diff --git a/mirror_lifecycle.go b/mirror_lifecycle.go
index 41d69df7e..e358e6e05 100644
--- a/mirror_lifecycle.go
+++ b/mirror_lifecycle.go
@@ -15,13 +15,19 @@
 package tessera
 
 import (
+	"bytes"
 	"context"
+	"encoding/gob"
+	"encoding/hex"
 	"errors"
 	"fmt"
+	"io"
 	"iter"
+	"log/slog"
 
 	"github.com/transparency-dev/tessera/api"
 	"github.com/transparency-dev/tessera/api/layout"
+	"github.com/transparency-dev/tessera/internal/parse"
 	"golang.org/x/mod/sumdb/note"
 )
 
@@ -133,12 +139,144 @@ type MirrorPackage struct {
 // AddEntries processes a stream of entry packages, verifies subtree consistency proofs,
 // and durably commits entries to the log.
 //
-// Returns the next required entry index, a recent pending checkpoint size, an opaque ticket for future invocations, and, optionally, a cosignature over a pending checkpoint whose size matches uploadEnd if one exists.
-func (mt *MirrorTarget) AddEntries(ctx context.Context, uploadStart, uploadEnd uint64, ticket []byte, next func() (*MirrorPackage, error)) (uint64, uint64, []byte, []byte, error) {
-	return 0, 0, nil, nil, errors.New("unimplemented")
+// Returns the next required entry index, a recent pending checkpoint size, an opaque
+// ticket for future invocations, and, optionally, a cosignature over a pending checkpoint
+// whose size matches uploadEnd if one exists.
+func (mt *MirrorTarget) AddEntries(ctx context.Context, uploadStart, uploadEnd uint64, ticketBytes []byte, next func() (*MirrorPackage, error)) (nextEntry uint64, pendingSize uint64, newTicket []byte, cosigs []byte, err error) {
+	curIntegratedSize, err := mt.reader.IntegratedSize(ctx)
+	if err != nil {
+		return 0, 0, nil, nil, fmt.Errorf("failed to read integrated size: %w", err)
+	}
+	var t *ticket
+	if t, err = mt.openTicket(ctx, ticketBytes); err != nil {
+		// Invalid or empty ticket, return a new one.
+		pendingCP, err := mt.cpSource(ctx)
+		if err != nil {
+			return 0, 0, nil, nil, fmt.Errorf("failed to get pending checkpoint: %v", err)
+		}
+		if len(pendingCP) == 0 {
+			return 0, 0, nil, nil, ErrNoPendingCheckpoint
+		}
+		t = &ticket{
+			PendingCP: pendingCP,
+		}
+		ticketBytes, err = mt.sealTicket(ctx, t)
+		if err != nil {
+			return 0, 0, nil, nil, fmt.Errorf("failed to create ticket: %v", err)
+		}
+
+		// If the client didn't provide a [valid] ticket, then we don't have a pending
+		// checkpoint to validate against, so we return a new ticket with the
+		// current checkpoint.
+		_, pendingSize, _, err := parse.CheckpointUnsafe(t.PendingCP)
+		if err != nil {
+			slog.ErrorContext(ctx, "Invalid pending checkpoint from source", slog.String("pending_checkpoint", string(t.PendingCP)), slog.String("error", err.Error()))
+			return 0, 0, nil, nil, fmt.Errorf("failed to parse pending checkpoint while creating ticket: %v", err)
+		}
+		return curIntegratedSize, pendingSize, ticketBytes, nil, ErrConflict
+	}
+
+	var pendingRoot []byte
+	_, pendingSize, pendingRoot, err = parse.CheckpointUnsafe(t.PendingCP)
+	if err != nil {
+		slog.ErrorContext(ctx, "Invalid pending checkpoint in ticket", slog.String("pending_checkpoint", string(t.PendingCP)), slog.String("error", err.Error()))
+		return 0, 0, nil, nil, fmt.Errorf("failed to parse pending checkpoint from ticket: %v", err)
+	}
+
+	// Handle 409 Conflicts:
+	//    - Zero-request check: If upload_start == 0 and upload_end == 0, the client is
+	//      requesting initial mirror information.
+	//    - upload_end:
+	//      * MUST be equal to the tree size of a known pending checkpoint value.
+	//      * MUST NOT be less than the mirror checkpoint's tree size.
+	//    - upload_start:
+	//      * MUST NOT be greater than the mirror's next expected entry index.
+	//      * MUST NOT be too far below the mirror's next entry index.
+	if (uploadStart == 0 && uploadEnd == 0) ||
+		(uploadEnd != pendingSize || uploadEnd < curIntegratedSize) ||
+		(uploadStart > curIntegratedSize) {
+		// TODO(al): add flexibility about re-writing some entries
+		return curIntegratedSize, pendingSize, ticketBytes, nil, ErrConflict
+	}
+
+
+	bi := func(yield func(api.EntryBundle) bool) {
+		for {
+			pkg, err := next()
+			if err != nil {
+				if err == io.EOF {
+					return
+				}
+				// TODO(al): handle this
+				slog.WarnContext(ctx, "NextPackage returned an error", slog.String("error", err.Error()))
+				return
+			}
+
+			// TODO(al): verify entries+proof under checkpoint (Failure -> 422 Unprocessable Entity).
+
+			if !yield(api.EntryBundle{Entries: pkg.Entries}) {
+				return
+			}
+		}
+	}
+
+	// TODO(al): Check uploadStart is aligned to EntryBundleWidth.
+	bundleIdx := uploadStart/layout.EntryBundleWidth
+
+	nextEntry, newRoot, err := mt.writer.IntegrateBundles(ctx, bundleIdx, bi)
+	switch {
+	case err != nil:
+		return 0, 0, nil, nil, err
+	case nextEntry == pendingSize:
+		if !bytes.Equal(pendingRoot, newRoot) {
+			slog.ErrorContext(ctx, "CORRUPTION DETECTED - pending root != calculated root", slog.String("calculated_root", hex.EncodeToString(newRoot)), slog.String("pending_checkpoint", string(t.PendingCP)))
+			return 0, 0, nil, nil, errors.New("internal error")
+		}
+		// This is a complete upload.
+		// TODO(al): 
+		// 		- cosign the pending checkpoint,
+		// 		- publish it IFF we not overwriting a larger checkpoint
+		// 		- If published, then return the cosig(s) to the caller.
+		return nextEntry, pendingSize, nil, []byte("— test cosig\n"), nil
+	case nextEntry > pendingSize:
+		// TODO(al): ticket is stale, probably need to update the ticket?
+		slog.WarnContext(ctx, "nextEntry > pendingSize", slog.Uint64("nextEntry", nextEntry), slog.Uint64("pendingSize", pendingSize))
+		return nextEntry, pendingSize, ticketBytes, nil, nil
+	default:
+		// Incomplete upload, return an updated ticket with the current checkpoint.
+		return nextEntry, pendingSize, ticketBytes, nil, nil
+	}
 }
 
 // IntegratedSize returns the size of the current integrated log.
 func (mt *MirrorTarget) IntegratedSize(ctx context.Context) (uint64, error) {
 	return mt.reader.IntegratedSize(ctx)
 }
+
+// ticket is the underlying structure of an add-entries ticket.
+type ticket struct {
+	// PendingCP holds the raw pending checkpoint bytes.
+	PendingCP []byte
+}
+
+func (mt *MirrorTarget) sealTicket(ctx context.Context, t *ticket) ([]byte, error) {
+	out := bytes.Buffer{}
+	if err := gob.NewEncoder(&out).Encode(t); err != nil {
+		return nil, fmt.Errorf("ticket encoding failed: %v", err)
+	}
+	// TODO(al): harden ticket & bind to this particular log mirror.
+	return out.Bytes(), nil
+}
+
+func (mt *MirrorTarget) openTicket(ctx context.Context, ticketBytes []byte) (*ticket, error) {
+	if len(ticketBytes) == 0 {
+		return nil, errors.New("empty ticket")
+	}
+	// TODO(al): harden ticket & verify it's for this particular log mirror.
+	var t ticket
+	if err := gob.NewDecoder(bytes.NewReader(ticketBytes)).Decode(&t); err != nil {
+		return nil, fmt.Errorf("ticket decoding failed: %v", err)
+	}
+	return &t, nil
+}
+	
diff --git a/mirror_lifecycle_test.go b/mirror_lifecycle_test.go
new file mode 100644
index 000000000..3292aa96b
--- /dev/null
+++ b/mirror_lifecycle_test.go
@@ -0,0 +1,192 @@
+// Copyright 2026 The Tessera authors. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package tessera
+
+import (
+	"context"
+	"encoding/base64"
+	"errors"
+	"fmt"
+	"io"
+	"iter"
+	"testing"
+
+	"github.com/transparency-dev/tessera/api"
+)
+
+type fakeMirrorWriter struct {
+	integrateFunc func(ctx context.Context, from uint64, bundles iter.Seq[api.EntryBundle]) (uint64, []byte, error)
+	sizeFunc      func(ctx context.Context) (uint64, error)
+}
+
+func (f *fakeMirrorWriter) IntegrateBundles(ctx context.Context, from uint64, bundles iter.Seq[api.EntryBundle]) (uint64, []byte, error) {
+	if f.integrateFunc != nil {
+		return f.integrateFunc(ctx, from, bundles)
+	}
+	return from, nil, nil
+}
+
+func (f *fakeMirrorWriter) IntegratedSize(ctx context.Context) (uint64, error) {
+	if f.sizeFunc != nil {
+		return f.sizeFunc(ctx)
+	}
+	return 0, nil
+}
+
+type fakeLogReader struct {
+	sizeFunc func(ctx context.Context) (uint64, error)
+}
+
+func (f *fakeLogReader) IntegratedSize(ctx context.Context) (uint64, error) {
+	if f.sizeFunc != nil {
+		return f.sizeFunc(ctx)
+	}
+	return 0, nil
+}
+func (f *fakeLogReader) ReadCheckpoint(ctx context.Context) ([]byte, error) { return nil, nil }
+func (f *fakeLogReader) ReadTile(ctx context.Context, level, index uint64, p uint8) ([]byte, error) {
+	return nil, nil
+}
+func (f *fakeLogReader) ReadEntryBundle(ctx context.Context, index uint64, p uint8) ([]byte, error) {
+	return nil, nil
+}
+func (f *fakeLogReader) NextIndex(ctx context.Context) (uint64, error) { return 0, nil }
+
+const (
+	testPendingCPOrigin = "test-origin"
+	testPendingCPSize   = uint64(200)
+	testPendingCPRoot   = "47DEQpj8HBSa+/TImW+5JCeuQeRkm5NMpJWZG3hSuFU="
+)
+
+var testPendingCP = fmt.Sprintf("%s\n%d\n%s\n— test-sig\n", testPendingCPOrigin, testPendingCPSize, testPendingCPRoot)
+
+func newTestMirrorTarget(size uint64) *MirrorTarget {
+	return &MirrorTarget{
+		writer: &fakeMirrorWriter{
+			sizeFunc: func(ctx context.Context) (uint64, error) { return size, nil },
+		},
+		reader: &fakeLogReader{
+			sizeFunc: func(ctx context.Context) (uint64, error) { return size, nil },
+		},
+		cpSource: func(ctx context.Context) ([]byte, error) {
+			return []byte(testPendingCP), nil
+		},
+	}
+}
+
+func TestMirrorTarget_AddEntries_NoTicket(t *testing.T) {
+	const (
+		testIntegratedSize = uint64(100)
+	)
+	ctx := context.Background()
+	mt := newTestMirrorTarget(testIntegratedSize)
+
+	nextEntry, pendingSize, newTicket, _, err := mt.AddEntries(ctx, 0, 0, nil, func() (*MirrorPackage, error) {
+		return nil, io.EOF
+	})
+	if !errors.Is(err, ErrConflict) {
+		t.Errorf("got %v, want ErrConflict", err)
+	}
+	if got, want := nextEntry, testIntegratedSize; got != want {
+		t.Errorf("got %d, want %d", got, want)
+	}
+	if got, want := pendingSize, testPendingCPSize; got != want {
+		t.Errorf("got %d, want %d", got, want)
+	}
+	if len(newTicket) == 0 {
+		t.Errorf("got empty ticket, want non-empty")
+	}
+}
+
+func TestMirrorTarget_AddEntries_RangeConflict(t *testing.T) {
+	const (
+		testUploadStart = uint64(100)
+		testUploadEnd   = uint64(250)
+		testPendingSize = uint64(200)
+		testIntegratedSize = uint64(150)
+	)
+	ctx := context.Background()
+	mt := &MirrorTarget{
+		writer: &fakeMirrorWriter{
+			sizeFunc: func(ctx context.Context) (uint64, error) { return testIntegratedSize, nil },
+		},
+		reader: &fakeLogReader{
+			sizeFunc: func(ctx context.Context) (uint64, error) { return testIntegratedSize, nil },
+		},
+		cpSource: func(ctx context.Context) ([]byte, error) { return []byte(testPendingCP), nil },
+	}
+
+	validTicket, err := mt.sealTicket(ctx, &ticket{PendingCP: []byte(testPendingCP)})
+	if err != nil {
+		t.Fatalf("sealTicket failed: %v", err)
+	}
+
+	// testUploadEnd  != pendingSize -> conflict
+	_, _, _, _, err = mt.AddEntries(ctx, testUploadStart, testUploadEnd, validTicket, func() (*MirrorPackage, error) {
+		return nil, io.EOF
+	})
+	if !errors.Is(err, ErrConflict) {
+		t.Errorf("want ErrConflict, got %v", err)
+	}
+}
+
+func TestMirrorTarget_AddEntries_CompleteUpload(t *testing.T) {
+	const (
+		testIntegratedSize = uint64(100)
+		testUploadStart    = uint64(100)
+		testUploadEnd      = uint64(200)
+	)
+
+	ctx := context.Background()
+	mt := &MirrorTarget{
+		writer: &fakeMirrorWriter{
+			integrateFunc: func(ctx context.Context, from uint64, bundles iter.Seq[api.EntryBundle]) (uint64, []byte, error) {
+				// Consume iterator
+				for range bundles {
+				}
+				decodedRoot, err := base64.StdEncoding.DecodeString(testPendingCPRoot)
+				if err != nil {
+					return 0, nil, fmt.Errorf("TEST ERROR: %v", err)
+				}
+				return testPendingCPSize, decodedRoot, nil
+			},
+		},
+		reader: &fakeLogReader{
+			sizeFunc: func(ctx context.Context) (uint64, error) { return testIntegratedSize, nil },
+		},
+		cpSource: func(ctx context.Context) ([]byte, error) { return []byte(testPendingCP), nil },
+	}
+
+	validTicket, err := mt.sealTicket(ctx, &ticket{PendingCP: []byte(testPendingCP)})
+	if err != nil {
+		t.Fatalf("sealTicket failed: %v", err)
+	}
+
+	nextEntry, pendingSize, _, cosigs, err := mt.AddEntries(ctx, testUploadStart, testUploadEnd, validTicket, func() (*MirrorPackage, error) {
+		return nil, io.EOF
+	})
+	if err != nil {
+		t.Errorf("unexpected error: %v", err)
+	}
+	if got, want := nextEntry, testUploadEnd; got != want {
+		t.Errorf("got %d, want %d", got, want)
+	}
+	if got, want := pendingSize, testUploadEnd; got != want {
+		t.Errorf("got %d, want %d", got, want)
+	}
+	if len(cosigs) == 0 {
+		t.Errorf("got empty cosigs, want non-empty")
+	}
+}