diff --git a/application/reconciliation/service.go b/application/reconciliation/service.go
new file mode 100644
index 0000000..c979868
--- /dev/null
+++ b/application/reconciliation/service.go
@@ -0,0 +1,53 @@
+// Package reconciliation provides read-only integrity checks for the GL.
+package reconciliation
+
+import (
+	"context"
+	"fmt"
+
+	"gl/domain/ledger"
+)
+
+type Repository interface {
+	List(context.Context, ledger.JournalFilter) ([]ledger.Journal, error)
+}
+
+type Report struct {
+	Scanned int
+	Valid   int
+	Invalid []Issue
+}
+
+type Issue struct {
+	JournalID string
+	Error     string
+}
+
+// Run scans all posted journals without mutating the ledger.
+func Run(ctx context.Context, repository Repository, pageSize int) (Report, error) {
+	if repository == nil {
+		return Report{}, fmt.Errorf("reconciliation repository is required")
+	}
+	if pageSize <= 0 || pageSize > 200 {
+		pageSize = 100
+	}
+	report := Report{Invalid: make([]Issue, 0)}
+	for offset := 0; ; offset += pageSize {
+		journals, err := repository.List(ctx, ledger.JournalFilter{Limit: pageSize, Offset: offset})
+		if err != nil {
+			return Report{}, fmt.Errorf("list journals at offset %d: %w", offset, err)
+		}
+		for _, journal := range journals {
+			report.Scanned++
+			if err := journal.Validate(); err != nil {
+				report.Invalid = append(report.Invalid, Issue{JournalID: journal.ID, Error: err.Error()})
+				continue
+			}
+			report.Valid++
+		}
+		if len(journals) < pageSize {
+			break
+		}
+	}
+	return report, nil
+}
diff --git a/application/reconciliation/service_test.go b/application/reconciliation/service_test.go
new file mode 100644
index 0000000..2f2aea5
--- /dev/null
+++ b/application/reconciliation/service_test.go
@@ -0,0 +1,43 @@
+package reconciliation
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	"gl/domain/ledger"
+)
+
+type repositoryStub struct {
+	pages [][]ledger.Journal
+}
+
+func (r repositoryStub) List(_ context.Context, filter ledger.JournalFilter) ([]ledger.Journal, error) {
+	index := filter.Offset / filter.Limit
+	if index >= len(r.pages) {
+		return nil, nil
+	}
+	return r.pages[index], nil
+}
+
+func validJournal() ledger.Journal {
+	return ledger.Journal{ID: "j1", SourceService: "wallet", IdempotencyKey: "k1", SourceTransactionID: "t1", EffectKind: "transfer", EventVersion: 1, OccurredAt: ledgerTestTime(), PayloadHash: "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa", Entries: []ledger.Entry{{LineNumber: 1, Account: ledger.AccountReference{Class: ledger.AccountClassTreasury, AssetID: 1}, Amount: mustAmount("-1")}, {LineNumber: 2, Account: ledger.AccountReference{Class: ledger.AccountClassTreasury, AssetID: 1}, Amount: mustAmount("1")}}}
+}
+
+func ledgerTestTime() (t time.Time)         { return time.Unix(1, 0).UTC() }
+func mustAmount(value string) ledger.Amount { amount, _ := ledger.ParseAmount(value); return amount }
+
+func TestRunScansAndReportsInvalidJournals(t *testing.T) {
+	valid := validJournal()
+	invalid := valid
+	invalid.Entries = append([]ledger.Entry(nil), valid.Entries...)
+	invalid.ID = "bad"
+	invalid.Entries[1].Amount = mustAmount("2")
+	report, err := Run(context.Background(), repositoryStub{pages: [][]ledger.Journal{{valid}, {invalid}}}, 1)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if report.Scanned != 2 || report.Valid != 1 || len(report.Invalid) != 1 || report.Invalid[0].JournalID != "bad" {
+		t.Fatalf("unexpected report: %+v", report)
+	}
+}