diff --git a/flush_error_test.go b/flush_error_test.go
new file mode 100644
index 0000000..1024db3
--- /dev/null
+++ b/flush_error_test.go
@@ -0,0 +1,91 @@
+package quickwit_test
+
+import (
+	"context"
+	"io"
+	"log/slog"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"sync"
+	"sync/atomic"
+	"testing"
+	"time"
+
+	"github.com/moonrhythm/quickwit"
+)
+
+type capturingHandler struct {
+	mu   *sync.Mutex
+	recs *[]slog.Record
+}
+
+func (h capturingHandler) Enabled(context.Context, slog.Level) bool { return true }
+func (h capturingHandler) Handle(_ context.Context, r slog.Record) error {
+	h.mu.Lock()
+	defer h.mu.Unlock()
+	*h.recs = append(*h.recs, r.Clone())
+	return nil
+}
+func (h capturingHandler) WithAttrs([]slog.Attr) slog.Handler { return h }
+func (h capturingHandler) WithGroup(string) slog.Handler       { return h }
+
+// The "flush failed, retrying indefinitely" line must carry the failure cause so
+// an operator can tell a 5xx/backpressure apart from a transport error.
+func TestFlushFailure_RetryLogIncludesCause(t *testing.T) {
+	var mu sync.Mutex
+	var recs []slog.Record
+	prev := slog.Default()
+	slog.SetDefault(slog.New(capturingHandler{mu: &mu, recs: &recs}))
+	defer slog.SetDefault(prev)
+
+	var attempts atomic.Int64
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		io.Copy(io.Discard, r.Body)
+		if attempts.Add(1) < 3 {
+			w.WriteHeader(http.StatusInternalServerError) // transient: two 500s
+			return
+		}
+		w.WriteHeader(http.StatusOK)
+	}))
+	defer server.Close()
+
+	c := quickwit.NewClient(server.URL + "/api/v1/test")
+	c.SetConcurrent(1)
+	c.SetBatchSize(1)
+	defer c.Close()
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+	if err := c.IngestSync(ctx, map[string]any{"index": 0}); err != nil {
+		t.Fatalf("IngestSync returned %v, want nil after retries", err)
+	}
+
+	mu.Lock()
+	defer mu.Unlock()
+	var found bool
+	for _, r := range recs {
+		if r.Message != "quickwit: flush failed, retrying indefinitely" {
+			continue
+		}
+		found = true
+		var cause string
+		r.Attrs(func(a slog.Attr) bool {
+			if a.Key == "error" {
+				cause = a.Value.String()
+				return false
+			}
+			return true
+		})
+		if cause == "" || cause == "<nil>" {
+			t.Errorf("retry log has no error cause, want the 5xx status; record=%+v", r)
+			continue
+		}
+		if !strings.Contains(cause, "500") {
+			t.Errorf("retry log error = %q, want it to name the 500 status", cause)
+		}
+	}
+	if !found {
+		t.Fatal("did not capture a 'flush failed, retrying indefinitely' log line")
+	}
+}
diff --git a/quickwit.go b/quickwit.go
index a4419d1..dab2b84 100644
--- a/quickwit.go
+++ b/quickwit.go
@@ -799,6 +799,12 @@ func (c *Client) loop() {
 	// reset at the top of every flush so it only reflects the most recent attempt.
 	var retryAfter time.Duration
 
+	// lastErr carries the reason the most recent flush failed out of flush() and
+	// into retryFlush()'s log line, so "flush failed, retrying indefinitely" says
+	// why (transport error, timeout, non-2xx status, …). Reset at the top of every
+	// flush like retryAfter, so it only reflects the most recent attempt.
+	var lastErr error
+
 	base := strings.TrimSuffix(c.endpoint, "/") + "/ingest"
 	// Two precomputed URLs: the default (commit=auto, used for fire-and-forget)
 	// and the tracked variant carrying the configured commit mode. They are equal
@@ -813,6 +819,7 @@ func (c *Client) loop() {
 		}
 
 		retryAfter = 0
+		lastErr = nil
 		buf.Reset()
 
 		// encodeFailures can only ever hold fire-and-forget items (ack == nil):
@@ -861,10 +868,12 @@ func (c *Client) loop() {
 			gzw.Reset(&gzBuf)
 			if _, err := gzw.Write(buf.Bytes()); err != nil {
 				slog.Error("quickwit: failed to gzip ingest body", "error", err)
+				lastErr = err
 				return false
 			}
 			if err := gzw.Close(); err != nil {
 				slog.Error("quickwit: failed to finalize gzip ingest body", "error", err)
+				lastErr = err
 				return false
 			}
 			body = gzBuf.Bytes()
@@ -878,6 +887,7 @@ func (c *Client) loop() {
 		defer cancel()
 		req, err := http.NewRequestWithContext(ctx, http.MethodPost, reqURL, bytes.NewReader(body))
 		if err != nil {
+			lastErr = err
 			return false
 		}
 		if useGzip {
@@ -887,6 +897,7 @@ func (c *Client) loop() {
 
 		resp, err := c.httpClient().Do(req)
 		if err != nil {
+			lastErr = fmt.Errorf("quickwit: ingest request failed: %w", err)
 			return false
 		}
 
@@ -895,6 +906,7 @@ func (c *Client) loop() {
 			resp.Body.Close()
 
 			slog.Error("quickwit: ingest status not ok", "status", resp.Status)
+			lastErr = fmt.Errorf("quickwit: ingest status %s", resp.Status)
 
 			if resp.StatusCode == http.StatusRequestEntityTooLarge {
 				if c.autoReduceBatchSize {
@@ -952,6 +964,7 @@ func (c *Client) loop() {
 		if readErr != nil {
 			// A truncated/incomplete 200 is ambiguous — retry rather than settle.
 			slog.Error("quickwit: failed to read ingest response", "error", readErr)
+			lastErr = fmt.Errorf("quickwit: failed to read ingest response: %w", readErr)
 			return false
 		}
 
@@ -1039,7 +1052,7 @@ func (c *Client) loop() {
 				}
 
 				attempt++
-				slog.Info("quickwit: flush failed, retrying indefinitely", "attempt", attempt)
+				slog.Info("quickwit: flush failed, retrying indefinitely", "attempt", attempt, "error", lastErr)
 
 				// Equal-jittered backoff, raised to a server-requested Retry-After
 				// when present. The sleep is interruptible so Close (or a long
@@ -1085,7 +1098,7 @@ func (c *Client) loop() {
 			}
 
 			attempt++
-			slog.Info("quickwit: flush failed while closing, retrying", "attempt", attempt)
+			slog.Info("quickwit: flush failed while closing, retrying", "attempt", attempt, "error", lastErr)
 			sleep := jitterBackoff(backoff)
 			if sleep > remaining {
 				sleep = remaining // never sleep past the deadline