diff --git a/e2e/acp_echo.go b/e2e/acp_echo.go
new file mode 100644
index 00000000..a7986e75
--- /dev/null
+++ b/e2e/acp_echo.go
@@ -0,0 +1,145 @@
+//go:build ignore
+
+package main
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"os"
+	"os/signal"
+	"strings"
+
+	acp "github.com/coder/acp-go-sdk"
+)
+
+// ScriptEntry defines a single entry in the test script.
+type ScriptEntry struct {
+	ExpectMessage   string `json:"expectMessage"`
+	ThinkDurationMS int64  `json:"thinkDurationMS"`
+	ResponseMessage string `json:"responseMessage"`
+}
+
+// acpEchoAgent implements the ACP Agent interface for testing.
+type acpEchoAgent struct {
+	script      []ScriptEntry
+	scriptIndex int
+	conn        *acp.AgentSideConnection
+	sessionID   acp.SessionId
+}
+
+var _ acp.Agent = (*acpEchoAgent)(nil)
+
+func main() {
+	if len(os.Args) != 2 {
+		fmt.Fprintln(os.Stderr, "Usage: acp_echo <script.json>")
+		os.Exit(1)
+	}
+
+	script, err := loadScript(os.Args[1])
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "Error loading script: %v\n", err)
+		os.Exit(1)
+	}
+
+	if len(script) == 0 {
+		fmt.Fprintln(os.Stderr, "Script is empty")
+		os.Exit(1)
+	}
+
+	sigCh := make(chan os.Signal, 1)
+	signal.Notify(sigCh, os.Interrupt)
+	go func() {
+		<-sigCh
+		os.Exit(0)
+	}()
+
+	agent := &acpEchoAgent{
+		script: script,
+	}
+
+	conn := acp.NewAgentSideConnection(agent, os.Stdout, os.Stdin)
+	agent.conn = conn
+
+	<-conn.Done()
+}
+
+func (a *acpEchoAgent) Initialize(_ context.Context, _ acp.InitializeRequest) (acp.InitializeResponse, error) {
+	return acp.InitializeResponse{
+		ProtocolVersion:   acp.ProtocolVersionNumber,
+		AgentCapabilities: acp.AgentCapabilities{},
+	}, nil
+}
+
+func (a *acpEchoAgent) Authenticate(_ context.Context, _ acp.AuthenticateRequest) (acp.AuthenticateResponse, error) {
+	return acp.AuthenticateResponse{}, nil
+}
+
+func (a *acpEchoAgent) Cancel(_ context.Context, _ acp.CancelNotification) error {
+	return nil
+}
+
+func (a *acpEchoAgent) NewSession(_ context.Context, _ acp.NewSessionRequest) (acp.NewSessionResponse, error) {
+	a.sessionID = "test-session"
+	return acp.NewSessionResponse{
+		SessionId: a.sessionID,
+	}, nil
+}
+
+func (a *acpEchoAgent) Prompt(ctx context.Context, params acp.PromptRequest) (acp.PromptResponse, error) {
+	// Extract text from prompt
+	var promptText string
+	for _, block := range params.Prompt {
+		if block.Text != nil {
+			promptText = block.Text.Text
+			break
+		}
+	}
+	promptText = strings.TrimSpace(promptText)
+
+	if a.scriptIndex >= len(a.script) {
+		return acp.PromptResponse{
+			StopReason: acp.StopReasonEndTurn,
+		}, nil
+	}
+
+	entry := a.script[a.scriptIndex]
+	expected := strings.TrimSpace(entry.ExpectMessage)
+
+	// Empty ExpectMessage matches any prompt
+	if expected != "" && expected != promptText {
+		return acp.PromptResponse{}, fmt.Errorf("expected message %q but got %q", expected, promptText)
+	}
+
+	a.scriptIndex++
+
+	// Send response via session update
+	if err := a.conn.SessionUpdate(ctx, acp.SessionNotification{
+		SessionId: params.SessionId,
+		Update:    acp.UpdateAgentMessageText(entry.ResponseMessage),
+	}); err != nil {
+		return acp.PromptResponse{}, err
+	}
+
+	return acp.PromptResponse{
+		StopReason: acp.StopReasonEndTurn,
+	}, nil
+}
+
+func (a *acpEchoAgent) SetSessionMode(_ context.Context, _ acp.SetSessionModeRequest) (acp.SetSessionModeResponse, error) {
+	return acp.SetSessionModeResponse{}, nil
+}
+
+func loadScript(scriptPath string) ([]ScriptEntry, error) {
+	data, err := os.ReadFile(scriptPath)
+	if err != nil {
+		return nil, fmt.Errorf("failed to read script file: %w", err)
+	}
+
+	var script []ScriptEntry
+	if err := json.Unmarshal(data, &script); err != nil {
+		return nil, fmt.Errorf("failed to parse script JSON: %w", err)
+	}
+
+	return script, nil
+}
diff --git a/e2e/echo_test.go b/e2e/echo_test.go
index 765521cf..fd44d32a 100644
--- a/e2e/echo_test.go
+++ b/e2e/echo_test.go
@@ -100,6 +100,34 @@ func TestE2E(t *testing.T) {
 		require.Equal(t, script[0].ExpectMessage, strings.TrimSpace(msgResp.Messages[1].Content))
 		require.Equal(t, script[0].ResponseMessage, strings.TrimSpace(msgResp.Messages[2].Content))
 	})
+
+	t.Run("acp_basic", func(t *testing.T) {
+		ctx, cancel := context.WithTimeout(context.Background(), testTimeout)
+		defer cancel()
+
+		script, apiClient := setup(ctx, t, &params{
+			cmdFn: func(ctx context.Context, t testing.TB, serverPort int, binaryPath, cwd, scriptFilePath string) (string, []string) {
+				return binaryPath, []string{
+					"server",
+					fmt.Sprintf("--port=%d", serverPort),
+					"--experimental-acp",
+					"--", "go", "run", filepath.Join(cwd, "acp_echo.go"), scriptFilePath,
+				}
+			},
+		})
+		messageReq := agentapisdk.PostMessageParams{
+			Content: "This is a test message.",
+			Type:    agentapisdk.MessageTypeUser,
+		}
+		_, err := apiClient.PostMessage(ctx, messageReq)
+		require.NoError(t, err, "Failed to send message via SDK")
+		require.NoError(t, waitAgentAPIStable(ctx, t, apiClient, operationTimeout, "post message"))
+		msgResp, err := apiClient.GetMessages(ctx)
+		require.NoError(t, err, "Failed to get messages via SDK")
+		require.Len(t, msgResp.Messages, 2)
+		require.Equal(t, script[0].ExpectMessage, strings.TrimSpace(msgResp.Messages[0].Content))
+		require.Equal(t, script[0].ResponseMessage, strings.TrimSpace(msgResp.Messages[1].Content))
+	})
 }
 
 type params struct {
diff --git a/e2e/testdata/acp_basic.json b/e2e/testdata/acp_basic.json
new file mode 100644
index 00000000..22dd8d98
--- /dev/null
+++ b/e2e/testdata/acp_basic.json
@@ -0,0 +1,6 @@
+[
+  {
+    "expectMessage": "This is a test message.",
+    "responseMessage": "Echo: This is a test message."
+  }
+]