Changelog

The latest release of the Agents SDK ↗ rewrites observability from scratch with diagnostics_channel, adds keepAlive() to prevent Durable Object eviction during long-running work, and introduces waitForMcpConnections so MCP tools are always available when onChatMessage runs.

The previous observability system used console.log() with a custom Observability.emit() interface. v0.7.0 replaces it with structured events published to diagnostics channels — silent by default, zero overhead when nobody is listening.

Every event has a type, payload, and timestamp. Events are routed to seven named channels:

Use the typed subscribe() helper from agents/observability for type-safe access:

import { subscribe } from "agents/observability";

const unsub = subscribe("rpc", (event) => {
  if (event.type === "rpc") {
    console.log(`RPC call: ${event.payload.method}`);
  }
  if (event.type === "rpc:error") {
    console.error(
      `RPC failed: ${event.payload.method} — ${event.payload.error}`,
    );
  }
});

// Clean up when done
unsub();

import { subscribe } from "agents/observability";

const unsub = subscribe("rpc", (event) => {
  if (event.type === "rpc") {
    console.log(`RPC call: ${event.payload.method}`);
  }
  if (event.type === "rpc:error") {
    console.error(
      `RPC failed: ${event.payload.method} — ${event.payload.error}`,
    );
  }
});

// Clean up when done
unsub();

In production, all diagnostics channel messages are automatically forwarded to Tail Workers — no subscription code needed in the agent itself:

export default {
  async tail(events) {
    for (const event of events) {
      for (const msg of event.diagnosticsChannelEvents) {
        // msg.channel is "agents:rpc", "agents:workflow", etc.
        console.log(msg.timestamp, msg.channel, msg.message);
      }
    }
  },
};

export default {
  async tail(events) {
    for (const event of events) {
      for (const msg of event.diagnosticsChannelEvents) {
        // msg.channel is "agents:rpc", "agents:workflow", etc.
        console.log(msg.timestamp, msg.channel, msg.message);
      }
    }
  },
};

The custom Observability override interface is still supported for users who need to filter or forward events to external services.

For the full event reference, refer to the Observability documentation.

Durable Objects are evicted after a period of inactivity (typically 70-140 seconds with no incoming requests, WebSocket messages, or alarms). During long-running operations — streaming LLM responses, waiting on external APIs, running multi-step computations — the agent can be evicted mid-flight.

keepAlive() prevents this by creating a 30-second heartbeat schedule. The alarm firing resets the inactivity timer. Returns a disposer function that cancels the heartbeat when called.

const dispose = await this.keepAlive();
try {
  const result = await longRunningComputation();
  await sendResults(result);
} finally {
  dispose();
}

const dispose = await this.keepAlive();
try {
  const result = await longRunningComputation();
  await sendResults(result);
} finally {
  dispose();
}

keepAliveWhile() wraps an async function with automatic cleanup — the heartbeat starts before the function runs and stops when it completes:

const result = await this.keepAliveWhile(async () => {
  const data = await longRunningComputation();
  return data;
});

const result = await this.keepAliveWhile(async () => {
  const data = await longRunningComputation();
  return data;
});

Key details:

• Multiple concurrent callers — Each keepAlive() call returns an independent disposer. Disposing one does not affect others.
• AIChatAgent built-in — AIChatAgent automatically calls keepAlive() during streaming responses. You do not need to add it yourself.
• Uses the scheduling system — The heartbeat does not conflict with your own schedules. It shows up in getSchedules() if you need to inspect it.

For the full API reference and when-to-use guidance, refer to Schedule tasks — Keeping the agent alive.

AIChatAgent now waits for MCP server connections to settle before calling onChatMessage. This ensures this.mcp.getAITools() returns the full set of tools, especially after Durable Object hibernation when connections are being restored in the background.

export class ChatAgent extends AIChatAgent {
  // Default — waits up to 10 seconds
  // waitForMcpConnections = { timeout: 10_000 };

  // Wait forever
  waitForMcpConnections = true;

  // Disable waiting
  waitForMcpConnections = false;
}

export class ChatAgent extends AIChatAgent {
  // Default — waits up to 10 seconds
  // waitForMcpConnections = { timeout: 10_000 };

  // Wait forever
  waitForMcpConnections = true;

  // Disable waiting
  waitForMcpConnections = false;
}

For lower-level control, call this.mcp.waitForConnections() directly inside onChatMessage instead.

• MCP deduplication by name and URL — addMcpServer with HTTP transport now deduplicates on both server name and URL. Calling it with the same name but a different URL creates a new connection. URLs are normalized before comparison (trailing slashes, default ports, hostname case).
• callbackHost optional for non-OAuth servers — addMcpServer no longer requires callbackHost when connecting to MCP servers that do not use OAuth.
• MCP URL security — Server URLs are validated before connection to prevent SSRF. Private IP ranges, loopback addresses, link-local addresses, and cloud metadata endpoints are blocked.
• Custom denial messages — addToolOutput now supports state: "output-error" with errorText for custom denial messages in human-in-the-loop tool approval flows.
• requestId in chat options — onChatMessage options now include a requestId for logging and correlating events.

To update to the latest version:

npm i agents@latest @cloudflare/ai-chat@latest