diff --git a/backend/internal/service/idempotency.go b/backend/internal/service/idempotency.go
index 2a86bd6038..4241468789 100644
--- a/backend/internal/service/idempotency.go
+++ b/backend/internal/service/idempotency.go
@@ -11,6 +11,7 @@ import (
 	"strings"
 	"sync"
 	"time"
+	"unicode/utf8"
 
 	infraerrors "github.com/Wei-Shaw/sub2api/internal/pkg/errors"
 	"github.com/Wei-Shaw/sub2api/internal/util/logredact"
@@ -454,11 +455,21 @@ func (c *IdempotencyCoordinator) marshalStoredResponse(data any) (string, error)
 	}
 	redacted := logredact.RedactText(string(raw))
 	if c.cfg.MaxStoredResponseLen > 0 && len(redacted) > c.cfg.MaxStoredResponseLen {
-		redacted = redacted[:c.cfg.MaxStoredResponseLen] + "...(truncated)"
+		redacted = truncateUTF8(redacted, c.cfg.MaxStoredResponseLen) + "...(truncated)"
 	}
 	return redacted, nil
 }
 
+func truncateUTF8(s string, maxBytes int) string {
+	if maxBytes <= 0 || len(s) <= maxBytes {
+		return s
+	}
+	for maxBytes > 0 && !utf8.ValidString(s[:maxBytes]) {
+		maxBytes--
+	}
+	return s[:maxBytes]
+}
+
 func (c *IdempotencyCoordinator) decodeStoredResponse(stored *string) (any, error) {
 	if stored == nil || strings.TrimSpace(*stored) == "" {
 		return map[string]any{}, nil
diff --git a/backend/internal/service/idempotency_test.go b/backend/internal/service/idempotency_test.go
index 6ff75d1c3e..e11c9e75f4 100644
--- a/backend/internal/service/idempotency_test.go
+++ b/backend/internal/service/idempotency_test.go
@@ -3,10 +3,12 @@ package service
 import (
 	"context"
 	"errors"
+	"strings"
 	"sync"
 	"sync/atomic"
 	"testing"
 	"time"
+	"unicode/utf8"
 
 	infraerrors "github.com/Wei-Shaw/sub2api/internal/pkg/errors"
 	"github.com/stretchr/testify/require"
@@ -441,6 +443,51 @@ func TestIdempotencyCoordinator_StoreUnavailableMetrics(t *testing.T) {
 	require.GreaterOrEqual(t, GetIdempotencyMetricsSnapshot().StoreUnavailableTotal, uint64(1))
 }
 
+type utf8RejectingIdempotencyRepo struct {
+	inMemoryIdempotencyRepo
+}
+
+func newUTF8RejectingIdempotencyRepo() *utf8RejectingIdempotencyRepo {
+	return &utf8RejectingIdempotencyRepo{inMemoryIdempotencyRepo: *newInMemoryIdempotencyRepo()}
+}
+
+func (r *utf8RejectingIdempotencyRepo) MarkSucceeded(ctx context.Context, id int64, responseStatus int, responseBody string, expiresAt time.Time) error {
+	if !utf8.ValidString(responseBody) {
+		return errors.New(`pq: invalid byte sequence for encoding "UTF8": 0xe8 0xb4 0x2e`)
+	}
+	return r.inMemoryIdempotencyRepo.MarkSucceeded(ctx, id, responseStatus, responseBody, expiresAt)
+}
+
+func TestIdempotencyCoordinator_TruncatedStoredResponseRemainsUTF8(t *testing.T) {
+	repo := newUTF8RejectingIdempotencyRepo()
+	cfg := DefaultIdempotencyConfig()
+	cfg.MaxStoredResponseLen = len(`{"message":"`) + 2
+	coordinator := NewIdempotencyCoordinator(repo, cfg)
+
+	opts := IdempotencyExecuteOptions{
+		Scope:          "test.scope.truncate_utf8",
+		Method:         "POST",
+		Route:          "/api/v1/accounts/import/codex-session",
+		ActorScope:     "admin:1",
+		RequireKey:     true,
+		IdempotencyKey: "truncate-utf8",
+		Payload:        map[string]any{"content": "codex-session"},
+	}
+
+	result, err := coordinator.Execute(context.Background(), opts, func(ctx context.Context) (any, error) {
+		return map[string]any{"message": strings.Repeat("\u8d26", 8)}, nil
+	})
+	require.NoError(t, err)
+	require.NotNil(t, result)
+
+	stored, err := repo.GetByScopeAndKeyHash(context.Background(), opts.Scope, HashIdempotencyKey(opts.IdempotencyKey))
+	require.NoError(t, err)
+	require.NotNil(t, stored)
+	require.NotNil(t, stored.ResponseBody)
+	require.True(t, utf8.ValidString(*stored.ResponseBody))
+	require.Contains(t, *stored.ResponseBody, "...(truncated)")
+}
+
 func TestDefaultIdempotencyCoordinatorAndTTLs(t *testing.T) {
 	SetDefaultIdempotencyCoordinator(nil)
 	require.Nil(t, DefaultIdempotencyCoordinator())