// orphans for a multi-file upload that fails mid-batch.
+//
+// Validation runs BEFORE BEGIN so a bad input shape (empty content,
+// over-cap size) doesn't even open a Tx. Once we're in the Tx, the only
+// failures expected are DB-side (broken connection, statement timeout)
+// — those abort cleanly via Rollback.
+func (p *PostgresStorage) PutBatch(ctx context.Context, workspaceID uuid.UUID, items []PutItem) ([]uuid.UUID, error) {
+	if len(items) == 0 {
+		return nil, nil
+	}
+	for i, it := range items {
+		if len(it.Content) == 0 {
+			return nil, fmt.Errorf("pendinguploads: item %d: empty content", i)
+		}
+		if len(it.Content) > MaxFileBytes {
+			return nil, ErrTooLarge
+		}
+		if it.Filename == "" {
+			return nil, fmt.Errorf("pendinguploads: item %d: empty filename", i)
+		}
+		if len(it.Filename) > 100 {
+			return nil, fmt.Errorf("pendinguploads: item %d: filename exceeds 100 chars", i)
+		}
+	}
+
+	tx, err := p.db.BeginTx(ctx, nil)
+	if err != nil {
+		return nil, fmt.Errorf("pendinguploads: begin tx: %w", err)
+	}
+	// Defer-rollback is safe even after a successful Commit — the second
+	// Rollback is a no-op (database/sql tracks tx state).
+	defer func() {
+		_ = tx.Rollback()
+	}()
+
+	out := make([]uuid.UUID, 0, len(items))
+	for i, it := range items {
+		var fid uuid.UUID
+		err := tx.QueryRowContext(ctx, `
+			INSERT INTO pending_uploads (workspace_id, content, size_bytes, filename, mimetype)
+			VALUES ($1, $2, $3, $4, $5)
+			RETURNING file_id
+		`, workspaceID, it.Content, int64(len(it.Content)), it.Filename, it.Mimetype).Scan(&fid)
+		if err != nil {
+			return nil, fmt.Errorf("pendinguploads: batch insert item %d: %w", i, err)
+		}
+		out = append(out, fid)
+	}
+
+	if err := tx.Commit(); err != nil {
+		return nil, fmt.Errorf("pendinguploads: commit batch: %w", err)
+	}
+	return out, nil
+}
+
 func (p *PostgresStorage) Get(ctx context.Context, fileID uuid.UUID) (Record, error) {
 	// The expires_at + acked_at filter in the WHERE clause means a
 	// caller sees ErrNotFound for absent / acked / expired without
diff --git a/workspace-server/internal/pendinguploads/storage_test.go b/workspace-server/internal/pendinguploads/storage_test.go
index e4db87f8..c6793c10 100644
--- a/workspace-server/internal/pendinguploads/storage_test.go
+++ b/workspace-server/internal/pendinguploads/storage_test.go
@@ -511,3 +511,223 @@ func TestSweepResult_TotalSumsCounts(t *testing.T) {
 		t.Errorf("zero Total = %d, want 0", z.Total())
 	}
 }
+
+// ----- PutBatch -------------------------------------------------------------
+//
+// PutBatch is the multi-file atomic insert path used by uploadPollMode in
+// chat_files.go. The contract that callers rely on:
+//
+//   - Either ALL rows commit, or NONE do — a per-row INSERT failure must
+//     leave the table unchanged (no orphaned rows from a half-applied batch).
+//   - Per-item validation runs BEFORE the Tx opens so a bad input shape
+//     never wastes a BEGIN round-trip.
+//   - Returned []uuid.UUID is in input order — handler maps response back
+//     to the multipart Files[i].
+//
+// sqlmock's ExpectBegin / ExpectQuery / ExpectCommit / ExpectRollback let us
+// pin the exact tx-lifecycle shape; if a future refactor swaps Begin for
+// BeginTx-with-options, the test fails until we re-pin.
+
+func TestPutBatch_HappyPath_AllCommitInOrder(t *testing.T) {
+	db, mock := newMockDB(t)
+	store := pendinguploads.NewPostgres(db)
+
+	wsID := uuid.New()
+	id1, id2, id3 := uuid.New(), uuid.New(), uuid.New()
+
+	mock.ExpectBegin()
+	mock.ExpectQuery(insertSQL).
+		WithArgs(wsID, []byte("aaa"), int64(3), "a.txt", "text/plain").
+		WillReturnRows(sqlmock.NewRows([]string{"file_id"}).AddRow(id1))
+	mock.ExpectQuery(insertSQL).
+		WithArgs(wsID, []byte("bbbb"), int64(4), "b.bin", "application/octet-stream").
+		WillReturnRows(sqlmock.NewRows([]string{"file_id"}).AddRow(id2))
+	mock.ExpectQuery(insertSQL).
+		WithArgs(wsID, []byte("ccccc"), int64(5), "c.pdf", "application/pdf").
+		WillReturnRows(sqlmock.NewRows([]string{"file_id"}).AddRow(id3))
+	mock.ExpectCommit()
+	// Rollback after Commit is a no-op in database/sql; sqlmock allows it
+	// when ExpectCommit was already matched, so we don't need to expect it.
+
+	got, err := store.PutBatch(context.Background(), wsID, []pendinguploads.PutItem{
+		{Content: []byte("aaa"), Filename: "a.txt", Mimetype: "text/plain"},
+		{Content: []byte("bbbb"), Filename: "b.bin", Mimetype: "application/octet-stream"},
+		{Content: []byte("ccccc"), Filename: "c.pdf", Mimetype: "application/pdf"},
+	})
+	if err != nil {
+		t.Fatalf("PutBatch: %v", err)
+	}
+	if len(got) != 3 || got[0] != id1 || got[1] != id2 || got[2] != id3 {
+		t.Errorf("ids out of order or missing: got %v want [%s %s %s]", got, id1, id2, id3)
+	}
+	if err := mock.ExpectationsWereMet(); err != nil {
+		t.Errorf("expectations: %v", err)
+	}
+}
+
+func TestPutBatch_EmptyItems_NoTxNoError(t *testing.T) {
+	db, _ := newMockDB(t) // zero expectations — must NOT round-trip
+	store := pendinguploads.NewPostgres(db)
+
+	got, err := store.PutBatch(context.Background(), uuid.New(), nil)
+	if err != nil {
+		t.Fatalf("expected nil error on empty batch, got %v", err)
+	}
+	if got != nil {
+		t.Errorf("expected nil ids on empty batch, got %v", got)
+	}
+}
+
+func TestPutBatch_RejectsEmptyContent_NoTx(t *testing.T) {
+	db, _ := newMockDB(t)
+	store := pendinguploads.NewPostgres(db)
+
+	_, err := store.PutBatch(context.Background(), uuid.New(), []pendinguploads.PutItem{
+		{Content: []byte("ok"), Filename: "a.txt"},
+		{Content: nil, Filename: "b.txt"},
+	})
+	if err == nil || !strings.Contains(err.Error(), "item 1") || !strings.Contains(err.Error(), "empty content") {
+		t.Fatalf("expected item-1 empty-content error, got %v", err)
+	}
+}
+
+func TestPutBatch_RejectsOversize_ReturnsErrTooLarge(t *testing.T) {
+	db, _ := newMockDB(t)
+	store := pendinguploads.NewPostgres(db)
+
+	too := make([]byte, pendinguploads.MaxFileBytes+1)
+	_, err := store.PutBatch(context.Background(), uuid.New(), []pendinguploads.PutItem{
+		{Content: []byte("ok"), Filename: "small.txt"},
+		{Content: too, Filename: "huge.bin"},
+	})
+	if !errors.Is(err, pendinguploads.ErrTooLarge) {
+		t.Fatalf("expected ErrTooLarge, got %v", err)
+	}
+}
+
+func TestPutBatch_RejectsEmptyFilename_NoTx(t *testing.T) {
+	db, _ := newMockDB(t)
+	store := pendinguploads.NewPostgres(db)
+
+	_, err := store.PutBatch(context.Background(), uuid.New(), []pendinguploads.PutItem{
+		{Content: []byte("hi"), Filename: ""},
+	})
+	if err == nil || !strings.Contains(err.Error(), "item 0") || !strings.Contains(err.Error(), "empty filename") {
+		t.Fatalf("expected item-0 empty-filename error, got %v", err)
+	}
+}
+
+func TestPutBatch_RejectsLongFilename_NoTx(t *testing.T) {
+	db, _ := newMockDB(t)
+	store := pendinguploads.NewPostgres(db)
+
+	long := strings.Repeat("z", 101)
+	_, err := store.PutBatch(context.Background(), uuid.New(), []pendinguploads.PutItem{
+		{Content: []byte("hi"), Filename: "ok.txt"},
+		{Content: []byte("hi"), Filename: long},
+	})
+	if err == nil || !strings.Contains(err.Error(), "item 1") || !strings.Contains(err.Error(), "exceeds 100 chars") {
+		t.Fatalf("expected item-1 too-long-filename error, got %v", err)
+	}
+}
+
+func TestPutBatch_BeginTxError_Wrapped(t *testing.T) {
+	db, mock := newMockDB(t)
+	store := pendinguploads.NewPostgres(db)
+
+	mock.ExpectBegin().WillReturnError(errors.New("conn refused"))
+
+	_, err := store.PutBatch(context.Background(), uuid.New(), []pendinguploads.PutItem{
+		{Content: []byte("hi"), Filename: "a.txt"},
+	})
+	if err == nil || !strings.Contains(err.Error(), "begin tx") {
+		t.Fatalf("expected wrapped begin-tx error, got %v", err)
+	}
+	if err := mock.ExpectationsWereMet(); err != nil {
+		t.Errorf("expectations: %v", err)
+	}
+}
+
+func TestPutBatch_RollsBackOnPerRowError_NoCommit(t *testing.T) {
+	// First INSERT succeeds, second errors. PutBatch MUST NOT issue
+	// Commit; the deferred Rollback unwinds row 1 so neither row commits.
+	// This is the contract that prevents orphan rows on a failed batch.
+	db, mock := newMockDB(t)
+	store := pendinguploads.NewPostgres(db)
+
+	wsID := uuid.New()
+	id1 := uuid.New()
+
+	mock.ExpectBegin()
+	mock.ExpectQuery(insertSQL).
+		WithArgs(wsID, []byte("aaa"), int64(3), "a.txt", "").
+		WillReturnRows(sqlmock.NewRows([]string{"file_id"}).AddRow(id1))
+	mock.ExpectQuery(insertSQL).
+		WithArgs(wsID, []byte("bb"), int64(2), "b.txt", "").
+		WillReturnError(errors.New("statement timeout"))
+	// Critical: Rollback expected, NOT Commit. If a future refactor
+	// accidentally swallows the per-row error and Commits anyway, this
+	// test fails because the unmet ExpectCommit-vs-Rollback shape diverges.
+	mock.ExpectRollback()
+
+	_, err := store.PutBatch(context.Background(), wsID, []pendinguploads.PutItem{
+		{Content: []byte("aaa"), Filename: "a.txt"},
+		{Content: []byte("bb"), Filename: "b.txt"},
+	})
+	if err == nil || !strings.Contains(err.Error(), "batch insert item 1") {
+		t.Fatalf("expected wrapped per-row insert error, got %v", err)
+	}
+	if err := mock.ExpectationsWereMet(); err != nil {
+		t.Errorf("expectations (must rollback, no commit): %v", err)
+	}
+}
+
+func TestPutBatch_RollsBackOnFirstRowError(t *testing.T) {
+	// Edge case: very first INSERT fails. No rows ever staged — but the
+	// Tx still needs to roll back to release the snapshot.
+	db, mock := newMockDB(t)
+	store := pendinguploads.NewPostgres(db)
+
+	wsID := uuid.New()
+	mock.ExpectBegin()
+	mock.ExpectQuery(insertSQL).
+		WithArgs(wsID, []byte("oops"), int64(4), "a.txt", "").
+		WillReturnError(errors.New("constraint violation"))
+	mock.ExpectRollback()
+
+	_, err := store.PutBatch(context.Background(), wsID, []pendinguploads.PutItem{
+		{Content: []byte("oops"), Filename: "a.txt"},
+	})
+	if err == nil || !strings.Contains(err.Error(), "batch insert item 0") {
+		t.Fatalf("expected wrapped item-0 insert error, got %v", err)
+	}
+	if err := mock.ExpectationsWereMet(); err != nil {
+		t.Errorf("expectations: %v", err)
+	}
+}
+
+func TestPutBatch_CommitError_Wrapped(t *testing.T) {
+	// Commit fails after every INSERT succeeded. Postgres has already
+	// rolled back the Tx by this point; we surface the error so the
+	// handler returns 500 and the client retries.
+	db, mock := newMockDB(t)
+	store := pendinguploads.NewPostgres(db)
+
+	wsID := uuid.New()
+	id1 := uuid.New()
+	mock.ExpectBegin()
+	mock.ExpectQuery(insertSQL).
+		WithArgs(wsID, []byte("hi"), int64(2), "a.txt", "").
+		WillReturnRows(sqlmock.NewRows([]string{"file_id"}).AddRow(id1))
+	mock.ExpectCommit().WillReturnError(errors.New("commit broken"))
+
+	_, err := store.PutBatch(context.Background(), wsID, []pendinguploads.PutItem{
+		{Content: []byte("hi"), Filename: "a.txt"},
+	})
+	if err == nil || !strings.Contains(err.Error(), "commit batch") {
+		t.Fatalf("expected wrapped commit error, got %v", err)
+	}
+	if err := mock.ExpectationsWereMet(); err != nil {
+		t.Errorf("expectations: %v", err)
+	}
+}
diff --git a/workspace-server/internal/pendinguploads/sweeper.go b/workspace-server/internal/pendinguploads/sweeper.go
index 84a56dab..b29a87ad 100644
--- a/workspace-server/internal/pendinguploads/sweeper.go
+++ b/workspace-server/internal/pendinguploads/sweeper.go
@@ -66,13 +66,13 @@ const sweepDeadline = 30 * time.Second
 // to exercise the ticker-driven sweep path without burning real wall-
 // clock time.
 func StartSweeper(ctx context.Context, storage Storage, ackRetention time.Duration) {
-	StartSweeperWithInterval(ctx, storage, ackRetention, SweepInterval)
+	startSweeperWithInterval(ctx, storage, ackRetention, SweepInterval)
 }
 
-// StartSweeperWithInterval is the test-friendly variant of StartSweeper
+// startSweeperWithInterval is the test-friendly variant of StartSweeper
 // — same loop, but the cadence is caller-specified. Production code
 // should use StartSweeper to keep the SweepInterval constant pinned.
-func StartSweeperWithInterval(ctx context.Context, storage Storage, ackRetention, interval time.Duration) {
+func startSweeperWithInterval(ctx context.Context, storage Storage, ackRetention, interval time.Duration) {
 	if storage == nil {
 		log.Println("pendinguploads sweeper: storage is nil — sweeper disabled")
 		return
diff --git a/workspace-server/internal/pendinguploads/sweeper_test.go b/workspace-server/internal/pendinguploads/sweeper_test.go
index 19ce26da..fb0c5aa0 100644
--- a/workspace-server/internal/pendinguploads/sweeper_test.go
+++ b/workspace-server/internal/pendinguploads/sweeper_test.go
@@ -44,6 +44,9 @@ func (f *fakeSweepStorage) MarkFetched(_ context.Context, _ uuid.UUID) error {
 func (f *fakeSweepStorage) Ack(_ context.Context, _ uuid.UUID) error {
 	return errors.New("not used")
 }
+func (f *fakeSweepStorage) PutBatch(_ context.Context, _ uuid.UUID, _ []pendinguploads.PutItem) ([]uuid.UUID, error) {
+	return nil, errors.New("not used")
+}
 func (f *fakeSweepStorage) Sweep(_ context.Context, ackRetention time.Duration) (pendinguploads.SweepResult, error) {
 	idx := int(f.calls.Load())
 	f.calls.Add(1)
@@ -180,7 +183,7 @@ func TestStartSweeperWithInterval_TickerFiresAdditionalCycles(t *testing.T) {
 	ctx, cancel := context.WithCancel(context.Background())
 	defer cancel()
 
-	go pendinguploads.StartSweeperWithInterval(ctx, store, time.Hour, 30*time.Millisecond)
+	go pendinguploads.StartSweeperWithIntervalForTest(ctx, store, time.Hour, 30*time.Millisecond)
 
 	// Immediate cycle + at least one tick-driven cycle.
 	store.waitForCycle(t, 2, 2*time.Second)
diff --git a/workspace-server/migrations/20260505200000_pending_uploads_acked_index.down.sql b/workspace-server/migrations/20260505200000_pending_uploads_acked_index.down.sql
new file mode 100644
index 00000000..2d84b00d
--- /dev/null
+++ b/workspace-server/migrations/20260505200000_pending_uploads_acked_index.down.sql
@@ -0,0 +1,2 @@
+-- Reversal of 20260505200000_pending_uploads_acked_index.up.sql.
+DROP INDEX IF EXISTS idx_pending_uploads_acked;
diff --git a/workspace-server/migrations/20260505200000_pending_uploads_acked_index.up.sql b/workspace-server/migrations/20260505200000_pending_uploads_acked_index.up.sql
new file mode 100644
index 00000000..f2beced2
--- /dev/null
+++ b/workspace-server/migrations/20260505200000_pending_uploads_acked_index.up.sql
@@ -0,0 +1,30 @@
+-- 20260505200000_pending_uploads_acked_index.up.sql
+--
+-- Adds the missing partial index for the acked-retention arm of the
+-- pendinguploads.Sweep query. The Phase 1 migration created two
+-- partial indexes both gated on `acked_at IS NULL` (workspace-fetch
+-- hot path + expires_at sweep arm); the third query path —
+-- `WHERE acked_at IS NOT NULL AND acked_at < now() - interval` — was
+-- left to a seq scan.
+--
+-- For a high-traffic deployment that's a real cost: the table
+-- accumulates one row per chat-attached file; the sweeper runs every
+-- 5 minutes and DELETEs rows past the 1-hour ack retention. A seq
+-- scan over 100K-1M acked rows holds an AccessShare lock for seconds
+-- on every cycle. Partial-indexing the inverse predicate reduces
+-- this to a btree range scan and lets the DELETE complete in
+-- low-millisecond range.
+--
+-- WHERE acked_at IS NOT NULL is intentionally inverse of the other
+-- two indexes — they cover the unacked working set; this covers the
+-- terminal-state set the sweeper visits. Disjoint subsets, so the
+-- two indexes don't overlap.
+--
+-- Caught in self-review on the parent RFC's Phase 4 PR; filed as
+-- a follow-up rather than a Phase 1 fix because the cost only
+-- materializes at a row count we don't expect to hit before the
+-- sweeper has had a chance to keep up.
+
+CREATE INDEX IF NOT EXISTS idx_pending_uploads_acked
+    ON pending_uploads (acked_at)
+    WHERE acked_at IS NOT NULL;