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2026-02-19-acp-coverage-roadmap.md

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title ACP Spec Coverage
author Bob <bob@dutifulbob.com>
date 2026-02-19

ACP Spec Coverage

What acpx implements from the ACP spec today and what is not yet implemented.

Supported Now

ACP Method acpx Feature Supported
initialize Handshake, capability negotiation yes
session/new sessions new yes
session/load Crash resume / reconnect yes
session/prompt prompt, exec, implicit prompt yes
session/update Streaming output (thinking, tools, text, diffs) yes
session/cancel Graceful cancel + acpx <agent> cancel yes
session/request_permission --approve-all, --approve-reads, --deny-all yes
session/set_mode acpx <agent> set-mode <mode> yes
session/set_config_option acpx <agent> set <key> <value> yes
session/set_model Legacy advertised model fallback yes
fs/read_text_file ACP client file read handler yes
fs/write_text_file ACP client file write handler yes
terminal/create ACP client terminal spawn handler yes
terminal/output ACP client terminal output handler yes
terminal/wait_for_exit ACP client terminal wait handler yes
terminal/kill ACP client terminal kill handler yes
terminal/release ACP client terminal release handler yes
authenticate Auth handshake handling yes

Supported Behavior Notes

Session cancel and controls

  • acpx <agent> cancel sends session/cancel through the queue path and keeps the session alive for follow-up prompts.
  • This is intentionally different from process-level SIGINT behavior, which can tear down the running process.
  • session/set_mode and session/set_config_option are exposed as set-mode and set commands and return agent-side validation errors when invalid.
  • Model config options are preferred; session/set_model remains available only when an adapter explicitly advertises legacy models metadata.

Filesystem client methods

  • acpx handles fs/read_text_file and fs/write_text_file as client-authority operations.
  • Permission behavior follows selected mode (--approve-all, --approve-reads, --deny-all).
  • Path sandboxing is applied to keep file operations scoped to cwd by default.

Terminal client methods

  • acpx handles full terminal lifecycle: create, output, wait_for_exit, kill, release.
  • The implementation includes process tracking, output buffering, and cleanup behavior for terminal IDs.

Authentication

  • acpx handles authenticate when adapters request it.
  • This keeps compatibility with adapters that rely on ACP auth handshake rather than out-of-band environment setup.

Not Yet Supported

ACP Method What it does Spec status
session/fork Branch a session into two unstable
session/list List sessions server-side unstable
session/resume Resume a paused session unstable
$/cancel_request Cancel any pending JSON-RPC request unstable

Not Yet Supported Notes

  • session/fork: would allow branching one conversation into parallel alternatives.
  • session/list: would expose adapter-side session inventory in addition to acpx local store listing.
  • session/resume: distinct from session/load; expected to support resume semantics without replay-like behavior.
  • $/cancel_request: transport-level JSON-RPC cancellation beyond session-scoped cancel.

ACP-Adjacent Features Not Yet Supported

Things acpx needs that aren't in the ACP spec:

  • Permission policies — Tool-kind/title policies now exist through --permission-policy; path/argument rules (allow reads to src/, deny writes to .env) are still not supported.
  • Multi-agent orchestration — Agent A prompts Agent B through acpx. Session bridging.
  • Webhooks / callbacks — Notify a URL when a prompt finishes. For CI/CD and automation pipelines.
  • Session export/import — Move sessions between machines.
  • Watch mode — Re-run prompt on file changes.
  • Cost/token tracking — Surface usage stats when agents/ACP expose them.