acpx-faq

Run coding agents (codex, claude, agy/Antigravity) through the acpx ACP CLI - the headless lane outside a herdr pane (no HERDR_ENV). Use before launching or prompting a subagent, and when a command fails, a session is not found, or a prompt is lost.

Drive coding agents headlessly through acpx (>= 0.15.1).

Syntax authority

Syntax comes from the binary, not this file. acpx --skill show acpx is the vendor reference (~29k chars) and the tiebreaker on command shape - do NOT dump it into the transcript. Retrieve in this order:

1. acpx --help / acpx <cmd> --help - the flags you are about to compose.
2. One topic: acpx --skill show acpx | rg -B2 -A12 '<term>', or a section: acpx --skill show acpx | sed -n '/^## Sessions/,/^## /p'.
3. Authoring something novel (a flow, an embedded runtime): dump ONCE to a scratch file and grep that file.

This file repeats nothing the reference covers. It carries only what the reference lacks: exit codes, failure semantics, traps, and per-agent recipes. If a syntax question is not answered here, it is answered there - go get it, never guess a flag.

When this lane

acpx is the headless out-of-process executor lane. Inside a herdr pane (HERDR_ENV=1) prefer herdr (herdr-faq): it gives you screen reads and dialog detection. Everywhere else (plain terminal, ssh, cron) herdr is untargetable and acpx is the lane. In-session fan-out stays on the harness's own subagent tool.

Invariants

1. Global flags precede the agent subcommand (--cwd, --model, --approve-all, --agent, --format, --timeout, --ttl, --mcp-config). The subcommand accepts only -s, --no-wait, -f (and exec also --config-option). A global flag after the agent exits 2 with error: unknown option.
2. -s <name> never creates a session. It resolves one, walking from --cwd up to the git root; no match exits 4. Create first with sessions ensure --name <n>.
3. status is not a turn signal. It probes the queue-owner process, never the agent, and keeps saying running after the turn has ended. Never gate automation on it.
4. Permission flags are not a sandbox. The claude adapter spawns its binary with --allow-dangerously-skip-permissions --setting-sources=project,local, so no permission request ever reaches acpx's policy layer (--deny-all is unreachable, not ignored); codex writes through its terminal capability. --no-terminal genuinely removes the terminal capability, but the only real filesystem control is which --cwd you hand it. Verify: ps -Ao args | grep claude-agent-sdk mid-turn.
5. [done] end_turn and exit 0 are not proof of success. A content-filter kill, an MCP load failure, or a truncated turn all end that way. Read the stream, or require the agent to write a result file you can check.
6. A persistent session leaves a resident process stack until you close it. Every -s prompt elects a queue owner holding npm exec -> node <adapter> -> <agent>, roughly 700 MB per executor. sessions close is the only supported teardown; the idle TTL (default 300s) is the backstop, and --ttl 0 removes it. See Teardown.

Per agent

Default to exec for dispatched work. exec runs a temporary session: no saved record, not queue-aware, so it never elects a queue owner and leaves nothing to clean up. Use -s <name> only to queue follow-ups onto a live run or to read sessions history afterwards - and then the closing sessions close is part of the recipe (invariant 6). One directory (ideally one worktree) per parallel executor: sessions key on cwd.

agy / Antigravity - the --agent escape hatch

acpx has no Antigravity adapter, and the built-in gemini agent is the public Gemini CLI - a different product, dead for Code Assist. The only route is Google's signed ACP server via --agent: the .par installed by Antigravity under ~/.local/lib/antigravity-acp/ (if missing, install Antigravity - never download it). localharness_external beside it must stay executable, or the server starts anyway and only logs Localharness not found.

bash

# macOS: --agent may point straight at the .par
D=/abs/real/dir                              # must exist: roots resolve through realpath
acpx --agent ~/.local/lib/antigravity-acp/agy_acp_server.par \
     --cwd "$D" --model gemini-3.7-flash-medium --timeout 1800 \
     exec 'Carry out $D/brief.md. Write your report to $D/report.md.'

• Default gemini-3.7-flash-medium (measured on par with Opus for rubric-driven bulk work and far faster; gemini-3.8-flash-high measured worse on the same task). Effort lives in the model id (gemini-3.7-flash-{low,medium,high}); no separate knob.

• agy reads sibling files unprompted - agents sharing a directory reproduce each other's output. One directory per agent, and checksum any rerun before believing an agreement number.

• Auth: with no auth type configured the server hangs on authenticate forever - it reads exactly like a network stall. One-time fix:

  bash

  mkdir -p ~/.gemini/antigravity-acp
  echo '{"auth":{"type":"oauth-personal"}}' > ~/.gemini/antigravity-acp/settings.json
  

  then complete its own sign-in once; the token persists. The server reads ~/.gemini/antigravity-acp/acp_token.json, never the CLI's token - a fully authenticated agy does nothing for this lane, and GEMINI_HOME relocates the whole tree (a second isolated lane). An entitlement problem is server-side with no client lever - do not "fix" it by switching to gemini-api-key or a GCP project (separate billing, not fixes).

• Never pass a positional agent with --agent - exit 2.

• Verification prompts: demand a bare reply and read the whole log. agy routes answers into brain artifact files, so a tail cannot tell "no tools" from "answered elsewhere". Add "Do not create any files".

• agy hides MCP failures inside a successful-looking turn - grep the output for MCP load failed; the exit code stays 0.

Linux / NixOS

Two silent differences from macOS:

1. --uid= is required, so --agent points at a wrapper (~/.local/bin/agy-acp-server):

   bash

   #!/usr/bin/env bash
   export SSL_CERT_FILE=/etc/ssl/certs/ca-certificates.crt   # NixOS; see 2
   exec ~/.local/lib/antigravity-acp/agy_acp_server.par --uid= "$@"
   

2. Without that SSL_CERT_FILE, NixOS gets a 502. The .par's embedded OpenSSL has no valid CA paths there; its local proxy reports 502 Bad Gateway: Failed to connect to backend API and the turn still ends [done] end_turn with exit 0 (invariant 5). The real CERTIFICATE_VERIFY_FAILED appears only under --alsologtostderr.

The invocation is otherwise the macOS one with --agent ~/.local/bin/agy-acp-server.

codex

bash

D=/abs/dir
acpx --cwd "$D" codex sessions ensure --name work            # idempotent; -s cannot create
acpx --cwd "$D" codex set model gpt-5.6-sol -s work
acpx --cwd "$D" codex set reasoning_effort high -s work
acpx --cwd "$D" --approve-all --timeout 5400 --format quiet --suppress-reads \
     codex -s work 'Carry out ./brief.md. Write your report to ./report.md.' > run.log 2>&1 &
# ...after the run finishes and you have read what you need:
acpx --cwd "$D" codex sessions close work   # ALWAYS - the run is not over until this returns

One-shot: exec --config-option reasoning_effort=low sets model and effort inline (0.14.0+), applied after --model and before the prompt.

• The codex adapter floats on ^1.1.5 - behavior can change without an acpx upgrade.
• OpenAI's content filter kills benign turns that end clean ([done] end_turn, exit 0). Vocabulary like race / sweep / exploit / attack in a filename, comment or prompt triggers it. Read the transcript tail before believing completion; recover by re-prompting the same -s <name> (context survives) with the artifact renamed neutrally and the remaining steps listed explicitly.
• Queue a follow-up onto a live session by prompting the same name again - it runs after the current turn. This is how you course-correct without relaunching.
• Handing off to a human TUI: sessions close first, then codex resume <sessionId> (id from sessions show - it is the ordinary codex rollout id).

claude

bash

D=/abs/dir
acpx --cwd "$D" --model claude-opus-5 claude sessions ensure -s work
acpx --cwd "$D" --model claude-opus-5 --approve-all --suppress-reads --timeout 2400 \
     claude -s work -f /abs/brief.md >> run.log 2>&1 &
# ...after the run finishes and you have read what you need:
acpx --cwd "$D" claude sessions close work  # ALWAYS - the run is not over until this returns

• No effort knob (set reasoning_effort returns Internal error; expected).
• --model and set model validate differently. set model refuses anything outside the advertised list (["default","opus[1m]","claude-fable-5[1m]","sonnet","haiku"]); the global --model at session creation passes the id through to the harness, so --model claude-opus-5 works. A genuinely unknown id still fails loudly: RUNTIME: Internal error: There's an issue with the selected model (<id>).
• Fable runs: --model 'claude-fable-5[1m]' (quote it - the brackets are globs).
• User-scope skills are excluded on purpose -> Unknown command: /polish. Fix with ACPX_CLAUDE_INCLUDE_USER_SETTINGS=1 - and settings bind at session creation, so recreate the session, do not re-prompt it.
• Second account: export CLAUDE_CONFIG_DIR and CLAUDE_SECURESTORAGE_CONFIG_DIR before acpx; the child inherits them.
• The claude adapter bundles an old claude (2.1.215) regardless of your PATH - newer harness features are absent; override with --agent at a newer adapter build.

Sessions

• Sessions key on (agent command, absolute cwd, optional name) - parallel executors need their own directories, which also keeps per-session model and effort settings from racing.
• sessions close is the teardown verb: marks the record closed, sends ACP session/close, takes down the owner and adapter processes (see Teardown). Also required before codex resume and sessions export.
• A session whose cwd you have deleted becomes an unreachable registry row - close cannot target it without its cwd; prune it.

Completion

status states (the vendor doc has no state table): running, idle, dead (owner gone or last exit abnormal), no-session. It is a local kill(pid,0)-style check that reads running for the whole idle TTL after a turn ends - a loop waiting for idle spins past real completion (invariant 3). What is actually correct:

• the foreground stream's terminating [done] end_turn, plus the process exit code;
• sessions show <n> (lastActivity, historyEntries) and sessions history <n>;
• best: a result file the brief required the agent to write - poll for the file.

--no-wait acknowledges enqueueing ([queued] <id>), which is neither delivery nor completion - behind a running turn the prompt waits for the turn boundary.

Supervise a backgrounded run by counting tool lines and mtime (rg -c '^\[tool\]' run.log, stat -f '%Sm' run.log); liveness: pgrep -fl 'acpx|codex-acp|claude-agent-acp'.

Teardown

sessions close <n> takes down the whole resident stack (acpx __queue-owner -> npm exec -> node <adapter> -> agent binary). Make it the last line of every scripted persistent run: forgotten owners hold ~700 MB each for hours, heartbeating queueDepth: 0 lock files.

• Verify: pgrep -fl 'acpx __queue-owner|claude-agent-acp|codex-acp' should print nothing once your runs are closed.
• A straggler gets a plain kill, never kill -9 on an owner. acpx (0.15.1) kills the adapter by PID only - there is no process-group kill in the ACP session path - so SIGKILL on an owner strands npm exec -> node -> <agent> reparented to PID 1. sessions close (or plain SIGTERM) takes the whole stack with it.
• Owners lingering with no --ttl 0 on the command line: check ~/.acpx/config.json - its ttl key sets the default idle TTL, and "ttl": 0 there disables the self-reap for every invocation.
• Closed records persist indefinitely; sessions prune --older-than 7 on a cadence.

MCP

--mcp-config <path> replaces the project/global mcpServers for that invocation (relative paths inside resolve from --cwd) - and it is absent from the vendor doc entirely.

The file shape is not the standard one. acpx wants a JSON array of named server objects; the standard object-keyed map throws an uncaught Node stack trace: Error: Invalid mcpServers in <path>: expected array.

jsonc

// RIGHT - an array, each entry carrying its own name
{ "mcpServers": [ { "name": "example", "type": "stdio", "command": "example", "args": ["mcp"] } ] }

A stdio server that dies only under agy is an MCP era mismatch, not an acpx fault. agy probes stdio with server/discover (MCP 2026-07-28 retired the initialize handshake); a legacy server that aborts on the unexpected request kills the pipe instead of answering an error, leaving nothing to fall back from: MCP load failed for <name>: ... expect initialized request, but received: ... "server/discover". The codex and claude adapters do not probe, which is the only reason the same config works for them. Fix the server (answer an error, or move to a current MCP SDK), or reach it over HTTP. The turn still exits 0 and ends [done] end_turn - grep for MCP load failed before trusting any run that used MCP.

• A live queue owner refuses a config change (it carries the config path plus a SHA-256 fingerprint): sessions close first.
• --mcp-config replaces the whole set - verify a config written for one adapter on another with a one-shot exec that asks the agent to name its own tools.
• ${...} in an MCP config expands in the launcher's environment (the child inherits it), so a variable like a session id resolves to your id. Never let an identity flow in through the environment.

Limits

• Incoming ACP messages cap at 64 MiB since 0.15.1 (previously unlimited); ACPX_MAX_ACP_MESSAGE_BYTES raises it, 0 disables.
• Terminal output retention is 64 KiB per call (ACPX_TERMINAL_MAX_OUTPUT_BYTES).
• Long unattended work: --timeout 5400, plus a wrapping shell timeout as belt-and-braces. Do NOT add --ttl 0 - the 300s default covers any gap within one run, and --ttl 0 removes the only automatic reaper (invariant 6). Reserve it for a human-driven warm session you will close by hand.
• --format quiet for fleets; --suppress-reads on every long run.
• Pass long briefs as -f <path> or a path in the prompt, never inlined - inlined content in a shell command is what a driving harness's classifier blocks on.

What acpx cannot do

Structural, not bugs. Do not design around them.

• An ACP session does not wake. An externally injected message lands in the transcript but starts no turn; the only inbound channel is acpx itself (-s <name>, optionally --no-wait).
• codex queue --thread does not reach an acpx-driven codex session (it does wake a plain interactive codex).
• acpx-spawned Claude sessions bind no IPC socket, so ListAgents / SendMessage cannot see or reach them.
• acpx cannot attach to a session it did not start.

Failures

Exit codes (absent from the vendor doc):

code	meaning
0	success (also cancel with nothing to cancel)
1	agent / protocol / runtime error - the catch-all
2	usage error: bad or conflicting flags, malformed --agent
3	--timeout exceeded
4	no session found by the directory walk
5	every permission request denied or cancelled
130	interrupted (cooperative cancel first)

Error catalog - string -> fix:

• No acpx session found (searched up to <cwd>) (exit 4) - invariant 2: sessions ensure --name <n> first; confirm --cwd.
• error: unknown option '--cwd' (exit 2) - global flag after the subcommand (invariant 1).
• Do not combine positional agent with --agent override (exit 2) - drop the positional claude/codex.
• Invalid mcpServers in <path>: expected array (uncaught stack trace) - see MCP.
• Invalid value for config option model: <id> (ACP -32603, adapter reported "Internal error") - set model outside the advertised list; use --model at session creation (see claude).
• RUNTIME: Internal error: There's an issue with the selected model (<id>) - the harness itself rejects the id: a real typo or an entitlement problem.
• Internal error from claude set reasoning_effort - no effort knob; expected.
• [error] RUNTIME: Authentication required - the agy ACP server was never authenticated; see agy.
• 502 Bad Gateway: Failed to connect to backend API (agy, exit 0) - missing CA bundle on NixOS; see Linux / NixOS.
• authenticate never returns (agy, no error, no URL) - no auth.type in ~/.gemini/antigravity-acp/settings.json; see agy.
• RUNTIME: This client is no longer supported for Gemini Code Assist - you used the built-in gemini agent; use --agent with the .par.
• already has an active writer (code -32600) - codex resume while acpx still owns the thread; sessions close <n> first.
• Missing --skill action. (exit 1) - --skill needs show|list|install|export|help.
• error: unknown option '--one-shot' (exit 2) - not a verb; exec is the one-shot form.
• Exit 3 = the wall clock ran out, not a stuck agent - check sessions show for lastActivity; an adapter-side timeout surfaces as exit 1 instead.

Silent failures worth an explicit check: content-filter kills and MCP load failures both end [done] end_turn with exit 0; a --no-wait prompt that "sent" may still be queued behind a running turn; status reads running long after completion; a session whose cwd no longer exists lingers until pruned.