---
description: Use tracing.startActiveSpan() and span.end() to instrument operations that outlive a single callback, such as stream pipelines.
title: Workers tracing — write custom spans with new startActiveSpan() and span.end() runtime APIs
image: https://developers.cloudflare.com/changelog/post/2026-07-28-start-active-span/og.png?v=b4fcc15affa3efee
---

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July 28, 2026

## Workers tracing — write custom spans with new startActiveSpan() and span.end() runtime APIs

[Workers](https://developers.cloudflare.com/workers/)

Copy as Markdown| [View as Markdown](https://developers.cloudflare.com/changelog/post/2026-07-28-start-active-span/index.md)| [Agent setup](https://developers.cloudflare.com/agent-setup/)

The Workers runtime now provides built-in `tracing.startActiveSpan()` and `span.end()` APIs, allowing you to write custom spans for operations that last beyond a single callback — for example, instrumenting a stream pipeline where the span should stay open until the stream is fully consumed.

This augments the [existing API for writing custom spans](https://developers.cloudflare.com/changelog/post/2026-06-16-custom-spans/), `tracing.enterSpan()`, which automatically ends a span when its callback is returned. With `startActiveSpan()`, the span remains open after the callback returns, and you call `span.end()` when the work is complete:

*src/index.jsjs*

```js
import { tracing } from "cloudflare:workers";

const encoder = new TextEncoder();

export default {
	fetch() {
		return tracing.startActiveSpan("stream-response", (span) => {
			let timer;

			const body = new ReadableStream({
				start(controller) {
					controller.enqueue(encoder.encode("Starting...\n"));

					timer = setTimeout(() => {
						controller.enqueue(encoder.encode("Complete.\n"));
						controller.close();

						span.setAttribute("stream.status", "complete");
						span.end();
					}, 1000);
				},

				cancel() {
					if (timer !== undefined) clearTimeout(timer);

					span.setAttribute("stream.status", "cancelled");
					span.end();
				},
			});

			return new Response(body, {
				headers: { "content-type": "text/plain" },
			});
		});
	},
};
```

*src/index.tsts*

```ts
import { tracing } from "cloudflare:workers";

const encoder = new TextEncoder();

export default {
	fetch(): Response {
		return tracing.startActiveSpan("stream-response", (span) => {
			let timer: ReturnType<typeof setTimeout> | undefined;

			const body = new ReadableStream<Uint8Array>({
				start(controller) {
					controller.enqueue(encoder.encode("Starting...\n"));

					timer = setTimeout(() => {
						controller.enqueue(encoder.encode("Complete.\n"));
						controller.close();

						span.setAttribute("stream.status", "complete");
						span.end();
					}, 1000);
				},

				cancel() {
					if (timer !== undefined) clearTimeout(timer);

					span.setAttribute("stream.status", "cancelled");
					span.end();
				},
			});

			return new Response(body, {
				headers: { "content-type": "text/plain" },
			});
		});
	},
};
```

For more details, refer to the [custom spans documentation](https://developers.cloudflare.com/workers/observability/traces/custom-spans/).

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```