diff --git a/sumdb/internal/sumweb/test.go b/sumdb/internal/sumweb/test.go
new file mode 100644
index 0000000..4cc98f9
--- /dev/null
+++ b/sumdb/internal/sumweb/test.go
@@ -0,0 +1,133 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package sumweb
+
+import (
+	"context"
+	"fmt"
+	"net/http"
+	"strings"
+	"sync"
+
+	"golang.org/x/exp/sumdb/internal/note"
+	"golang.org/x/exp/sumdb/internal/tlog"
+)
+
+// NewTestServer constructs a new TestServer
+// that will sign its tree with the given signer key
+// (see golang.org/x/exp/sumdb/internal/note)
+// and fetch new records as needed by calling gosum.
+func NewTestServer(signer string, gosum func(path, vers string) ([]byte, error)) *TestServer {
+	return &TestServer{signer: signer, gosum: gosum}
+}
+
+// A TestServer is an in-memory implementation of Server for testing.
+type TestServer struct {
+	signer string
+	gosum  func(path, vers string) ([]byte, error)
+
+	mu      sync.Mutex
+	hashes  testHashes
+	records [][]byte
+	lookup  map[string]int64
+}
+
+// testHashes implements tlog.HashReader, reading from a slice.
+type testHashes []tlog.Hash
+
+func (h testHashes) ReadHashes(indexes []int64) ([]tlog.Hash, error) {
+	var list []tlog.Hash
+	for _, id := range indexes {
+		list = append(list, h[id])
+	}
+	return list, nil
+}
+
+func (s *TestServer) NewContext(r *http.Request) (context.Context, error) {
+	return nil, nil
+}
+
+func (s *TestServer) Signed(ctx context.Context) ([]byte, error) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	size := int64(len(s.records))
+	h, err := tlog.TreeHash(size, s.hashes)
+	if err != nil {
+		return nil, err
+	}
+	text := tlog.FormatTree(tlog.Tree{N: size, Hash: h})
+	signer, err := note.NewSigner(s.signer)
+	if err != nil {
+		return nil, err
+	}
+	return note.Sign(&note.Note{Text: string(text)}, signer)
+}
+
+func (s *TestServer) ReadRecords(ctx context.Context, id, n int64) ([][]byte, error) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	var list [][]byte
+	for i := int64(0); i < n; i++ {
+		if id+i >= int64(len(s.records)) {
+			return nil, fmt.Errorf("missing records")
+		}
+		list = append(list, s.records[id+i])
+	}
+	return list, nil
+}
+
+func (s *TestServer) FindKey(ctx context.Context, key string) (int64, error) {
+	s.mu.Lock()
+	id, ok := s.lookup[key]
+	s.mu.Unlock()
+	if ok {
+		return id, nil
+	}
+
+	// Look up module and compute go.sum lines.
+	i := strings.Index(key, "@")
+	if i < 0 {
+		return 0, fmt.Errorf("invalid lookup key %q", key)
+	}
+	path, vers := key[:i], key[i+1:]
+	data, err := s.gosum(path, vers)
+	if err != nil {
+		return 0, err
+	}
+
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	// We ran the fetch without the lock.
+	// If another fetch happened and committed, use it instead.
+	id, ok = s.lookup[key]
+	if ok {
+		return id, nil
+	}
+
+	// Add record.
+	id = int64(len(s.records))
+	s.records = append(s.records, data)
+	if s.lookup == nil {
+		s.lookup = make(map[string]int64)
+	}
+	s.lookup[key] = id
+	hashes, err := tlog.StoredHashesForRecordHash(id, tlog.RecordHash([]byte(data)), s.hashes)
+	if err != nil {
+		panic(err)
+	}
+	s.hashes = append(s.hashes, hashes...)
+
+	return id, nil
+}
+
+func (s *TestServer) ReadTileData(ctx context.Context, t tlog.Tile) ([]byte, error) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	return tlog.ReadTileData(t, s.hashes)
+}