feat: serve experimental ACP v2 (#1400)

## Summary

The draft ACP v2 is no longer opt-in. The adapter always connects
through the SDK's protocol router (as codex-acp does), so a client that
asks for protocol version 2 or later in `initialize` gets v2, and every
other client gets v1 as before. v2 is still experimental and follows the
draft as it changes.

- Removes `CLAUDE_AGENT_ACP_EXPERIMENTAL_V2`, which only turned on what
is now the default.
- Adds `CLAUDE_AGENT_ACP_PROTOCOL` to limit the adapter to one version:
  - unset, empty, or `auto`: both, picked by the client's `initialize`.
- `v1`: a client that asks for v2 is negotiated down to v1. This is the
escape hatch if a client's v2 handling breaks.
- `v2`: a v1 client's `initialize` is rejected with `-32022 Unsupported
protocol version`.
- Any other value fails at startup, so a typo like
`CLAUDE_AGENT_ACP_PROTOCOL=1` doesn't silently serve both.
- Fixes the comment in `src/acp-subagents.ts` that said v2 clients don't
get subagents.

Note: a client that already sends `protocolVersion >= 2` switches to the
v2 surface as soon as this lands on `main`, which publishes a preview.

## Testing

- New routing tests: down-negotiation to v1, rejecting v1 when limited
to v2, env parsing.
- `npm run check`, `npm run build`, `npm run test:run` pass.
- Ran the built binary over stdio with each setting: default → v2 for a
v2 client, `v1` → protocol version 1, `v2` → `-32022` for a v1 client,
invalid value → exits with an error at startup.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -95,10 +95,13 @@ root session. Clients that use the historical `_meta["subagent-transcript"]` cap
 
 ### Experimental ACP v2
 
-Started with `CLAUDE_AGENT_ACP_EXPERIMENTAL_V2=1`, the adapter also serves the
-[draft ACP v2](https://agentclientprotocol.com/protocol/v2/overview): a client that asks for
-protocol version 2 or later in `initialize` gets v2, and every other client gets v1, as before.
-The draft can still change, and this surface changes with it.
+The adapter also serves the [draft ACP v2](https://agentclientprotocol.com/protocol/v2/overview):
+a client that asks for protocol version 2 or later in `initialize` gets v2, and every other client
+gets v1. The draft can still change, and this surface changes with it.
+
+`CLAUDE_AGENT_ACP_PROTOCOL` limits the adapter to one version. With `v1`, a client that asks
+for v2 is negotiated down to v1. With `v2`, the adapter rejects the `initialize` of a v1 client.
+`auto`, the default, serves both.
 
 On v2 the adapter serves the session baseline (`session/new`, `session/list`, `session/resume`
 with replay, `session/close`, `session/prompt`, `session/cancel`), `session/delete`,

src/acp-subagents.ts

@@ -17,7 +17,8 @@ export { AIR_NATIVE_SUBAGENT_SESSIONS_CAPABILITY } from "./air-extension.js";
  * - `rfd`: the subagents RFD (agentclientprotocol/agent-client-protocol#1992,
  *   as revised): `subagent_update` and `session_message`, which the SDK types.
  *   A client that declares `clientCapabilities.subagents` and is not AIR gets
- *   it. (ACP v2 clients do not get subagents yet.)
+ *   it. (An ACP v2 client gets it without declaring anything, since subagents
+ *   are part of v2's baseline.)
  * - `air`: the earlier draft of that RFD, which AIR implements:
  *   `subagent_spawned` and `subagent_state_update`, typed below. AIR gets it,
  *   whichever capability it declares (`docs/air-extensions.md`).

src/index.ts

@@ -2,7 +2,7 @@
 
 import { claudeCliPath } from "./acp-agent.js";
 import { applyManagedPolicyEnv } from "./managed-policy.js";
-import { serveAcp } from "./serve.js";
+import { acpProtocolsFromEnv, serveAcp } from "./serve.js";
 import packageJson from "../package.json" with { type: "json" };
 import { appendFileSync, mkdirSync } from "node:fs";
 import { join } from "node:path";
@@ -83,9 +83,8 @@ if (process.argv.includes("--cli")) {
     : undefined;
   logger?.log("Claude ACP started");
 
-  // The draft ACP v2 is opt-in while it is a draft.
   const acp = serveAcp(logger, {
-    experimentalV2: process.env.CLAUDE_AGENT_ACP_EXPERIMENTAL_V2 === "1",
+    protocols: acpProtocolsFromEnv(process.env.CLAUDE_AGENT_ACP_PROTOCOL),
   });
 
   async function shutdown() {

src/serve.ts

@@ -1,19 +1,32 @@
-import { ndJsonStream } from "@agentclientprotocol/sdk/unstable";
-import * as v2 from "@agentclientprotocol/sdk/experimental/v2/unstable";
+import * as v2 from "@agentclientprotocol/sdk/experimental/v2";
 import { v1AgentApp, type ClaudeAcpAgent, type Logger } from "./acp-agent.js";
 import { nodeToWebReadable, nodeToWebWritable } from "./utils.js";
 import { v2AgentApp } from "./v2/agent.js";
 
 /**
- * Serves ACP on stdio.
- *
- * With `experimentalV2`, the connection goes through {@link acpProtocolRouter}.
- * Without it, the adapter serves ACP v1 alone and never routes by protocol
- * version, exactly as before.
+ * The ACP versions that the adapter serves: `v1` or `v2` alone, or both with
+ * `auto`, where the client's `initialize` picks the version.
  */
+export type AcpProtocols = "auto" | "v1" | "v2";
+
+/**
+ * Reads `CLAUDE_AGENT_ACP_PROTOCOL`: unset, empty, or `auto` serves both
+ * versions, and `v1` or `v2` serves that version alone. Throws on any other
+ * value, so that a typo such as `CLAUDE_AGENT_ACP_PROTOCOL=1` fails at startup
+ * instead of silently serving both.
+ */
+export function acpProtocolsFromEnv(value: string | undefined): AcpProtocols {
+  if (value === undefined || value === "") return "auto";
+  if (value === "auto" || value === "v1" || value === "v2") return value;
+  throw new Error(
+    `CLAUDE_AGENT_ACP_PROTOCOL must be "auto", "v1", or "v2", not ${JSON.stringify(value)}`,
+  );
+}
+
+/** Serves ACP on stdio through {@link acpProtocolRouter}. */
 export function serveAcp(
   logger: Logger | undefined,
-  options: { experimentalV2: boolean },
+  options: { protocols: AcpProtocols },
 ): { closed: Promise<void>; dispose(): Promise<void> } {
   const output = nodeToWebWritable(process.stdout);
   const input = nodeToWebReadable(process.stdin);
@@ -23,9 +36,9 @@ export function serveAcp(
   const onAgent = (created: ClaudeAcpAgent) => {
     agent = created;
   };
-  const { closed } = options.experimentalV2
-    ? acpProtocolRouter(logger, onAgent).connect(v2.ndJsonStream(output, input))
-    : v1AgentApp(logger, onAgent).connect(ndJsonStream(output, input));
+  const { closed } = acpProtocolRouter(logger, onAgent, options.protocols).connect(
+    v2.ndJsonStream(output, input),
+  );
   return {
     closed,
     dispose: async () => {
@@ -36,16 +49,19 @@ export function serveAcp(
 
 /**
  * Routes a connection to the protocol version that the client negotiates in
- * `initialize`: the experimental draft ACP v2 for a client that requests
- * version 2 or later, and ACP v1 otherwise. `onAgent` receives the agent of
- * the connection once the router has opened it.
+ * `initialize`. With `auto`, a client that requests version 2 or later gets
+ * the draft ACP v2, and every other client gets ACP v1. With `v1`, a v2
+ * client is negotiated down to v1. With `v2`, the router rejects the
+ * `initialize` of a v1 client as an unsupported protocol version. `onAgent`
+ * receives the agent of the connection once the router has opened it.
  */
 export function acpProtocolRouter(
   logger: Logger | undefined,
   onAgent: (agent: ClaudeAcpAgent) => void,
+  protocols: AcpProtocols = "auto",
 ): v2.AgentProtocolRouter {
-  return v2
-    .agentProtocolRouter()
-    .withV1(v1AgentApp(logger, onAgent))
-    .withV2(v2AgentApp(logger, onAgent));
+  const router = v2.agentProtocolRouter();
+  if (protocols !== "v2") router.withV1(v1AgentApp(logger, onAgent));
+  if (protocols !== "v1") router.withV2(v2AgentApp(logger, onAgent));
+  return router;
 }

src/tests/acp-v2.test.ts

@@ -21,7 +21,7 @@ import type { ClaudeAcpAgent } from "../acp-agent.js";
 import type { AuthStatusUpdateNotification } from "../auth-status.js";
 import { clientSupportsCompactionUpdates } from "../context-compaction.js";
 import { v2DiffContent } from "../diff.js";
-import { acpProtocolRouter } from "../serve.js";
+import { type AcpProtocols, acpProtocolRouter, acpProtocolsFromEnv } from "../serve.js";
 import { clientSupportsNotices } from "../session-notices.js";
 import { v1PermissionResponse, v2PermissionRequest } from "../v2/permission.js";
 import { v1PromptRequest } from "../v2/prompt.js";
@@ -157,10 +157,10 @@ afterEach(async () => {
 });
 
 /** Connects a fresh router to a client, and returns the client end of the stream. */
-function connectRouter(): v2.Stream {
+function connectRouter(protocols?: AcpProtocols): v2.Stream {
   const toAgent = new TransformStream<v2.AnyWireMessage, v2.AnyWireMessage>();
   const toClient = new TransformStream<v2.AnyWireMessage, v2.AnyWireMessage>();
-  acpProtocolRouter(undefined, (agent) => agents.push(agent)).connect({
+  acpProtocolRouter(undefined, (agent) => agents.push(agent), protocols).connect({
     readable: toAgent.readable,
     writable: toClient.writable,
   });
@@ -316,6 +316,52 @@ describe("ACP protocol routing", () => {
       expect((method as v2.AuthMethodTerminal).args?.[0]).toBe("--cli");
     }
   });
+
+  it("negotiates a v2 client down to v1 when limited to v1", async () => {
+    // On the raw stream, since the v2 client app cannot read the v1 response
+    // that a real client would switch to v1 on.
+    const stream = connectRouter("v1");
+    const writer = stream.writable.getWriter();
+    await writer.write({
+      jsonrpc: "2.0",
+      id: 1,
+      method: "initialize",
+      params: { protocolVersion: v2.PROTOCOL_VERSION, info: V2_CLIENT_INFO, capabilities: {} },
+    });
+    const reader = stream.readable.getReader();
+    let response: v2.AnyWireMessage | undefined;
+    while (!response) {
+      const { value, done } = await reader.read();
+      if (done) break;
+      if (!Array.isArray(value) && "id" in value && value.id === 1) response = value;
+    }
+    await writer.close();
+
+    expect(response).toMatchObject({ id: 1, result: { protocolVersion: 1 } });
+  });
+
+  it("rejects a v1 client when limited to v2", async () => {
+    const initialize = v1
+      .client({ name: "v1-test-client" })
+      .connectWith(connectRouter("v2") as unknown as v1.Stream, (agent) =>
+        agent.request(v1.methods.agent.initialize, {
+          protocolVersion: v1.PROTOCOL_VERSION,
+          clientCapabilities: {},
+        }),
+      );
+
+    await expect(initialize).rejects.toThrow(/Unsupported protocol version/);
+    expect(agents).toHaveLength(0);
+  });
+
+  it("reads the protocols to serve from CLAUDE_AGENT_ACP_PROTOCOL", () => {
+    expect(acpProtocolsFromEnv(undefined)).toBe("auto");
+    expect(acpProtocolsFromEnv("")).toBe("auto");
+    expect(acpProtocolsFromEnv("auto")).toBe("auto");
+    expect(acpProtocolsFromEnv("v1")).toBe("v1");
+    expect(acpProtocolsFromEnv("v2")).toBe("v2");
+    expect(() => acpProtocolsFromEnv("1")).toThrow(/CLAUDE_AGENT_ACP_PROTOCOL/);
+  });
 });
 
 /** The commands of every session of the mocked SDK, in v2 form. */