https://github.com/agentruntimecontrolprotocol/swift-sdk

Swift reference SDK for ARCP (Agent Runtime Control Protocol).
https://github.com/agentruntimecontrolprotocol/swift-sdk

agent-protocol agent-runtime-control-protocol agents ai-agents arcp durable-execution llm mcp sdk streaming swift

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Swift reference SDK for ARCP (Agent Runtime Control Protocol).

• Host: GitHub
• URL: https://github.com/agentruntimecontrolprotocol/swift-sdk
• Owner: agentruntimecontrolprotocol
• License: apache-2.0
• Created: 2026-05-10T16:45:39.000Z (about 2 months ago)
• Default Branch: main
• Last Pushed: 2026-06-12T13:50:25.000Z (18 days ago)
• Last Synced: 2026-06-12T15:23:37.369Z (17 days ago)
• Topics: agent-protocol, agent-runtime-control-protocol, agents, ai-agents, arcp, durable-execution, llm, mcp, sdk, streaming, swift
• Language: Swift
• Homepage: https://github.com/agentruntimecontrolprotocol/spec
• Size: 572 KB
• Stars: 1
• Watchers: 0
• Forks: 0
• Open Issues: 30
• Metadata Files:
  • Readme: README.md
  • Changelog: CHANGELOG.md
  • Contributing: CONTRIBUTING.md
  • License: LICENSE

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README

          
ARCP Swift SDK


Swift SDK for the Agent Runtime Control Protocol (ARCP) — submit, observe, and control long-running agent jobs from Swift.




  

  

  

  

  





  Specification ·

  Concepts ·

  Install ·

  Quick start ·

  Guides ·

  API reference



---

The `ARCP` Swift package is the Swift reference implementation of [ARCP](https://github.com/agentruntimecontrolprotocol/spec/blob/main/docs/draft-arcp-1.1.md), the Agent Runtime Control Protocol. It covers both sides of the wire — `ARCPClient` for submitting and observing jobs, `ARCPRuntime` for hosting agents through a `ToolHandler` / `JobContext` pair — 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).

## Installation

Requires Swift 6.1 or later and macOS 14+. The SDK ships as a single Swift Package Manager package named `ARCP` with one executable product, `arcp`, for the bundled CLI. Add the dependency to your `Package.swift`:

```swift

dependencies: [

    .package(url: "https://github.com/agentruntimecontrolprotocol/swift-sdk.git", from: "1.1.0"),

],

targets: [

    .target(name: "MyApp", dependencies: [

        .product(name: "ARCP", package: "swift-sdk"),

    ]),

]

```

## Quick start

Connect to a runtime, submit a job, stream its events to completion:

```swift

import ARCP

import NIOPosix

let group = MultiThreadedEventLoopGroup(numberOfThreads: 1)

let transport = try await WebSocketClient.connect(

    url: "wss://runtime.example.com/arcp",

    eventLoopGroup: group

)

let client = try await ARCPClient.open(

    transport: transport,

    auth: AuthBlock(scheme: .bearer, token: ProcessInfo.processInfo.environment["ARCP_TOKEN"]),

    client: IdentityBlock(kind: "quickstart", version: "1.0.0"),

    capabilities: Capabilities(streaming: true, durableJobs: true, subscriptions: true)

)

let result = try await client.invoke(

    tool: "data-analyzer",

    arguments: .object(["dataset": .string("s3://example/sales.csv")])

)

switch result.outcome {

case .completed(let payload):

    print("final:", payload.result ?? payload.summary ?? .null)

case .failed(let error):

    print("failed [\(error.code.rawValue)]:", error.message)

case .cancelled(let payload):

    print("cancelled:", payload.reason)

}

await client.close()

try await group.shutdownGracefully()

```

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.

## Concepts

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 `session.open`/`session.accepted` handshake, and is *resumable*: if the transport drops, you reconnect and the runtime replays buffered events from your last seen message id (`resume` with `after_message_id`). Resume-token rotation is not yet implemented; resume past the retained window returns `RESUME_WINDOW_EXPIRED`. Jobs outlive the session that started them. See [§6](https://github.com/agentruntimecontrolprotocol/spec/blob/main/docs/draft-arcp-1.1.md).

- **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`, or `timed_out`. See [§7](https://github.com/agentruntimecontrolprotocol/spec/blob/main/docs/draft-arcp-1.1.md).

- **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](https://github.com/agentruntimecontrolprotocol/spec/blob/main/docs/draft-arcp-1.1.md).

- **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`). Lease enforcement is **agent-cooperative**: the runtime validates `lease_constraints.expires_at` at submission (UTC, future) and exposes `checkLeaseExpiration`, `checkModelUse`, and `charge` on `JobContext`, which a handler MUST invoke before authority-bearing operations. The runtime does not yet mediate `fs.*`/`net.fetch`/`tool.call`/`agent.delegate` itself, so it cannot unilaterally stop a handler that skips the checks. Leases may carry a budget and an expiry, and the §9.4 subset rules are validated by `LeaseSubsetting`. See [§9](https://github.com/agentruntimecontrolprotocol/spec/blob/main/docs/draft-arcp-1.1.md).

- **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](https://github.com/agentruntimecontrolprotocol/spec/blob/main/docs/draft-arcp-1.1.md).

The SDK models each of these as first-class objects; the rest of this README shows how.

## Guides

### Sessions and resume

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.

```swift

import ARCP

let client = try await ARCPClient.open(

    transport: transport,

    auth: AuthBlock(scheme: .bearer, token: token),

    client: IdentityBlock(kind: "resumable", version: "1.0.0"),

    capabilities: Capabilities(streaming: true, durableJobs: true)

)

// Track the last envelope we saw so we can resume gap-free.

var lastSeen: MessageId?

let drainer = Task {

    for await envelope in client.unhandled {

        lastSeen = envelope.id

    }

}

// ... transport drops; reconnect over a fresh transport ...

let resumed = try await ARCPClient.open(

    transport: try await WebSocketClient.connect(url: url, eventLoopGroup: group),

    auth: AuthBlock(scheme: .bearer, token: token),

    client: IdentityBlock(kind: "resumable", version: "1.0.0"),

    capabilities: Capabilities(streaming: true, durableJobs: true)

)

try await resumed.send(

    Envelope(

        sessionId: resumed.info.sessionId, jobId: jobId,

        payload: .resume(ResumePayload(afterMessageId: lastSeen, includeOpenStreams: true))

    )

)

// The runtime replays every envelope with id > lastSeen, then resumes live streaming.

drainer.cancel()

```

### Submitting jobs

Submit a job with an agent (optionally version-pinned as `name@version`), an input, and an optional lease request, idempotency key, and runtime limit.

```swift

let invocation = try await client.invoke(

    tool: "weekly-report@2.1.0",

    arguments: .object(["week": .string("2026-W19")]),

    costBudget: .from(["USD": 1.00]),

    leaseConstraints: LeaseConstraints(expiresAt: Date().addingTimeInterval(60)),

    idempotencyKey: IdempotencyKey("weekly-report-2026-W19")

)

print("job_id =", invocation.jobId?.rawValue ?? "(pending)")

print("outcome =", invocation.outcome)

```

### Consuming events

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.

```swift

let invocation = try await client.invoke(

    tool: "summarizer",

    arguments: .object(["text": .string(corpus)])

)

// Stream structured progress while the job runs.

let progressTask = Task {

    for await progress in invocation.progress {

        let percent = progress.percent.map { String(format: "%.0f%%", $0) } ?? "?"

        print("progress \(percent) — \(progress.message ?? "")")

    }

}

// Every event the client didn't consume internally surfaces here in order.

for await envelope in await client.unhandled {

    switch envelope.payload {

    case .log(let log):

        print("[\(log.level)] \(log.message)")

    case .metric(let m):

        print("metric \(m.name)=\(m.value)\(m.unit.map { " \($0)" } ?? "")")

    case .jobCompleted, .jobFailed, .jobCancelled:

        break

    default:

        continue

    }

}

progressTask.cancel()

```

### Leases and budgets

Request capabilities, a budget, and an expiry; read budget-remaining metrics as they arrive; handle the runtime's enforcement decisions.

```swift

let invocation = try await client.invoke(

    tool: "web-research",

    arguments: .object(["iterations": .int(8), "perCallUSD": .double(0.3)]),

    costBudget: .from(["USD": 1.00]),

    leaseConstraints: LeaseConstraints(expiresAt: Date().addingTimeInterval(600))

)

// Surface cost.budget.remaining metrics as they arrive.

let watcher = Task {

    for await env in await client.unhandled {

        if case .metric(let m) = env.payload, m.name == "cost.budget.remaining" {

            let unit = m.unit ?? ""

            print(String(format: "budget remaining: %.2f %@", m.value, unit))

        }

    }

}

if case .failed(let error) = invocation.outcome {

    // BUDGET_EXHAUSTED and LEASE_EXPIRED are never retryable.

    print("job ended [\(error.code.rawValue)]: \(error.message)")

}

watcher.cancel()

```

### Subscribing to jobs

Attach read-only to a job submitted elsewhere and observe its live stream (with optional history replay) without cancel authority.

```swift

let observer = try await ARCPClient.open(

    transport: try await WebSocketClient.connect(url: url, eventLoopGroup: group),

    auth: AuthBlock(scheme: .bearer, token: token),

    client: IdentityBlock(kind: "dashboard", version: "1.0.0"),

    capabilities: Capabilities(subscriptions: true)

)

let filter = SubscriptionFilter(jobIds: [jobId])

try await observer.send(

    Envelope(

        sessionId: observer.info.sessionId,

        payload: .subscribe(

            SubscribePayload(filter: filter, since: SubscriptionSince(afterMessageId: nil))

        )

    )

)

for await envelope in await observer.unhandled {

    if case .subscribeEvent(let payload) = envelope.payload {

        print("event:", payload.event)

    }

}

```

### Error handling

Catch the typed error taxonomy and respect the `retryable` flag — `LEASE_EXPIRED` and `BUDGET_EXHAUSTED` are never retryable; a naive retry fails identically.

```swift

do {

    let invocation = try await client.invoke(tool: "flaky", arguments: .null)

    if case .failed(let error) = invocation.outcome {

        switch error.code {

        case .leaseExpired, .budgetExhausted:

            throw ARCPError.failedPrecondition(detail: "renew lease/budget and resubmit")

        default:

            if error.retryable == true {

                // safe to retry with backoff (e.g. internal, unavailable, deadlineExceeded)

            }

        }

    }

} catch let error as ARCPError {

    if error.isRetryable {

        // backoff and retry

    }

    throw error

}

```

## Feature support

ARCP features this SDK negotiates during the `session.open`/`session.accepted` handshake:

| Feature flag | Status |

|---|---|

| `heartbeat` | Supported |

| `ack` | Planned |

| `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 |

## Transport

ARCP is transport-agnostic. This SDK ships a `WebSocketTransport` (client-side, built on `WebSocketKit` / SwiftNIO), a `StdioTransport` (NDJSON framing over stdin/stdout for in-process child runtimes), and a `MemoryTransport` (in-process pair for tests and samples). WebSocket is the default for networked runtimes; stdio is used for in-process child runtimes. Select one by constructing the corresponding `Transport` — `WebSocketClient.connect(url:eventLoopGroup:)`, `StdioTransport(inbound:outbound:)`, or `MemoryTransport.makePair()` — and passing it to `ARCPClient.open(transport:auth:client:capabilities:)`.

## API reference

Full API reference — every type, method, and event payload — is in [`docs/`](docs/) and at .

## Versioning and compatibility

This SDK speaks **ARCP v1.1 (draft)**. The SDK follows semantic versioning independently of the protocol; the negotiated runtime version is available on `client.info.runtimeIdentity.version`. 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.

## Contributing

See [`CONTRIBUTING.md`](CONTRIBUTING.md). Protocol questions and proposed changes belong in the [spec repository](https://github.com/agentruntimecontrolprotocol/spec); SDK bugs and feature requests belong here.

## License

Apache-2.0 — see [`LICENSE`](LICENSE).