PHP SDK for the Agent Runtime Control Protocol (ARCP) — submit, observe, and control long-running agent jobs from PHP.
Specification · Concepts · Install · Quick start · Guides · API reference
arcp/arcp is the PHP reference implementation of ARCP, the Agent Runtime Control Protocol. It covers both sides of the wire — Arcp\Client\ARCPClient for submitting and observing jobs, Arcp\Runtime\ARCPRuntime for hosting agents — so either side can talk to any conformant peer in any language without hand-rolling the envelope, sequencing, or lease enforcement.
ARCP itself is a transport-agnostic wire protocol for long-running AI agent jobs. It owns the parts of agent infrastructure that don't change between products — sessions, durable event streams, capability leases, budgets, resume — and stays out of the parts that do. ARCP wraps the agent function; it does not define how agents are built, how tools are exposed (that's MCP), or how telemetry is exported (that's OpenTelemetry).
Requires PHP 8.4 or newer and Composer 2.x. The SDK ships as a single Composer package; the bin/arcp CLI is registered automatically so vendor/bin/arcp is available in any project that pulls it in.
composer require arcp/arcpThe pdo_sqlite, mbstring, and json extensions are required (all bundled with most PHP distributions). The runtime uses Amp v3 + fibers; no extra extension is needed for async.
Connect to a runtime, submit a job, stream its events to completion:
<?php
declare(strict_types=1);
require __DIR__ . '/vendor/autoload.php';
use Amp\Websocket\Client\WebsocketHandshake;
use function Amp\Websocket\Client\connect;
use Arcp\Client\ARCPClient;
use Arcp\Envelope\Envelope;
use Arcp\Envelope\MessageCatalog;
use Arcp\Json\EnvelopeSerializer;
use Arcp\Messages\Execution\JobProgress;
use Arcp\Messages\Session\Auth;
use Arcp\Messages\Session\Capabilities;
use Arcp\Messages\Session\PeerInfo;
use Arcp\Transport\WebSocketTransport;
$serializer = new EnvelopeSerializer(MessageCatalog::create());
$transport = new WebSocketTransport(
connect(new WebsocketHandshake('wss://runtime.example.com/arcp')),
$serializer,
);
$client = new ARCPClient($transport);
$client->open(
Auth::bearer((string) getenv('ARCP_TOKEN')),
new PeerInfo('quickstart', '1.0.0'),
new Capabilities(streaming: true, subscriptions: true),
);
$client->subscribe(
['types' => ['job.progress', 'log', 'metric']],
static function (Envelope $env): void {
if ($env->payload instanceof JobProgress) {
printf("[progress %d%%] %s\n", $env->payload->percent, $env->payload->message ?? '');
}
},
);
$result = $client->invokeTool('data-analyzer', ['dataset' => 's3://example/sales.csv']);
printf("final: %s\n", json_encode($result->value));
$client->close();This is the whole shape of the SDK: open a session, submit work, consume an ordered event stream, get a terminal result or error. Everything below is detail on those four moves.
ARCP organizes everything around four concerns — identity, durability, authority, and observability — expressed through five core objects:
- Session — a connection between a client and a runtime. A session carries identity (a bearer token), negotiates a feature set in a
hello/welcomehandshake, and is resumable: if the transport drops, you reconnect with a resume token and the runtime replays buffered events. Jobs outlive the session that started them. See §6. - Job — one unit of agent work submitted into a session. A job has an identity, an optional idempotency key, a resolved agent version, and a lifecycle that ends in exactly one terminal state:
success,error,cancelled, ortimed_out. See §7. - Event — the ordered, session-scoped stream a job emits: logs, thoughts, tool calls and results, status, metrics, artifact references, progress, and streamed result chunks. Events carry strictly monotonic sequence numbers so the stream survives reconnects gap-free. See §8.
- Lease — the authority a job runs under, expressed as capability grants (
fs.read,fs.write,net.fetch,tool.call,agent.delegate,cost.budget,model.use). The runtime enforces the lease at every operation boundary; a job can never act outside it. Leases may carry a budget and an expiry, and may be subset and handed to sub-agents via delegation. See §9. - Subscription — read-only attachment to a job started elsewhere (e.g. a dashboard watching a job a CLI submitted). A subscriber observes the live event stream but cannot cancel or mutate the job. Distinct from resume, which continues the original session and carries cancel authority. See §7.6.
The SDK models each of these as first-class objects; the rest of this README shows how.
Open a session, negotiate features, and reconnect transparently after a transport drop using the resume token — jobs keep running server-side while you're gone.
use Amp\Websocket\Client\WebsocketHandshake;
use function Amp\Websocket\Client\connect;
use Arcp\Client\ARCPClient;
use Arcp\Envelope\Envelope;
use Arcp\Envelope\MessageCatalog;
use Arcp\Ids\MessageId;
use Arcp\Json\EnvelopeSerializer;
use Arcp\Messages\Control\Resume;
use Arcp\Messages\Session\Auth;
use Arcp\Messages\Session\Capabilities;
use Arcp\Messages\Session\PeerInfo;
use Arcp\Transport\WebSocketTransport;
$serializer = new EnvelopeSerializer(MessageCatalog::create());
$openTransport = static fn (): WebSocketTransport => new WebSocketTransport(
connect(new WebsocketHandshake('wss://runtime.example.com/arcp')),
$serializer,
);
$client = new ARCPClient($openTransport());
$accepted = $client->open(
Auth::bearer((string) getenv('ARCP_TOKEN')),
new PeerInfo('resumable', '1.0.0'),
new Capabilities(streaming: true, durableJobs: true),
);
$sessionId = $accepted->sessionId;
$lastMessageId = null;
// ... transport drops ...
$resumed = new ARCPClient($openTransport());
$resumed->open(Auth::bearer((string) getenv('ARCP_TOKEN')), new PeerInfo('resumable', '1.0.0'), new Capabilities(streaming: true, durableJobs: true));
$resumed->session->transport->send(new Envelope(
id: MessageId::random(),
payload: new Resume(afterMessageId: (string) $lastMessageId, includeOpenStreams: true),
timestamp: $resumed->clock->now(),
sessionId: $sessionId,
));
// The runtime replays every envelope with id > $lastMessageId, then resumes live streaming.Submit a job with an agent (optionally version-pinned as name@version), an input, and an optional lease request, idempotency key, and runtime limit.
use Arcp\Ids\IdempotencyKey;
$result = $client->invokeTool(
tool: 'weekly-report@2.1.0',
arguments: [
'week' => '2026-W19',
'lease' => ['net.fetch' => ['s3://reports/**']],
'expires_at' => (new DateTimeImmutable('+60 seconds'))->format(DateTimeInterface::RFC3339_EXTENDED),
],
deadlineSeconds: 300.0,
idempotencyKey: new IdempotencyKey('weekly-report-2026-W19'),
);
printf("resolved value = %s\n", json_encode($result->value));Iterate the ordered event stream — log, thought, tool_call, tool_result, status, metric, artifact_ref, progress, result_chunk — and optionally acknowledge progress so the runtime can release buffered events early.
use Arcp\Envelope\Envelope;
use Arcp\Messages\Execution\JobProgress;
use Arcp\Messages\Execution\ResultChunk;
use Arcp\Messages\Telemetry\EventEmit;
use Arcp\Messages\Telemetry\LogRecord;
use Arcp\Messages\Telemetry\Metric;
$client->subscribe(
['session_id' => [(string) $client->session->sessionId]],
static function (Envelope $env) use ($client): void {
match (true) {
$env->payload instanceof LogRecord => printf("[log] %s\n", $env->payload->message),
$env->payload instanceof Metric => printf("[metric] %s=%s %s\n", $env->payload->name, $env->payload->value, $env->payload->unit ?? ''),
$env->payload instanceof JobProgress => printf("[progress %d%%] %s\n", $env->payload->percent, $env->payload->message ?? ''),
$env->payload instanceof ResultChunk => $client->resultChunks->push($env->payload),
$env->payload instanceof EventEmit => printf("[event] %s\n", $env->payload->eventType),
default => null,
};
},
);Request capabilities, a budget, and an expiry; read budget-remaining metrics as they arrive; handle the runtime's enforcement decisions.
use Arcp\Envelope\Envelope;
use Arcp\Errors\BudgetExhaustedException;
use Arcp\Messages\Telemetry\Metric;
$client->subscribe(
['types' => ['metric']],
static function (Envelope $env): void {
if ($env->payload instanceof Metric && $env->payload->name === 'cost.budget.remaining') {
printf("budget remaining: %.2f %s\n", $env->payload->value, $env->payload->unit ?? '');
}
},
);
try {
$client->invokeTool('web-research', [
'iterations' => 8,
'per_call_usd' => 0.30,
'lease' => [
'tool.call' => ['search.*', 'fetch.*'],
'cost.budget' => ['USD:1.00'],
'model.use' => ['anthropic/claude-*'],
],
'expires_at' => (new DateTimeImmutable('+10 minutes'))->format(DateTimeInterface::RFC3339_EXTENDED),
]);
} catch (BudgetExhaustedException $e) {
// BUDGET_EXHAUSTED / LEASE_EXPIRED are never retryable: resubmit with a fresh lease.
fwrite(STDERR, "job ended: {$e->code()->value}\n");
}Attach read-only to a job submitted elsewhere and observe its live stream (with optional history replay) without cancel authority.
use Arcp\Envelope\Envelope;
$listing = $observer->listJobs(['status' => ['running']], limit: 10);
$first = $listing->jobs[0] ?? null;
if ($first === null) {
return;
}
$subscriptionId = $observer->subscribe(
[
'job_id' => [$first['job_id']],
'since_message_id' => null, // omit to start live; set for backfill
],
static function (Envelope $env): void {
printf("[seq=%s] %s\n", (string) $env->id, $env->type());
},
);
// ... later ...
$observer->unsubscribe($subscriptionId);Catch the typed error taxonomy and respect the retryable flag — LEASE_EXPIRED and BUDGET_EXHAUSTED are never retryable; a naive retry fails identically.
use Arcp\Errors\ARCPException;
use Arcp\Errors\BudgetExhaustedException;
use Arcp\Errors\ErrorCode;
use Arcp\Errors\LeaseExpiredException;
try {
$client->invokeTool('flaky');
} catch (BudgetExhaustedException | LeaseExpiredException $e) {
throw $e; // resubmit with a fresh lease / budget instead
} catch (ARCPException $e) {
if ($e->isRetryable()) {
// safe to retry with backoff (e.g. UNAVAILABLE, DEADLINE_EXCEEDED, INTERNAL)
return;
}
fprintf(STDERR, "fatal: %s (%s)\n", $e->code()->value, $e->getMessage());
throw $e;
}ARCP features this SDK negotiates during the hello/welcome handshake:
| Feature flag | Status |
|---|---|
heartbeat |
Supported |
ack |
Partial |
list_jobs |
Supported |
subscribe |
Supported |
lease_expires_at |
Supported |
cost.budget |
Supported |
model.use |
Supported |
provisioned_credentials |
Supported |
progress |
Supported |
result_chunk |
Supported |
agent_versions |
Supported |
ARCP is transport-agnostic. This SDK ships a WebSocket transport (default), an stdio transport for in-process child runtimes, and an in-memory transport for tests. WebSocket is the default for networked runtimes; stdio is used for in-process child runtimes. Select one by constructing the corresponding Arcp\Transport\Transport implementation (new WebSocketTransport($connection, $serializer), new StdioTransport(...), MemoryTransport::pair()) and passing it to new ARCPClient($transport); the bundled bin/arcp serve --host H --port P exposes a WebSocket runtime that mirrors the same protocol on the wire.
Full API reference — every type, method, and event payload — is in docs/.
This SDK speaks ARCP v1.1 (draft). The SDK follows semantic versioning independently of the protocol; the protocol version it negotiates is shown above and in session.hello. A runtime advertising a different ARCP MAJOR is not guaranteed compatible. Feature mismatches degrade gracefully: the effective feature set is the intersection of what the client and runtime advertise, and the SDK will not use a feature outside it.
See CONTRIBUTING.md. Protocol questions and proposed changes belong in the spec repository; SDK bugs and feature requests belong here.
Apache-2.0 — see LICENSE.