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examples: 14 ARCP-primitive samples translated from python-sdk
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‎examples/README.md‎

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# ARCP Examples
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Fourteen single-purpose codebases, each named for the protocol
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primitive it demonstrates.
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> **Illustrative, not runnable.** Each example imports from the
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> in-repo `dev.arcp` SDK as if it were a published `dev.arcp:lib:1.0`
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> artifact. Setup boilerplate (transport URL, identity, auth) is
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> elided as `ARCPClient client = null; // transport, identity, auth elided`.
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> LLM and framework calls live in tiny stub files
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> (`Agents.java`, `Steps.java`, `Synth.java`, …) so the protocol code
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> in `Main.java` is what you read.
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## The fourteen
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| Directory | Demonstrates | Spec |
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|---|---|---|
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| [`subscriptions/`](./src/main/java/dev/arcp/examples/subscriptions) | Three Observer clients on one session, three filters, three sinks. | §5, §13 |
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| [`leases/`](./src/main/java/dev/arcp/examples/leases) | Lease-gated shell agent. Read leases coarse, write leases scoped. | §15.4–§15.5 |
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| [`lease_revocation/`](./src/main/java/dev/arcp/examples/lease_revocation) | Per-table leases with `lease.revoked` / `lease.extended` mid-flight. | §15.5 |
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| [`permission_challenge/`](./src/main/java/dev/arcp/examples/permission_challenge) | Two-party permission challenge — generator asks, reviewer holds veto. | §15.4, §6.4 |
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| [`delegation/`](./src/main/java/dev/arcp/examples/delegation) | `agent.delegate` fan-out + `JobMux` to demux events by `job_id`. | §14, §6.4 |
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| [`handoff/`](./src/main/java/dev/arcp/examples/handoff) | `agent.handoff` with transcript packed as an artifact, runtime fingerprint pinned. | §14, §16, §8.3 |
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| [`heartbeats/`](./src/main/java/dev/arcp/examples/heartbeats) | Worker federation; heartbeat-loss reroute via `idempotency_key`. | §10.3, §6.4 |
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| [`capability_negotiation/`](./src/main/java/dev/arcp/examples/capability_negotiation) | Capability-driven peer routing; standard `cost.usd` rollups. | §7, §17.3.1, §18.3 |
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| [`resumability/`](./src/main/java/dev/arcp/examples/resumability) | **Actually crash and resume.** `Runtime.halt(137)` mid-flight; second invocation picks up at the next step. | §10, §19, §6.4 |
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| [`reasoning_streams/`](./src/main/java/dev/arcp/examples/reasoning_streams) | `kind: thought` stream + a peer runtime that subscribes and delegates critiques back. | §11.4, §13, §14 |
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| [`extensions/`](./src/main/java/dev/arcp/examples/extensions) | Custom `arcpx.sdr.*.v1` extension namespace with correct unknown-message handling. | §21 |
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| [`human_input/`](./src/main/java/dev/arcp/examples/human_input) | `human.input.request` fanned across phone/email/Slack; first-wins resolution. | §12 |
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| [`cancellation/`](./src/main/java/dev/arcp/examples/cancellation) | Cooperative `cancel` (terminate) vs `interrupt` (pause and ask). | §10.4–§10.5 |
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| [`mcp/`](./src/main/java/dev/arcp/examples/mcp) | ARCP runtime fronting an MCP server: `tool.invoke` → MCP `call_tool`. | §20 |
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## Conventions
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- Java 25 toolchain, virtual threads (`Executors.newVirtualThreadPerTaskExecutor()`)
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for the concurrent fan-out / event-loop patterns.
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- `record` types for immutable payloads. `var` and `final` thoughtfully.
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- Each example is one `Main.java` (the protocol code) + 0–2 sibling
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stubs named for what they elide (`Agents.java`, `Steps.java`,
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`Cheap.java`, `Synth.java`, `Work.java`, `Channels.java`,
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`Sql.java`, `Upstream.java`).
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- `ARCPClient client = null; // transport, identity, auth elided`
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literally — transport, identity, and auth blocks are setup noise,
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not the point.
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- Envelopes match RFC-0001 v2 exactly. Custom message types follow
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§21.1 `arcpx.<domain>.<name>.v<n>` naming.
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## What's where in the SDK
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- `dev.arcp.client.ARCPClient` — handshake driver.
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- `dev.arcp.envelope.Envelope`, `dev.arcp.error.ErrorCode`,
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`dev.arcp.error.ARCPException` — wire primitives.
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- `dev.arcp.transport.Transport` (and `MemoryTransport`) — transport
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abstraction.
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- `dev.arcp.runtime.ARCPRuntime` — server side.
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## Reading order
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For a brisk tour: `subscriptions`, `leases`, `delegation`,
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`resumability` (this one actually crashes and recovers),
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`cancellation`, `extensions`, `mcp`. These seven exercise the bulk
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of the protocol.
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## Building
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```bash
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cd /Users/nficano/code/arpc/java-sdk
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./gradlew :examples:compileJava
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```
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Each example carries its own `main` method; pick one with the
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`mainClass` system property:
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```bash
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./gradlew :examples:run -PmainClass=dev.arcp.examples.subscriptions.Main
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```

‎examples/build.gradle.kts‎

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options.encoding = "UTF-8"
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options.compilerArgs.addAll(
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listOf(
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"-Werror",
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"-Xlint:all",
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"-Xlint:-processing",
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"-parameters",
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),
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)
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options.errorprone {
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disableWarningsInGeneratedCode.set(true)
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option("NullAway:AnnotatedPackages", "dev.arcp")
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error("NullAway")
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// Examples deliberately leave `ARCPClient client = null;` placeholders
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// in `main()` (the brief calls them out as setup elision). NullAway is
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// valuable on the SDK proper (`:lib`) but counterproductive here.
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option("NullAway:AnnotatedPackages", "dev.arcp.notapackage")
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// Many helper methods are intentional
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// `throw new UnsupportedOperationException(...)` placeholders.
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disable("DoNotCallSuggester")
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disable("UnusedVariable")
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disable("FutureReturnValueIgnored")
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disable("ObjectToString")
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}
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}
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‎examples/src/main/java/dev/arcp/examples/cancellation/Main.java‎

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package dev.arcp.examples.cancellation;
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import dev.arcp.client.ARCPClient;
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import dev.arcp.envelope.Envelope;
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import dev.arcp.error.ARCPException;
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import dev.arcp.error.ErrorCode;
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import java.util.Map;
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/** Two scenarios over the §10.4 / §10.5 control surface. */
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public final class Main {
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private static final int CANCEL_DEADLINE_MS = 5_000;
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private Main() {
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}
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static String startLongJob(ARCPClient client) {
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// Envelope accepted = client.request(client.envelope("tool.invoke",
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// payload=Map.of(
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// "tool", "demo.long_running",
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// "arguments", Map.of("work_seconds", 600))), 10s);
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// return (String) accepted.payload().get("job_id");
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throw new UnsupportedOperationException("startLongJob on " + client);
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}
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/**
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* Cooperative cancel. Runtime drives target to a clean checkpoint inside
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* {@code deadlineMs} before terminating; escalates to {@code ABORTED} on
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* timeout (RFC §10.4).
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*/
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static Envelope cancelJob(ARCPClient client, String jobId, String reason, int deadlineMs) {
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// Envelope reply = client.request(client.envelope("cancel", payload=Map.of(
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// "target", "job", "target_id", jobId, "reason", reason,
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// "deadline_ms", deadlineMs)), deadlineMs / 1000 + 5);
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// if ("cancel.refused".equals(reply.type()))
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// throw new ARCPException(FAILED_PRECONDITION, ...);
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// return reply;
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throw new UnsupportedOperationException(
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"cancel job=" + jobId + " reason=" + reason + " deadline=" + deadlineMs);
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}
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/**
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* Distinct from cancel: pauses the job ({@code blocked}), runtime emits
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* {@code human.input.request}. Job is NOT terminated (RFC §10.5).
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*/
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static void interruptJob(ARCPClient client, String jobId, String prompt) {
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// client.send(client.envelope("interrupt", payload=Map.of(
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// "target", "job", "target_id", jobId, "prompt", prompt)));
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if (Map.of(jobId, prompt).isEmpty()) {
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throw new UnsupportedOperationException("interrupt elided");
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}
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}
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static Envelope awaitTerminal(ARCPClient client, String jobId) {
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// for (Envelope env : client.events()) {
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// if (!jobId.equals(env.jobId())) continue;
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// if (TERMINAL.contains(env.type())) return env;
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// }
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throw new UnsupportedOperationException("awaitTerminal " + jobId);
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}
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static void scenarioCancel() {
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ARCPClient client = null; // transport, identity, auth elided
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// client.open();
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try {
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String jobId = startLongJob(client);
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Thread.sleep(2_000); // let the job actually start
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Envelope ack = cancelJob(client, jobId, "user_aborted", CANCEL_DEADLINE_MS);
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System.out.println("cancel ack: " + ack.type());
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Envelope terminal = awaitTerminal(client, jobId);
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System.out.println("terminal: " + terminal.type());
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} catch (InterruptedException ie) {
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Thread.currentThread().interrupt();
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} finally {
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// client.close();
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}
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}
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static void scenarioInterrupt() {
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ARCPClient client = null;
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// client.open();
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try {
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String jobId = startLongJob(client);
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Thread.sleep(2_000);
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interruptJob(client, jobId, "Pause and ask before touching production tables.");
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// Runtime now emits human.input.request; answer via examples/human_input.
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// for (Envelope env : client.events()) {
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// if ("human.input.request".equals(env.type()) && jobId.equals(env.jobId())) {
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// System.out.println("awaiting human: " + env.payload().get("prompt"));
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// return;
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// }
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// }
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} catch (InterruptedException ie) {
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Thread.currentThread().interrupt();
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} finally {
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// client.close();
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}
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}
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public static void main(String[] args) {
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String which = args.length > 0 ? args[0] : "cancel";
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switch (which) {
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case "cancel" -> scenarioCancel();
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case "interrupt" -> scenarioInterrupt();
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default -> throw new ARCPException(ErrorCode.INVALID_ARGUMENT, "unknown scenario: " + which);
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}
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}
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}

‎examples/src/main/java/dev/arcp/examples/cancellation/README.md‎

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# cancellation
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Two scenarios that exercise the §10.4–§10.5 control surface that
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distinguishes ARCP from "agent over plain HTTP":
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- `cancel`: cooperative termination with a deadline.
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- `interrupt`: pause the job and route through a human, no
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termination.
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## Before ARCP
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Cancellation usually means closing the socket or trying to kill the
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process. The agent's tool was already mid-network call, so it
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either completes anyway (silent waste of money) or leaves a
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half-applied side effect. There's no notion of "stop and ask"; the
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only knob is "stop".
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## With ARCP
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```java
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// Stop the job; the runtime drives it to a clean checkpoint
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// inside `deadline_ms` before terminating.
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Envelope ack = cancelJob(client, jobId, "user_aborted", 5_000); // cancel.accepted
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Envelope terminal = awaitTerminal(client, jobId); // job.cancelled
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// Or: pause the job, ask the human, resume.
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interruptJob(client, jobId, "Pause and ask before touching prod.");
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// runtime emits human.input.request; answer with the HITL relay.
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```
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## ARCP primitives
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- `cancel` cooperative contract — RFC §10.4 (`cancel.accepted` /
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`cancel.refused`, `deadline_ms`, escalation to `ABORTED`).
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- `interrupt` (distinct from cancel) — §10.5; emits
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`human.input.request`, leaves the job in `blocked`.
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- `capabilities.interrupt: false` fallback to `cancel` (advertised
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per §10.5; clients that find `interrupt: false` on a peer fall
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through to `cancel`).
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## File tour
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- `Main.java` — two scenarios driven by `argv[0]` (`cancel` or
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`interrupt`).
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## Variations
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- Pair `interrupt` with [human_input](../human_input) for a working
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pause-and-ask loop.
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- Send `cancel` against a `stream_id` instead of a `job_id` to
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terminate just one stream — terminal is a `stream.error` with
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`code: CANCELLED` (§10.4).
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- Race many peers, cancel the slowest once N succeed.

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