diff --git a/go/genkit/logging.go b/go/genkit/logging.go
index 69d97c4bf..1b1dc9a3f 100644
--- a/go/genkit/logging.go
+++ b/go/genkit/logging.go
@@ -39,3 +39,8 @@ func logger(ctx context.Context) *slog.Logger {
 	}
 	return slog.Default()
 }
+
+// DebugLog is a helper function for plugins to log debugging info.
+func DebugLog(ctx context.Context, msg string, args ...any) {
+	logger(ctx).Debug(msg, args...)
+}
diff --git a/go/plugins/localvec/localvec.go b/go/plugins/localvec/localvec.go
new file mode 100644
index 000000000..c95f24eb5
--- /dev/null
+++ b/go/plugins/localvec/localvec.go
@@ -0,0 +1,248 @@
+// Copyright 2024 Google LLC
+//
+// 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 localvec is a local vector database for development and testing.
+// The database is stored in a file in the local file system.
+// Production code should use a real vector database.
+package localvec
+
+import (
+	"cmp"
+	"context"
+	"crypto/md5"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io/fs"
+	"math"
+	"os"
+	"path/filepath"
+	"slices"
+
+	"github.com/google/genkit/go/ai"
+	"github.com/google/genkit/go/genkit"
+)
+
+// New returns a new local vector database. This will register a new
+// retriever with genkit, and also return it.
+// This retriever may only be used by a single goroutine at a time.
+// This is based on js/plugins/dev-local-vectorstore/src/index.ts.
+func New(ctx context.Context, dir, name string, embedder ai.Embedder, embedderOptions any) (ai.Retriever, error) {
+	r, err := newRetriever(ctx, dir, name, embedder, embedderOptions)
+	if err != nil {
+		return nil, err
+	}
+	ai.RegisterRetriever("devLocalVectorStore/"+name, r)
+	return r, nil
+}
+
+// retriever implements the [ai.Retriever] interface
+// for a local vector database.
+type retriever struct {
+	filename        string
+	embedder        ai.Embedder
+	embedderOptions any
+	data            map[string]dbValue
+}
+
+// dbValue is the type of a document stored in the database.
+type dbValue struct {
+	Doc       *ai.Document `json:"doc"`
+	Embedding []float32    `json:"embedding"`
+}
+
+// newRetriever returns a new ai.Retriever to register.
+func newRetriever(ctx context.Context, dir, name string, embedder ai.Embedder, embedderOptions any) (ai.Retriever, error) {
+	if err := os.MkdirAll(dir, 0o755); err != nil {
+		return nil, err
+	}
+	dbname := "__db_" + name + ".json"
+	filename := filepath.Join(dir, dbname)
+	f, err := os.Open(filename)
+	var data map[string]dbValue
+	if err != nil {
+		if !errors.Is(err, fs.ErrNotExist) {
+			return nil, err
+		}
+	} else {
+		defer f.Close()
+		decoder := json.NewDecoder(f)
+		if err := decoder.Decode(&data); err != nil {
+			return nil, err
+		}
+	}
+
+	r := &retriever{
+		filename:        filename,
+		embedder:        embedder,
+		embedderOptions: embedderOptions,
+		data:            data,
+	}
+	return r, nil
+}
+
+// Index implements the genkit [ai.Retriever.Index] method.
+func (r *retriever) Index(ctx context.Context, req *ai.IndexerRequest) error {
+	for _, doc := range req.Documents {
+		ereq := &ai.EmbedRequest{
+			Document: doc,
+			Options:  r.embedderOptions,
+		}
+		vals, err := r.embedder.Embed(ctx, ereq)
+		if err != nil {
+			return fmt.Errorf("localvec index embedding failed: %v", err)
+		}
+
+		id, err := docID(doc)
+		if err != nil {
+			return err
+		}
+
+		if _, ok := r.data[id]; ok {
+			genkit.DebugLog(ctx, "localvec skipping document because already present", "id", id)
+			continue
+		}
+
+		if r.data == nil {
+			r.data = make(map[string]dbValue)
+		}
+
+		r.data[id] = dbValue{
+			Doc:       doc,
+			Embedding: vals,
+		}
+	}
+
+	// Update the file every time we add documents.
+	tmpname := r.filename + ".tmp"
+	f, err := os.Create(tmpname)
+	if err != nil {
+		return err
+	}
+	encoder := json.NewEncoder(f)
+	if err := encoder.Encode(r.data); err != nil {
+		return err
+	}
+	if err := f.Close(); err != nil {
+		return err
+	}
+
+	return nil
+}
+
+// RetrieverOptions may be passed in the Options field
+// of [ai.RetrieverRequest] to pass options to the retriever.
+// The Options field should be either nil or a value of type *RetrieverOptions.
+type RetrieverOptions struct {
+	K int `json:"k,omitempty"` // number of entries to return
+}
+
+// Retrieve implements the genkit [ai.Retriever.Retrieve] method.
+func (r *retriever) Retrieve(ctx context.Context, req *ai.RetrieverRequest) (*ai.RetrieverResponse, error) {
+	// Use the embedder to convert the document we want to
+	// retrieve into a vector.
+	ereq := &ai.EmbedRequest{
+		Document: req.Document,
+		Options:  r.embedderOptions,
+	}
+	vals, err := r.embedder.Embed(ctx, ereq)
+	if err != nil {
+		return nil, fmt.Errorf("localvec retrieve embedding failed: %v", err)
+	}
+
+	type scoredDoc struct {
+		score float64
+		doc   *ai.Document
+	}
+	scoredDocs := make([]scoredDoc, 0, len(r.data))
+	for _, dbv := range r.data {
+		score := similarity(vals, dbv.Embedding)
+		scoredDocs = append(scoredDocs, scoredDoc{
+			score: score,
+			doc:   dbv.Doc,
+		})
+	}
+
+	slices.SortFunc(scoredDocs, func(a, b scoredDoc) int {
+		// We want to sort by descending score,
+		// so pass b.score first to reverse the default ordering.
+		return cmp.Compare(b.score, a.score)
+	})
+
+	k := 3
+	if options, _ := req.Options.(*RetrieverOptions); options != nil {
+		k = options.K
+	}
+	k = min(k, len(scoredDocs))
+
+	docs := make([]*ai.Document, 0, k)
+	for i := 0; i < k; i++ {
+		docs = append(docs, scoredDocs[i].doc)
+	}
+
+	resp := &ai.RetrieverResponse{
+		Documents: docs,
+	}
+	return resp, nil
+}
+
+// similarity computes the [cosine similarity] between two vectors.
+//
+// [cosine similarity]: https://en.wikipedia.org/wiki/Cosine_similarity
+func similarity(vals1, vals2 []float32) float64 {
+	l2norm := func(v float64, s, t float64) (float64, float64) {
+		if v == 0 {
+			return s, t
+		}
+		a := math.Abs(v)
+		if a > t {
+			r := t / v
+			s = 1 + s*r*r
+			t = a
+		} else {
+			r := v / t
+			s = s + r*r
+		}
+		return s, t
+	}
+
+	dot := float64(0)
+	s1 := float64(1)
+	t1 := float64(0)
+	s2 := float64(1)
+	t2 := float64(0)
+
+	for i, v1f := range vals1 {
+		v1 := float64(v1f)
+		v2 := float64(vals2[i])
+		dot += v1 * v2
+		s1, t1 = l2norm(v1, s1, t1)
+		s2, t2 = l2norm(v2, s2, t2)
+	}
+
+	l1 := t1 * math.Sqrt(s1)
+	l2 := t2 * math.Sqrt(s2)
+
+	return dot / (l1 * l2)
+}
+
+// docID returns the ID to use for a Document.
+// This is intended to be the same as the genkit Typescript computation.
+func docID(doc *ai.Document) (string, error) {
+	b, err := json.Marshal(doc)
+	if err != nil {
+		return "", fmt.Errorf("localvec: error marshaling document: %v", err)
+	}
+	return fmt.Sprintf("%02x", md5.Sum(b)), nil
+}
diff --git a/go/plugins/localvec/localvec_test.go b/go/plugins/localvec/localvec_test.go
new file mode 100644
index 000000000..47baa4da2
--- /dev/null
+++ b/go/plugins/localvec/localvec_test.go
@@ -0,0 +1,100 @@
+// Copyright 2024 Google LLC
+//
+// 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 localvec
+
+import (
+	"context"
+	"math"
+	"strings"
+	"testing"
+
+	"github.com/google/genkit/go/ai"
+	"github.com/google/genkit/go/internal/fakeembedder"
+)
+
+func TestLocalVec(t *testing.T) {
+	ctx := context.Background()
+
+	// Make two very similar vectors and one different vector.
+	// Arrange for a fake embedder to return those vector
+	// when provided with documents.
+
+	const dim = 32
+	v1 := make([]float32, dim)
+	v2 := make([]float32, dim)
+	v3 := make([]float32, dim)
+	for i := range v1 {
+		v1[i] = float32(i)
+		v2[i] = float32(i)
+		v3[i] = float32(dim - i)
+	}
+	v2[0] = 1
+
+	d1 := ai.DocumentFromText("hello1", nil)
+	d2 := ai.DocumentFromText("hello2", nil)
+	d3 := ai.DocumentFromText("goodbye", nil)
+
+	embedder := fakeembedder.New()
+	embedder.Register(d1, v1)
+	embedder.Register(d2, v2)
+	embedder.Register(d3, v3)
+
+	r, err := newRetriever(ctx, t.TempDir(), "testLocalVec", embedder, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	indexerReq := &ai.IndexerRequest{
+		Documents: []*ai.Document{d1, d2, d3},
+	}
+	err = r.Index(ctx, indexerReq)
+	if err != nil {
+		t.Fatalf("Index operation failed: %v", err)
+	}
+
+	retrieverOptions := &RetrieverOptions{
+		K: 2,
+	}
+
+	retrieverReq := &ai.RetrieverRequest{
+		Document: d1,
+		Options:  retrieverOptions,
+	}
+	retrieverResp, err := r.Retrieve(ctx, retrieverReq)
+	if err != nil {
+		t.Fatalf("Retrieve operation failed: %v", err)
+	}
+
+	docs := retrieverResp.Documents
+	if len(docs) != 2 {
+		t.Errorf("got %d results, expected 2", len(docs))
+	}
+	for _, d := range docs {
+		text := d.Content[0].Text()
+		if !strings.HasPrefix(text, "hello") {
+			t.Errorf("returned doc text %q does not start with %q", text, "hello")
+		}
+	}
+}
+
+func TestSimilarity(t *testing.T) {
+	x := []float32{5, 23, 2, 5, 9}
+	y := []float32{3, 21, 2, 5, 14}
+	got := similarity(x, y)
+	want := 0.975
+	if math.Abs(got-want) > 0.001 {
+		t.Errorf("got %f, want %f", got, want)
+	}
+}