feat: add experimental ACP mode (--experimental-acp)

chat/src/components/chat-provider.tsx

@@ -304,9 +304,6 @@ export function ChatProvider({ children }: PropsWithChildren) {
       });
     } finally {
       if (type === "user") {
-        setMessages((prevMessages) =>
-          prevMessages.filter((m) => !isDraftMessage(m))
-        );
         setLoading(false);
       }
     }

cmd/attach/attach.go

@@ -129,7 +129,42 @@ func WriteRawInputOverHTTP(ctx context.Context, url string, msg string) error {
 	return nil
 }
 
+// statusResponse is used to parse the /status endpoint response.
+type statusResponse struct {
+	Status    string `json:"status"`
+	AgentType string `json:"agent_type"`
+	ACPMode   bool   `json:"acp_mode"`
+}
+
+func checkACPMode(remoteUrl string) error {
+	resp, err := http.Get(remoteUrl + "/status")
+	if err != nil {
+		return xerrors.Errorf("failed to check server status: %w", err)
+	}
+	defer func() { _ = resp.Body.Close() }()
+
+	if resp.StatusCode != http.StatusOK {
+		return xerrors.Errorf("unexpected %d response from server: %s", resp.StatusCode, resp.Status)
+	}
+
+	var status statusResponse
+	if err := json.NewDecoder(resp.Body).Decode(&status); err != nil {
+		return xerrors.Errorf("failed to decode server status: %w", err)
+	}
+
+	if status.ACPMode {
+		return xerrors.New("attach is not supported in ACP mode. The server is running with --experimental-acp which uses JSON-RPC instead of terminal emulation.")
+	}
+
+	return nil
+}
+
 func runAttach(remoteUrl string) error {
+	// Check if server is running in ACP mode (attach not supported)
+	if err := checkACPMode(remoteUrl); err != nil {
+		return err
+	}
+
 	ctx, cancel := context.WithCancel(context.Background())
 	defer cancel()
 	stdin := int(os.Stdin.Fd())

cmd/server/server.go

@@ -19,6 +19,7 @@ import (
 	"github.com/coder/agentapi/lib/httpapi"
 	"github.com/coder/agentapi/lib/logctx"
 	"github.com/coder/agentapi/lib/msgfmt"
+	st "github.com/coder/agentapi/lib/screentracker"
 	"github.com/coder/agentapi/lib/termexec"
 )
 
@@ -104,11 +105,33 @@ func runServer(ctx context.Context, logger *slog.Logger, argsToPass []string) er
 	}
 
 	printOpenAPI := viper.GetBool(FlagPrintOpenAPI)
+	experimentalACP := viper.GetBool(FlagExperimentalACP)
+
+	if printOpenAPI && experimentalACP {
+		return xerrors.Errorf("flags --%s and --%s are mutually exclusive", FlagPrintOpenAPI, FlagExperimentalACP)
+	}
+
+	var agentIO st.AgentIO
+	var transport = "pty"
 	var process *termexec.Process
+	var acpWait func() error
+
 	if printOpenAPI {
-		process = nil
+		agentIO = nil
+	} else if experimentalACP {
+		acpResult, err := httpapi.SetupACP(ctx, httpapi.SetupACPConfig{
+			Program:     agent,
+			ProgramArgs: argsToPass[1:],
+		})
+		if err != nil {
+			return xerrors.Errorf("failed to setup ACP: %w", err)
+		}
+		acpIO := acpResult.AgentIO
+		acpWait = acpResult.Wait
+		agentIO = acpIO
+		transport = "acp"
 	} else {
-		process, err = httpapi.SetupProcess(ctx, httpapi.SetupProcessConfig{
+		proc, err := httpapi.SetupProcess(ctx, httpapi.SetupProcessConfig{
 			Program:        agent,
 			ProgramArgs:    argsToPass[1:],
 			TerminalWidth:  termWidth,
@@ -118,11 +141,14 @@ func runServer(ctx context.Context, logger *slog.Logger, argsToPass []string) er
 		if err != nil {
 			return xerrors.Errorf("failed to setup process: %w", err)
 		}
+		process = proc
+		agentIO = proc
 	}
 	port := viper.GetInt(FlagPort)
 	srv, err := httpapi.NewServer(ctx, httpapi.ServerConfig{
 		AgentType:      agentType,
-		Process:        process,
+		AgentIO:        agentIO,
+		Transport:      transport,
 		Port:           port,
 		ChatBasePath:   viper.GetString(FlagChatBasePath),
 		AllowedHosts:   viper.GetStringSlice(FlagAllowedHosts),
@@ -138,19 +164,34 @@ func runServer(ctx context.Context, logger *slog.Logger, argsToPass []string) er
 	}
 	logger.Info("Starting server on port", "port", port)
 	processExitCh := make(chan error, 1)
-	go func() {
-		defer close(processExitCh)
-		if err := process.Wait(); err != nil {
-			if errors.Is(err, termexec.ErrNonZeroExitCode) {
-				processExitCh <- xerrors.Errorf("========\n%s\n========\n: %w", strings.TrimSpace(process.ReadScreen()), err)
-			} else {
-				processExitCh <- xerrors.Errorf("failed to wait for process: %w", err)
+	// Wait for process exit in PTY mode
+	if process != nil {
+		go func() {
+			defer close(processExitCh)
+			if err := process.Wait(); err != nil {
+				if errors.Is(err, termexec.ErrNonZeroExitCode) {
+					processExitCh <- xerrors.Errorf("========\n%s\n========\n: %w", strings.TrimSpace(process.ReadScreen()), err)
+				} else {
+					processExitCh <- xerrors.Errorf("failed to wait for process: %w", err)
+				}
 			}
-		}
-		if err := srv.Stop(ctx); err != nil {
-			logger.Error("Failed to stop server", "error", err)
-		}
-	}()
+			if err := srv.Stop(ctx); err != nil {
+				logger.Error("Failed to stop server", "error", err)
+			}
+		}()
+	}
+	// Wait for process exit in ACP mode
+	if acpWait != nil {
+		go func() {
+			defer close(processExitCh)
+			if err := acpWait(); err != nil {
+				processExitCh <- xerrors.Errorf("ACP process exited: %w", err)
+			}
+			if err := srv.Stop(ctx); err != nil {
+				logger.Error("Failed to stop server", "error", err)
+			}
+		}()
+	}
 	if err := srv.Start(); err != nil && err != context.Canceled && err != http.ErrServerClosed {
 		return xerrors.Errorf("failed to start server: %w", err)
 	}
@@ -180,16 +221,17 @@ type flagSpec struct {
 }
 
 const (
-	FlagType           = "type"
-	FlagPort           = "port"
-	FlagPrintOpenAPI   = "print-openapi"
-	FlagChatBasePath   = "chat-base-path"
-	FlagTermWidth      = "term-width"
-	FlagTermHeight     = "term-height"
-	FlagAllowedHosts   = "allowed-hosts"
-	FlagAllowedOrigins = "allowed-origins"
-	FlagExit           = "exit"
-	FlagInitialPrompt  = "initial-prompt"
+	FlagType            = "type"
+	FlagPort            = "port"
+	FlagPrintOpenAPI    = "print-openapi"
+	FlagChatBasePath    = "chat-base-path"
+	FlagTermWidth       = "term-width"
+	FlagTermHeight      = "term-height"
+	FlagAllowedHosts    = "allowed-hosts"
+	FlagAllowedOrigins  = "allowed-origins"
+	FlagExit            = "exit"
+	FlagInitialPrompt   = "initial-prompt"
+	FlagExperimentalACP = "experimental-acp"
 )
 
 func CreateServerCmd() *cobra.Command {
@@ -228,6 +270,7 @@ func CreateServerCmd() *cobra.Command {
 		// localhost:3284 is the default origin when you open the chat interface in your browser. localhost:3000 and 3001 are used during development.
 		{FlagAllowedOrigins, "o", []string{"http://localhost:3284", "http://localhost:3000", "http://localhost:3001"}, "HTTP allowed origins. Use '*' for all, comma-separated list via flag, space-separated list via AGENTAPI_ALLOWED_ORIGINS env var", "stringSlice"},
 		{FlagInitialPrompt, "I", "", "Initial prompt for the agent. Recommended only if the agent doesn't support initial prompt in interaction mode. Will be read from stdin if piped (e.g., echo 'prompt' | agentapi server -- my-agent)", "string"},
+		{FlagExperimentalACP, "", false, "Use experimental ACP transport instead of PTY", "bool"},
 	}
 
 	for _, spec := range flagSpecs {

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
+}

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 {

e2e/testdata/acp_basic.json

@@ -0,0 +1,6 @@
+[
+  {
+    "expectMessage": "This is a test message.",
+    "responseMessage": "Echo: This is a test message."
+  }
+]

go.mod

@@ -6,6 +6,7 @@ require (
 	github.com/ActiveState/termtest/xpty v0.6.0
 	github.com/acarl005/stripansi v0.0.0-20180116102854-5a71ef0e047d
 	github.com/charmbracelet/bubbletea v1.3.4
+	github.com/coder/acp-go-sdk v0.6.3
 	github.com/coder/agentapi-sdk-go v0.0.0-20250505131810-560d1d88d225
 	github.com/coder/quartz v0.1.2
 	github.com/danielgtaylor/huma/v2 v2.32.0