Add native Go Resend current parity (#114)

* Add native Go Resend current parity

- Implements native Resend emails, domains, API keys, audiences, contacts, inbox UI, and explicit JSON seed handling in the Go runtime.

- Adds official Resend SDK external E2E coverage plus CI wiring for the native Go service.

- Updates Resend docs, skills, and service listings for native runtime support.

* Reject unsupported native seed configs

Author: ctate

.github/workflows/ci.yml

@@ -53,6 +53,22 @@ jobs:
           test "${ready:-}" = "1"
           AWS_EMULATOR_E2E_URL=http://127.0.0.1:4017 pnpm --dir packages/@emulators/aws exec vitest run src/__tests__/aws-sdk.e2e.test.ts -t "sqs|iam|sts"
 
+      - name: Native Resend SDK E2E
+        run: |
+          go build -o /tmp/emulate-native ./cmd/emulate
+          /tmp/emulate-native start --service resend --port 4018 &
+          server_pid=$!
+          trap 'kill "$server_pid" 2>/dev/null || true' EXIT
+          for attempt in {1..50}; do
+            if curl -fsS http://127.0.0.1:4018/_emulate/health >/dev/null; then
+              ready=1
+              break
+            fi
+            sleep 0.2
+          done
+          test "${ready:-}" = "1"
+          RESEND_EMULATOR_E2E_URL=http://127.0.0.1:4018 pnpm --dir packages/@emulators/resend exec vitest run src/__tests__/resend.test.ts -t "real resend SDK E2E"
+
       - run: node scripts/sync-versions.mjs --check
 
       - run: pnpm format:check

README.md

@@ -16,7 +16,12 @@ All services start with sensible defaults. No config file needed:
 - **Slack** on `http://localhost:4003`
 - **Apple** on `http://localhost:4004`
 - **Microsoft** on `http://localhost:4005`
-- **AWS** on `http://localhost:4006`
+- **Okta** on `http://localhost:4006`
+- **AWS** on `http://localhost:4007`
+- **Resend** on `http://localhost:4008`
+- **Stripe** on `http://localhost:4009`
+- **MongoDB Atlas** on `http://localhost:4010`
+- **Clerk** on `http://localhost:4011`
 
 ## CLI
 
@@ -141,7 +146,7 @@ afterAll(() => Promise.all([github.close(), vercel.close()]))
 
 | Option | Default | Description |
 |--------|---------|-------------|
-| `service` | *(required)* | Service name: `'vercel'`, `'github'`, `'google'`, `'slack'`, `'apple'`, `'microsoft'`, or `'aws'` |
+| `service` | *(required)* | Service name: `'vercel'`, `'github'`, `'google'`, `'slack'`, `'apple'`, `'microsoft'`, `'okta'`, `'aws'`, `'resend'`, `'stripe'`, `'mongoatlas'`, or `'clerk'` |
 | `port` | `4000` | Port for the HTTP server |
 | `seed` | none | Inline seed data (same shape as YAML config) |
 | `baseUrl` | none | Override advertised base URL. Per-service `baseUrl` in seed config takes highest priority, then this option, then `EMULATE_BASE_URL` env var (supports `{service}`), then `PORTLESS_URL` (supports `{service}`, automatically set by the `portless` CLI wrapper), then `http://localhost:<port>`. |
@@ -693,6 +698,19 @@ Manual STS requests can use `POST /sts/` with an `Action` form parameter. In the
 
 - `GetCallerIdentity`, `AssumeRole`
 
+## Resend
+
+Resend email API emulation with local capture for sent emails, domains, API keys, audiences, contacts, and an inbox UI. Set `RESEND_BASE_URL` before importing the official Resend Node.js SDK and the SDK will send to the emulator.
+
+The experimental native Go runtime serves the same current Resend routes, supports explicit JSON seed configs for Resend through `--seed`, and is verified against the official `resend` SDK for emails, batch email sends, domains, API keys, and legacy audience contacts.
+
+- `POST /emails`, `POST /emails/batch`, `GET /emails`, `GET /emails/:id`, `POST /emails/:id/cancel`
+- `POST /domains`, `GET /domains`, `GET /domains/:id`, `DELETE /domains/:id`, `POST /domains/:id/verify`
+- `POST /api-keys`, `GET /api-keys`, `DELETE /api-keys/:id`
+- `POST /audiences`, `GET /audiences`, `DELETE /audiences/:id`
+- `POST /audiences/:audience_id/contacts`, `GET /audiences/:audience_id/contacts`, `DELETE /audiences/:audience_id/contacts/:id`
+- `GET /inbox`, `GET /inbox/:id`
+
 ## Next.js Integration
 
 Embed emulators directly in your Next.js app so they run on the same origin. This solves the Vercel preview deployment problem where OAuth callback URLs change with every deployment.

apps/web/app/docs/page.mdx

@@ -16,11 +16,12 @@ All services start with sensible defaults. No config file needed:
 - **Slack** on `http://localhost:4003`
 - **Apple** on `http://localhost:4004`
 - **Microsoft** on `http://localhost:4005`
-- **AWS** on `http://localhost:4006`
-- **Okta** on `http://localhost:4007`
-- **MongoDB Atlas** on `http://localhost:4008`
-- **Resend** on `http://localhost:4009`
-- **Stripe** on `http://localhost:4010`
+- **Okta** on `http://localhost:4006`
+- **AWS** on `http://localhost:4007`
+- **Resend** on `http://localhost:4008`
+- **Stripe** on `http://localhost:4009`
+- **MongoDB Atlas** on `http://localhost:4010`
+- **Clerk** on `http://localhost:4011`
 
 ## CLI
 

apps/web/app/docs/programmatic-api/page.mdx

@@ -59,7 +59,7 @@ afterAll(() => Promise.all([github.close(), vercel.close()]))
     <tr>
       <td><code>service</code></td>
       <td><em>(required)</em></td>
-      <td>Service name: <code>'vercel'</code>, <code>'github'</code>, <code>'google'</code>, <code>'slack'</code>, <code>'apple'</code>, <code>'microsoft'</code>, <code>'aws'</code>, <code>'okta'</code>, <code>'mongoatlas'</code>, <code>'resend'</code>, or <code>'stripe'</code></td>
+      <td>Service name: <code>'vercel'</code>, <code>'github'</code>, <code>'google'</code>, <code>'slack'</code>, <code>'apple'</code>, <code>'microsoft'</code>, <code>'okta'</code>, <code>'aws'</code>, <code>'resend'</code>, <code>'stripe'</code>, <code>'mongoatlas'</code>, or <code>'clerk'</code></td>
     </tr>
     <tr>
       <td><code>port</code></td>

apps/web/app/docs/resend/page.mdx

@@ -2,6 +2,10 @@
 
 Resend email API emulation with email sending, domain management, API keys, audiences, contacts, and a local inbox for captured messages.
 
+Set `RESEND_BASE_URL` before importing the official `resend` Node.js SDK and the SDK will call the emulator without code changes.
+
+The experimental native Go runtime implements the current Resend routes below, supports explicit JSON seed configs for Resend through `--seed`, and is verified against the official `resend` SDK for emails, batch email sends, domains, API keys, and legacy audience contacts.
+
 ## Emails
 
 - `POST /emails` — send single email

cmd/emulate/main.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"context"
+	"encoding/json"
 	"errors"
 	"flag"
 	"fmt"
@@ -19,6 +20,7 @@ import (
 
 	coreconfig "github.com/vercel-labs/emulate/internal/core/config"
 	emuruntime "github.com/vercel-labs/emulate/internal/runtime"
+	"github.com/vercel-labs/emulate/internal/services/resend"
 )
 
 var version = "dev"
@@ -100,24 +102,46 @@ func runStart(ctx context.Context, args []string, stdout io.Writer, stderr io.Wr
 		fmt.Fprintln(stderr, "The native Go runtime does not support --portless yet.")
 		return 1
 	}
+	var seedServices []string
+	var resendSeed *resend.SeedConfig
 	if *seedValue != "" {
-		fmt.Fprintln(stderr, "The native Go runtime does not support --seed yet.")
-		return 1
+		loaded, err := coreconfig.Load(coreconfig.LoadOptions{Path: *seedValue})
+		if err != nil {
+			fmt.Fprintf(stderr, "Failed to load seed config: %v\n", err)
+			return 1
+		}
+		if unsupported := unsupportedNativeSeedServices(loaded.Data); len(unsupported) > 0 {
+			fmt.Fprintf(stderr, "The native Go runtime only supports --seed for resend. Unsupported seed config services: %s\n", strings.Join(unsupported, ", "))
+			return 1
+		}
+		seedServices = coreconfig.InferServices(loaded.Data, nativeSeedServiceNames())
+		if raw, ok := loaded.Data["resend"]; ok {
+			var cfg resend.SeedConfig
+			if err := json.Unmarshal(raw, &cfg); err != nil {
+				fmt.Fprintf(stderr, "Failed to parse resend seed config: %v\n", err)
+				return 1
+			}
+			resendSeed = &cfg
+		}
 	}
 	services, err := parseServices(*serviceValue)
 	if err != nil {
 		fmt.Fprintln(stderr, err)
 		return 1
 	}
+	if *serviceValue == "" && len(seedServices) > 0 {
+		services = seedServices
+	}
 
 	baseURL := *baseURLValue
 	if baseURL == "" {
 		baseURL = fmt.Sprintf("http://localhost:%d", port)
 	}
 	server := emuruntime.NewServer(emuruntime.ServerOptions{
-		Version:  version,
-		BaseURL:  baseURL,
-		Services: services,
+		Version:    version,
+		BaseURL:    baseURL,
+		Services:   services,
+		ResendSeed: resendSeed,
 	})
 	httpServer := &nethttp.Server{
 		Handler:           server.Handler,
@@ -252,7 +276,7 @@ func printHelp(w io.Writer) {
 	fmt.Fprintln(w, "\nStart options:")
 	fmt.Fprintln(w, "  -p, --port <port>          Base port")
 	fmt.Fprintln(w, "  -s, --service <services>   Comma-separated services to enable")
-	fmt.Fprintln(w, "      --seed <file>          Path to seed config file (not supported in native Go yet)")
+	fmt.Fprintln(w, "      --seed <file>          Path to JSON seed config file (YAML not supported in native Go yet)")
 	fmt.Fprintln(w, "      --base-url <url>       Override advertised base URL")
 	fmt.Fprintln(w, "      --portless             Serve over HTTPS via portless (not supported in native Go yet)")
 	fmt.Fprintln(w, "\nThe published TypeScript CLI remains the default user-facing runtime.")
@@ -265,7 +289,7 @@ func printStartHelp(w io.Writer) {
 	fmt.Fprintln(w, "\nOptions:")
 	fmt.Fprintln(w, "  -p, --port <port>          Base port")
 	fmt.Fprintln(w, "  -s, --service <services>   Comma-separated services to enable")
-	fmt.Fprintln(w, "      --seed <file>          Path to seed config file (not supported in native Go yet)")
+	fmt.Fprintln(w, "      --seed <file>          Path to JSON seed config file (YAML not supported in native Go yet)")
 	fmt.Fprintln(w, "      --base-url <url>       Override advertised base URL")
 	fmt.Fprintln(w, "      --portless             Serve over HTTPS via portless (not supported in native Go yet)")
 }
@@ -305,6 +329,28 @@ func parseServices(value string) ([]string, error) {
 	return services, nil
 }
 
+func nativeSeedServiceNames() []string {
+	return []string{"resend"}
+}
+
+func unsupportedNativeSeedServices(data map[string]json.RawMessage) []string {
+	supported := map[string]bool{}
+	for _, service := range nativeSeedServiceNames() {
+		supported[service] = true
+	}
+
+	unsupported := make([]string, 0)
+	for _, service := range emuruntime.ServiceNames() {
+		if supported[service] {
+			continue
+		}
+		if _, ok := data[service]; ok {
+			unsupported = append(unsupported, service)
+		}
+	}
+	return unsupported
+}
+
 func getenv(name string, fallback string) string {
 	if value := os.Getenv(name); value != "" {
 		return value

cmd/emulate/main_test.go

@@ -158,6 +158,97 @@ func TestRunStartServesHealthEndpoint(t *testing.T) {
 	}
 }
 
+func TestRunStartSeedsResendFromJSONConfig(t *testing.T) {
+	tempDir := t.TempDir()
+	seedPath := filepath.Join(tempDir, "emulate.config.json")
+	if err := os.WriteFile(seedPath, []byte(`{"resend":{"domains":[{"name":"example.com"}],"contacts":[{"email":"test@example.com","first_name":"Test"}]}}`), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	port := freePort(t)
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	var stdout, stderr bytes.Buffer
+	done := make(chan int, 1)
+	go func() {
+		done <- runWithContext(ctx, []string{"start", "--port", strconv.Itoa(port), "--seed", seedPath}, &stdout, &stderr)
+	}()
+
+	healthURL := fmt.Sprintf("http://127.0.0.1:%d%s", port, emuruntime.HealthPath)
+	var health struct {
+		OK       bool     `json:"ok"`
+		Services []string `json:"services"`
+	}
+	waitForJSON(t, healthURL, &health)
+	if !health.OK || len(health.Services) != 1 || health.Services[0] != "resend" {
+		t.Fatalf("unexpected health body: %#v", health)
+	}
+
+	var domains struct {
+		Data []struct {
+			Name   string `json:"name"`
+			Status string `json:"status"`
+		} `json:"data"`
+	}
+	waitForJSON(t, fmt.Sprintf("http://127.0.0.1:%d/domains", port), &domains)
+	if len(domains.Data) != 1 || domains.Data[0].Name != "example.com" || domains.Data[0].Status != "verified" {
+		t.Fatalf("unexpected seeded domains: %#v", domains)
+	}
+
+	cancel()
+	select {
+	case code := <-done:
+		if code != 0 {
+			t.Fatalf("start exited with %d, stderr: %s", code, stderr.String())
+		}
+	case <-time.After(2 * time.Second):
+		t.Fatal("start did not shut down after context cancellation")
+	}
+}
+
+func TestRunStartRejectsYAMLSeedConfig(t *testing.T) {
+	tempDir := t.TempDir()
+	seedPath := filepath.Join(tempDir, "emulate.config.yaml")
+	if err := os.WriteFile(seedPath, []byte("resend:\n  domains:\n    - name: example.com\n"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	var stdout, stderr bytes.Buffer
+	code := run([]string{"start", "--seed", seedPath}, &stdout, &stderr)
+	if code == 0 {
+		t.Fatal("start with YAML seed exited successfully")
+	}
+	if !strings.Contains(stderr.String(), "YAML config loading is not implemented") {
+		t.Fatalf("unexpected stderr: %s", stderr.String())
+	}
+}
+
+func TestRunStartRejectsUnsupportedNativeSeedServices(t *testing.T) {
+	tempDir := t.TempDir()
+	seedPath := filepath.Join(tempDir, "emulate.config.json")
+	if err := os.WriteFile(seedPath, []byte(`{"aws":{"s3":{"buckets":[{"name":"example"}]}}}`), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	for _, args := range [][]string{
+		{"start", "--seed", seedPath},
+		{"start", "--service", "aws", "--seed", seedPath},
+	} {
+		t.Run(strings.Join(args[:2], " "), func(t *testing.T) {
+			var stdout, stderr bytes.Buffer
+			code := run(args, &stdout, &stderr)
+			if code == 0 {
+				t.Fatal("start with unsupported seed service exited successfully")
+			}
+			errText := stderr.String()
+			if !strings.Contains(errText, "only supports --seed for resend") || !strings.Contains(errText, "aws") {
+				t.Fatalf("unexpected stderr: %s", errText)
+			}
+		})
+	}
+}
+
 func TestRunTopLevelHelpIncludesFullStartOptions(t *testing.T) {
 	var stdout, stderr bytes.Buffer
 	code := run([]string{"--help"}, &stdout, &stderr)

internal/core/ui/ui.go

@@ -292,9 +292,13 @@ body{font-family:'Geist',-apple-system,BlinkMacSystemFont,sans-serif;background:
 .inspector-table th,.inspector-table td{text-align:left;padding:8px 12px;border-bottom:1px solid #0a3300;font-size:.8125rem;}
 .inspector-table th{color:#1a8c00;font-weight:600;font-size:.75rem;text-transform:uppercase;letter-spacing:.04em;}
 .inspector-table td{color:#33ff00;}
+.inspector-table a{color:#33ff00;text-decoration:none;}
+.inspector-table a:hover{text-decoration:underline;}
 .inspector-table tbody tr{transition:background .1s;}
 .inspector-table tbody tr:hover{background:#0a3300;}
 .inspector-empty{color:#1a8c00;text-align:center;padding:20px 0;font-size:.8125rem;}
+.email-preview-frame{width:100%;min-height:300px;border:1px solid #0a3300;border-radius:8px;background:#fff;}
+.email-preview-text{white-space:pre-wrap;word-break:break-word;color:#33ff00;font:inherit;}
 `
 }
 

internal/runtime/server.go

@@ -7,15 +7,17 @@ import (
 	"github.com/vercel-labs/emulate/internal/core/store"
 	"github.com/vercel-labs/emulate/internal/core/ui"
 	"github.com/vercel-labs/emulate/internal/services/aws"
+	"github.com/vercel-labs/emulate/internal/services/resend"
 )
 
 const HealthPath = "/_emulate/health"
 
 type ServerOptions struct {
-	Version  string
-	BaseURL  string
-	Services []string
-	Store    *store.Store
+	Version    string
+	BaseURL    string
+	Services   []string
+	Store      *store.Store
+	ResendSeed *resend.SeedConfig
 }
 
 type Server struct {
@@ -69,6 +71,12 @@ func NewServer(options ServerOptions) *Server {
 			BaseURL:        options.BaseURL,
 		})
 	}
+	if serviceEnabled(services, "resend") {
+		resend.Register(router, resend.Options{
+			Store: runtimeStore,
+			Seed:  options.ResendSeed,
+		})
+	}
 	router.NotFound(func(c *corehttp.Context) {
 		c.JSON(http.StatusNotFound, map[string]any{"message": "Not Found"})
 	})

internal/runtime/server_test.go

@@ -167,3 +167,33 @@ func TestNewHandlerDoesNotMountAWSWhenDisabled(t *testing.T) {
 		t.Fatalf("status = %d, body = %s", res.Code, res.Body.String())
 	}
 }
+
+func TestNewHandlerMountsResendWhenEnabled(t *testing.T) {
+	handler := NewHandler(ServerOptions{Services: []string{"resend"}})
+
+	res := httptest.NewRecorder()
+	req := httptest.NewRequest(http.MethodPost, "/emails", strings.NewReader(`{"from":"a@example.com","to":"b@example.com","subject":"Hello"}`))
+	req.Header.Set("Content-Type", "application/json")
+	req.Header.Set("Authorization", "Bearer re_test_token")
+	handler.ServeHTTP(res, req)
+
+	if res.Code != http.StatusOK {
+		t.Fatalf("status = %d, body = %s", res.Code, res.Body.String())
+	}
+	if !strings.Contains(res.Body.String(), `"id"`) {
+		t.Fatalf("unexpected body: %s", res.Body.String())
+	}
+}
+
+func TestNewHandlerDoesNotMountResendWhenDisabled(t *testing.T) {
+	handler := NewHandler(ServerOptions{Services: []string{"github"}})
+
+	res := httptest.NewRecorder()
+	req := httptest.NewRequest(http.MethodPost, "/emails", strings.NewReader(`{"from":"a@example.com","to":"b@example.com","subject":"Hello"}`))
+	req.Header.Set("Content-Type", "application/json")
+	handler.ServeHTTP(res, req)
+
+	if res.Code != http.StatusNotFound {
+		t.Fatalf("status = %d, body = %s", res.Code, res.Body.String())
+	}
+}