agents >=0.22.0 changed what `Agent` does. Agent now directly extends Cloudflare's DurableObject, with the required PartyServer runtime vendored into agents/lifecycle. Silent: code written for older versions still runs, without an error.

const fs = require("fs");
const path = require("path");
const root = "/w/node_modules/agents";
const pkg = JSON.parse(fs.readFileSync(path.join(root, "package.json"), "utf8"));
function distFiles() {
  const dist = path.join(root, "dist");
  return fs.readdirSync(dist).filter((f) => f.endsWith(".js")).map((f) => ({ file: f, src: fs.readFileSync(path.join(dist, f), "utf8") }));
}
let found = null;
for (const f of distFiles()) {
  const m = /var Agent = class Agent extends (\w+) \{/.exec(f.src);
  if (m) {
    found = { file: f.file, base: m[1], src: f.src };
    break;
  }
}
if (!found) {
  console.error("ERROR: could not find the Agent class in agents " + pkg.version);
  process.exit(2);
}
const fromWorkers = /import \{[^}]*\bDurableObject\b[^}]*\} from "cloudflare:workers";/.test(found.src);
if (found.base === "DurableObject" && fromWorkers) {
  console.log("OK: agents " + pkg.version + " Agent extends DurableObject from cloudflare:workers (" + found.file + ")");
  process.exit(0);
}
const fromPartyserver = /import \{[^}]*\bServer\b[^}]*\} from "partyserver";/.test(found.src);
if (found.base === "Server" && fromPartyserver) {
  console.error("FAIL: Agent extends Server from partyserver, not Cloudflare's DurableObject");
  process.exit(1);
}
console.error("ERROR: unexpected Agent base class " + found.base + " (DurableObject from cloudflare:workers=" + fromWorkers + ")");
process.exit(2);

const fs = require("fs");
const path = require("path");
const root = "/w/node_modules/agents";
const pkg = JSON.parse(fs.readFileSync(path.join(root, "package.json"), "utf8"));
function distFiles() {
  const dist = path.join(root, "dist");
  return fs.readdirSync(dist).filter((f) => f.endsWith(".js")).map((f) => ({ file: f, src: fs.readFileSync(path.join(dist, f), "utf8") }));
}
let found = null;
for (const f of distFiles()) {
  const m = /var Agent = class Agent extends (\w+) \{/.exec(f.src);
  if (m) {
    found = { file: f.file, base: m[1], src: f.src };
    break;
  }
}
if (!found) {
  console.error("ERROR: could not find the Agent class in agents " + pkg.version);
  process.exit(2);
}
const fromWorkers = /import \{[^}]*\bDurableObject\b[^}]*\} from "cloudflare:workers";/.test(found.src);
if (found.base === "DurableObject" && fromWorkers) {
  console.log("OK: agents " + pkg.version + " Agent extends DurableObject from cloudflare:workers (" + found.file + ")");
  process.exit(0);
}
const fromPartyserver = /import \{[^}]*\bServer\b[^}]*\} from "partyserver";/.test(found.src);
if (found.base === "Server" && fromPartyserver) {
  console.error("FAIL: Agent extends Server from partyserver, not Cloudflare's DurableObject");
  process.exit(1);
}
console.error("ERROR: unexpected Agent base class " + found.base + " (DurableObject from cloudflare:workers=" + fromWorkers + ")");
process.exit(2);