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Custom spans

Cloudflare Workers automatically instruments platform operations like fetch calls, KV reads, and D1 queries. Custom spans let you extend this visibility into your own application logic, so you can trace custom code paths alongside the built-in instrumentation.

The custom spans API is available in two ways — both provide the same methods and behave identically:

• import { tracing } from "cloudflare:workers" — works anywhere in your codebase, including utility functions, libraries, and modules that do not have access to the handler context.
• ctx.tracing — available on the ExecutionContext passed to your handler, convenient when you are already working within a handler.

There are two span creation methods:

• enterSpan() — creates a span that automatically ends when the callback returns or its returned promise settles. Use this for most instrumentation.
• startActiveSpan() — creates a span that you end manually by calling span.end(). Use this when the span must outlive the callback, such as when instrumenting streams or other long-lived operations.

Custom spans require tracing to be enabled on your Worker. If you have not already done so, set observability.traces.enabled to true in your Wrangler configuration file:

{
  "$schema": "./node_modules/wrangler/config-schema.json",
  "observability": {
    "traces": {
      "enabled": true
    }
  }
}

[observability.traces]
enabled = true

Use tracing.enterSpan() to wrap a section of code in a named span. The span automatically becomes a child of whichever span is currently active, and ends when the callback returns or its returned promise settles.

The following example uses both access methods — the cloudflare:workers import and ctx.tracing — to show that they are interchangeable:

import { tracing } from "cloudflare:workers";

export default {
	async fetch(request, env, ctx) {
		// Using the import
		return tracing.enterSpan("handleRequest", async (span) => {
			span.setAttribute("url.path", new URL(request.url).pathname);

			const user = await ctx.tracing.enterSpan("auth", async () => {
				// Using ctx.tracing
				return authenticate(request, env);
			});

			return buildResponse(user);
		});
	},
};

import { tracing } from "cloudflare:workers";

export default {
	async fetch(request: Request, env: Env, ctx: ExecutionContext) {
		// Using the import
		return tracing.enterSpan("handleRequest", async (span) => {
			span.setAttribute("url.path", new URL(request.url).pathname);

			const user = await ctx.tracing.enterSpan("auth", async () => {
				// Using ctx.tracing
				return authenticate(request, env);
			});

			return buildResponse(user);
		});
	},
};

Creates a new span and runs callback inside it. The span is automatically ended when the callback returns (synchronous or asynchronous) or throws.

Parameters:

Returns: The return value of callback.

Behavior:

• The new span is a child of whichever span is currently active on the async context. If no span is active, it becomes a child of the request's root span.
• Nested enterSpan calls and runtime-created spans (such as fetch or KV operations) that run inside the callback automatically become children of this span.
• The span ends when the callback returns synchronously, throws synchronously, or when its returned promise fulfills or rejects.

// Synchronous callback — span ends when the function returns
const result = tracing.enterSpan("parse", (span) => {
	span.setAttribute("format", "json");
	return JSON.parse(body);
});

// Async callback — span ends when the promise settles
const data = await tracing.enterSpan("fetchData", async (span) => {
	const res = await fetch("https://api.example.com/data");
	span.setAttribute("http.response.status_code", res.status);
	return res.json();
});

// Forwarding arguments
const doubled = tracing.enterSpan("compute", (span, x) => x * 2, 21);

Creates a new span, makes it the active span while callback runs, and returns the callback result without automatically ending the span. You must call span.end() explicitly when the operation is complete.

Parameters:

Returns: The return value of callback.

Behavior:

• Unlike enterSpan, the span is not automatically ended when the callback returns or throws. You are responsible for calling span.end().
• If you forget to call span.end(), the span is still submitted when the request-owned span object is destroyed, as a backstop. Do not rely on this behavior — always call span.end() explicitly.

Use startActiveSpan when you need a span to cover an operation that extends beyond a single callback — for example, instrumenting a stream pipeline where the span should remain open until the stream is fully consumed:

import { tracing } from "cloudflare:workers";

export default {
	async fetch(request, env, ctx) {
		const body = request.body;
		if (!body) return new Response("No body", { status: 400 });

		// The span is active during the callback, so the pipeThrough
		// operation is correctly nested. The span stays open after
		// the callback returns, until flush() calls span.end().
		const stream = tracing.startActiveSpan("process-stream", (span) => {
			span.setAttribute(
				"request.content_type",
				request.headers.get("content-type") ?? "unknown",
			);

			return body.pipeThrough(
				new TransformStream({
					transform(chunk, controller) {
						// Process each chunk
						controller.enqueue(chunk);
					},
					flush() {
						span.setAttribute("stream.status", "complete");
						span.end();
					},
					cancel() {
						span.setAttribute("stream.status", "cancelled");
						span.end();
					},
				}),
			);
		});

		return new Response(stream);
	},
};

import { tracing } from "cloudflare:workers";

export default {
	async fetch(request: Request, env: Env, ctx: ExecutionContext) {
		const body = request.body;
		if (!body) return new Response("No body", { status: 400 });

		// The span is active during the callback, so the pipeThrough
		// operation is correctly nested. The span stays open after
		// the callback returns, until flush() calls span.end().
		const stream = tracing.startActiveSpan("process-stream", (span) => {
			span.setAttribute(
				"request.content_type",
				request.headers.get("content-type") ?? "unknown",
			);

			return body.pipeThrough(
				new TransformStream({
					transform(chunk, controller) {
						// Process each chunk
						controller.enqueue(chunk);
					},
					flush() {
						span.setAttribute("stream.status", "complete");
						span.end();
					},
					cancel() {
						span.setAttribute("stream.status", "cancelled");
						span.end();
					},
				}),
			);
		});

		return new Response(stream);
	},
};

You can also capture the span reference for later use without streams:

let capturedSpan;
const value = tracing.startActiveSpan("manual-operation", (span) => {
	capturedSpan = span;
	span.setAttribute("phase", "started");
	return computeResult();
});

// The span is still open here — you can set more attributes
capturedSpan.setAttribute("phase", "complete");
capturedSpan.end(); // Now the span is submitted

The Span object is passed into the enterSpan and startActiveSpan callbacks. It provides methods to annotate the span with metadata and control its lifecycle.

Sets an attribute on the span.

Attributes appear alongside the span in your traces and OpenTelemetry exports.

span.setAttribute("user.plan", "enterprise");
span.setAttribute("item.count", 42);
span.setAttribute("cache.hit", true);

A readonly boolean indicating whether this invocation is being traced. When the request is not sampled (based on your head_sampling_rate), isTraced is false and enterSpan still runs the callback but does not record any telemetry.

You can use this to skip expensive attribute computation when the request is not being traced:

tracing.enterSpan("process", (span) => {
	if (span.isTraced) {
		span.setAttribute(
			"request.body.preview",
			JSON.stringify(body).slice(0, 200),
		);
	}
	return processBody(body);
});

Ends the span and submits its attributes to the tracing system. This method is idempotent — calling it multiple times has no effect after the first call. After end() is called, span.isTraced returns false and any further setAttribute calls are silently ignored, including calls from in-flight async work that has not yet completed.

• For spans created with enterSpan, you do not need to call end() — the runtime calls it automatically. Calling end() yourself is safe but has no effect since the runtime has already ended the span.
• For spans created with startActiveSpan, you must call end() to submit the span.

let mySpan;
const result = tracing.startActiveSpan("manual-op", (span) => {
	mySpan = span;
	span.setAttribute("step", "processing");
	return doWork();
});

// Later, when the work is truly complete:
mySpan.end(); // Span is submitted
mySpan.end(); // No-op, safe to call again

Spans nest automatically based on the JavaScript async context. Any enterSpan call or platform operation (such as fetch and env.MY_KV.get()) that runs inside a callback becomes a child of the enclosing span.

import { tracing } from "cloudflare:workers";

async function handleOrder(env, orderId) {
	return tracing.enterSpan("handleOrder", async (span) => {
		span.setAttribute("order.id", orderId);

		// This KV read is automatically a child of "handleOrder"
		const order = await env.ORDERS_KV.get(orderId, "json");

		// This nested span is also a child of "handleOrder"
		const total = tracing.enterSpan("calculateTotal", (innerSpan) => {
			innerSpan.setAttribute("item.count", order.items.length);
			return order.items.reduce((sum, item) => sum + item.price, 0);
		});

		// This fetch is a child of "handleOrder"
		await fetch("https://api.example.com/notify", {
			method: "POST",
			body: JSON.stringify({ orderId, total }),
		});

		return new Response(JSON.stringify({ orderId, total }));
	});
}

import { tracing } from "cloudflare:workers";

async function handleOrder(env: Env, orderId: string) {
	return tracing.enterSpan("handleOrder", async (span) => {
		span.setAttribute("order.id", orderId);

		// This KV read is automatically a child of "handleOrder"
		const order = await env.ORDERS_KV.get(orderId, "json");

		// This nested span is also a child of "handleOrder"
		const total = tracing.enterSpan("calculateTotal", (innerSpan) => {
			innerSpan.setAttribute("item.count", order.items.length);
			return order.items.reduce(
				(sum: number, item: any) => sum + item.price,
				0,
			);
		});

		// This fetch is a child of "handleOrder"
		await fetch("https://api.example.com/notify", {
			method: "POST",
			body: JSON.stringify({ orderId, total }),
		});

		return new Response(JSON.stringify({ orderId, total }));
	});
}

console.log() and other console methods emit log events that are automatically attributed to the currently active span. This means log output from inside an enterSpan or startActiveSpan callback is associated with that span in your traces and OpenTelemetry exports.

tracing.enterSpan("processPayment", async (span) => {
	console.log("Starting payment processing"); // attributed to "processPayment"
	const result = await chargeCard(token, amount);
	console.log("Payment complete", result.id); // also attributed to "processPayment"
});

The full type declarations for the custom spans API:

declare module "cloudflare:workers" {
	namespace tracing {
		function enterSpan<T, A extends unknown[]>(
			name: string,
			callback: (span: Span, ...args: A) => T,
			...args: A
		): T;

		function startActiveSpan<T, A extends unknown[]>(
			name: string,
			callback: (span: Span, ...args: A) => T,
			...args: A
		): T;
	}

	class Span {
		readonly isTraced: boolean;
		setAttribute(
			key: string,
			value: string | number | boolean | undefined,
		): void;
		end(): void;
	}
}

The same API is available on the handler context as ctx.tracing, with the same types.

Both methods set the span as the active context parent only during the callback. After the callback returns, the span is no longer the active parent. With enterSpan, this distinction does not matter because the span is also ended. With startActiveSpan, the span remains open but is no longer the context parent — new spans created after the callback returns are not children of this span.

• No manual parent-child wiring. Parent-child relationships are determined by the JavaScript async context automatically.
• No setAttributes (bulk set) yet. Use individual setAttribute calls. Bulk setting is planned for a future release.
• No spanContext() (trace/span IDs) yet. Access to trace and span identifiers for manual propagation across boundaries is planned for a future release.
• No setOutcome yet. Setting span outcome status is planned for a future release.

For other tracing limitations, refer to the known limitations page.