feat: add experimental ACP mode (--experimental-acp) by johnstcn · Pull Request #186 · coder/agentapi

johnstcn · 2026-02-13T21:27:36Z

Depends on #185
Fixescoder/internal#1333

Add support for Agent Control Protocol (ACP) as an alternative to terminal emulation. ACP uses JSON-RPC over stdin/stdout pipes.

Introduce AgentIO interface to abstract PTY vs ACP transports
Add ACPConversation implementing Conversation interface
Add --experimental-acp flag (mutually exclusive with --print-openapi)
Add e2e test with mock ACP agent

Created using Mux (Opus 4.5)

- Add Emitter interface to screentracker package - Remove OnSnapshot from PTYConversationConfig, accept Emitter in NewPTY - Rename EventEmitter methods: EmitMessages, EmitStatus, EmitScreen - Accept agentType at NewEventEmitter construction instead of per-call - Update server.go wiring, all tests pass

github-actions · 2026-02-13T21:29:52Z

✅ Preview binaries are ready!

To test with modules: agentapi_version = "agentapi_186" or download from: https://github.com/coder/agentapi/releases/tag/agentapi_186

Add support for Agent Control Protocol (ACP) as an alternative to terminal emulation. ACP uses JSON-RPC over stdin/stdout pipes. - Introduce AgentIO interface to abstract PTY vs ACP transports - Add ACPConversation implementing Conversation interface - Add --experimental-acp flag (mutually exclusive with --print-openapi) - Add e2e test with mock ACP agent - Block `attach` when using --experimental-acp (no terminal) - Update chat UI to show ACP tool calls Other changes: - chat: Fix redundant draft filtering from finally block Created using Mux (Opus 4.5)

johnstcn · 2026-02-16T10:01:48Z

- setMessages((prevMessages) =>
- prevMessages.filter((m) => !isDraftMessage(m))
- );


review: this was causing a 'flicker' when sending a message in the UI

johnstcn · 2026-02-16T10:02:27Z

+	}
+
+	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.")


review: It should be eventually supported but I'm not sure what form it should take yet.

johnstcn · 2026-02-16T10:48:13Z

+func (c *acpClient) RequestPermission(ctx context.Context, params acp.RequestPermissionRequest) (acp.RequestPermissionResponse, error) {
+	// Auto-approve all permissions for Phase 1
+	return acp.RequestPermissionResponse{
+ Outcome: acp.RequestPermissionOutcome{
+ Selected: &acp.RequestPermissionOutcomeSelected{OptionId: "allow"},
+ },
+	}, nil
+}


review: we need to add support for allow/deny; will be handled in a future PR

Copilot

Pull request overview

This PR adds experimental support for Agent Control Protocol (ACP) as an alternative transport to PTY-based terminal emulation. ACP uses JSON-RPC over stdin/stdout pipes for cleaner communication without terminal escape sequences.

Changes:

Introduces AgentIO interface abstraction for PTY vs ACP transports
Implements ACPConversation and ACPAgentIO for ACP protocol support
Adds --experimental-acp CLI flag with validation against --print-openapi
Adds Backend field to status API response and blocks attach command in ACP mode

Reviewed changes

Copilot reviewed 16 out of 17 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
x/acpio/acpio.go	Core ACP I/O implementation with JSON-RPC connection handling
x/acpio/acp_conversation.go	ACP conversation tracker with async message handling
x/acpio/acp_conversation_test.go	Comprehensive unit tests for ACP conversation
lib/httpapi/setup.go	ACP process setup and lifecycle management
lib/httpapi/server.go	Server integration with transport abstraction
lib/httpapi/server_test.go	Updated tests to use AgentIO interface
lib/httpapi/models.go	Added Backend field to status response
cmd/server/server.go	CLI flag and transport selection logic
cmd/attach/attach.go	ACP mode detection and rejection for attach command
openapi.json	API schema update with backend field
go.mod/go.sum	Added acp-go-sdk dependency
lib/acp/doc.go	Package documentation for ACP support
e2e/acp_echo.go	Mock ACP agent for e2e testing
e2e/echo_test.go	E2E test for ACP mode
e2e/testdata/acp_basic.json	Test script data
chat/src/components/chat-provider.tsx	Removed draft message cleanup (appears to be a bug)

Comments suppressed due to low confidence (1)

lib/httpapi/server_test.go:37

The tests don't set the Transport field in ServerConfig, which means it will be an empty string. The server code will default to PTY transport when config.Transport != "acp", but the Backend field in the status response will be an empty string instead of "pty". This could cause issues for clients that check the backend field.

Consider setting a default value "pty" for config.Transport when it's empty, or explicitly setting Transport in all tests.

	srv, err := httpapi.NewServer(ctx, httpapi.ServerConfig{
AgentType: msgfmt.AgentTypeClaude,
AgentIO: nil,
Port: 0,
ChatBasePath: "/chat",
AllowedHosts: []string{"*"},
AllowedOrigins: []string{"*"},
})

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Copilot · 2026-02-17T18:07:57Z

+	go func() {
+ select {
+ case <-ctx.Done():
+ logger.Info("Context done, closing ACP agent")
+ _ = stdin.Close()
+ _ = stdout.Close()
+ // Try graceful shutdown first
+ _ = cmd.Process.Signal(syscall.SIGTERM)
+ // Force kill after timeout
+ time.AfterFunc(5*time.Second, func() {
+ _ = cmd.Process.Kill()
+ })
+ case <-done:
+ // Process exited normally, nothing to clean up
+ return
+ }
+	}()


The cleanup goroutine in SetupACP has a potential race condition with the AfterFunc timer. If the context is cancelled, SIGTERM is sent, and then exactly 5 seconds later the cleanup goroutine receives from the done channel (normal exit), the AfterFunc will still fire and call Kill() on an already-exited process.
Consider using a sync mechanism to cancel the AfterFunc timer when the process exits normally, or checking if the process has exited before calling Kill().

Copilot · 2026-02-17T18:07:58Z

+ case <-ctx.Done():
+ logger.Info("Context done, closing ACP agent")
+ _ = stdin.Close()
+ _ = stdout.Close()
+ // Try graceful shutdown first
+ _ = cmd.Process.Signal(syscall.SIGTERM)
+ // Force kill after timeout
+ time.AfterFunc(5*time.Second, func() {
+ _ = cmd.Process.Kill()
+ })


When the context is cancelled, the cleanup goroutine closes stdin/stdout pipes and signals SIGTERM to the process. However, the main goroutine (lines 185-194) is simultaneously calling acpResult.Wait() which calls cmd.Wait(). Closing the pipes while the ACP connection might still be active could cause the ACP SDK to encounter unexpected EOF errors or other I/O errors.
Consider coordinating the shutdown sequence so that the ACP connection can be gracefully closed before the pipes are closed, or ensure that the ACP SDK properly handles pipe closure during context cancellation.

Previously, when Send() failed, only empty agent messages were removed. If the agent had streamed partial content before the error, that partial message would incorrectly remain in the conversation. Now we remove the agent message on error regardless of whether it has content, ensuring the conversation state stays consistent.

Reorder the shutdown sequence to send SIGTERM first, giving the agent a chance to shutdown gracefully before losing I/O.

- Add Clock field to SetupACPConfig - Default to quartz.NewReal() if Clock is nil - Replace time.AfterFunc with config.Clock.AfterFunc for testability

The cleanup goroutine in SetupACPProcess was leaking because it didn't return after handling ctx.Done(). After scheduling the kill timer, the goroutine would block forever on the done channel.

- Add ctx and cancel fields to ACPConversation struct - Update NewACPConversation to accept context.Context as first param - Create cancellable context in constructor using context.WithCancel - Add Stop() method that calls c.cancel() - Check c.ctx.Err() in executePrompt before each message part - Update server.go to pass ctx to NewACPConversation - Update all test calls to pass context.Background()

johnstcn · 2026-02-19T09:50:07Z

Breaking into smaller PRs for ease of review:

johnstcn added 6 commits February 12, 2026 18:31

refactor: use functional options for NewEventEmitter
bc79eda

address PR review comments
1cd4dca

use uint for subscription buffer size
a116d63

Merge branch 'event-emitter-7z1e' into cj/refactor/event-emitter
ca5d5d4

johnstcn self-assigned this Feb 13, 2026

johnstcn force-pushed the cj/exp/acp branch from 4be9ef1 to 253f1f3Compare February 13, 2026 21:43

johnstcn force-pushed the cj/exp/acp branch from 253f1f3 to 8c8da69Compare February 13, 2026 22:11

fixup! feat: add experimental ACP mode (--experimental-acp)
e9d8dd3

johnstcn commented Feb 16, 2026
View reviewed changes

johnstcn added 2 commits February 17, 2026 16:26

self-review
1c3c9aa

more self-review fixes
35f4f5f

johnstcn marked this pull request as ready for review February 17, 2026 17:57

CopilotAI review requested due to automatic review settings February 17, 2026 17:57

Copilotstarted reviewing on behalf of johnstcn February 17, 2026 17:58 View session

CopilotAI reviewed Feb 17, 2026
View reviewed changes

johnstcn added 6 commits February 17, 2026 22:46

Send SIGTERM before closing pipes in ACP shutdown
eeea49a
Reorder the shutdown sequence to send SIGTERM first, giving the agent a chance to shutdown gracefully before losing I/O.

Clean up draft messages in sendMessage error paths
01c1b7a

refactor: move draft cleanup to finally block in sendMessage
8a44a8f

Use quartz.Clock instead of time.AfterFunc in SetupACP
fe02d07
- Add Clock field to SetupACPConfig - Default to quartz.NewReal() if Clock is nil - Replace time.AfterFunc with config.Clock.AfterFunc for testability

acpio: check parent ctx and return early
97f7a17

johnstcn changed the base branch from cj/refactor/event-emitter to mainFebruary 18, 2026 11:13

johnstcn added 3 commits February 18, 2026 11:15

Merge branch 'main' into cj/exp/acp
8ad6c6f

fix: add missing return after context cancellation cleanup
9982505
The cleanup goroutine in SetupACPProcess was leaking because it didn't return after handling ctx.Done(). After scheduling the kill timer, the goroutine would block forever on the done channel.

johnstcn marked this pull request as draft February 19, 2026 09:32

johnstcn closed this Feb 19, 2026

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feat: add experimental ACP mode (--experimental-acp) - #186

feat: add experimental ACP mode (--experimental-acp)#186
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