@@ -47,7 +47,7 @@ started. Invalid options throw.
 | `serviceName`    | string | `unknown_service:node`      | Service name in exported resource attributes. The default follows the OpenTelemetry [semantic conventions](https://opentelemetry.io/docs/specs/semconv/general/trace/) for low-cardinality service names. |
 | `sdkName`        | string | `microtel`                   | `telemetry.sdk.name` resource attribute, and the name of the instrumentation scope on exported spans. |
 | `sdkVersion`    | string | this package's version      | `telemetry.sdk.version` resource attribute, and the scope version. |
-| `filter`         | string | all modules                  | Comma-separated list of modules to instrument. Supported values: `node:http`, `node:http/client`, `node:http/server`. |
+| `filter`         | string | all modules                  | Comma-separated list of modules to instrument. Supported values: `node:http`, `node:http/client`, `node:http/server`, and `node:undici`. `node:fetch` is an alias for `node:undici`: both cover all undici requests, including `fetch()`, and either one enables them. |
 | `exclude`        | string | none                         | Comma-separated list of modules to exclude from instrumentation. Same values as `filter`; takes precedence over it. |
 | `maxBufferSize`  | number | `100`                        | Number of spans buffered before an immediate flush is triggered. Must be a positive integer. |
 | `flushInterval`  | number | `10000`                      | Milliseconds between periodic flushes. Must be a positive integer. |
@@ -121,11 +121,14 @@ port that is not a decimal number is omitted.
 
 ### Undici/Fetch client
 
-Not implemented yet. The instrumentation subscribes to the
-`undici:request:*` diagnostics channels and produces one CLIENT span per
-request, with the same attributes as the HTTP client spans plus
-`server.port`; the plan is documented as a TODO in
-`lib/instrumentations.js`.
+A span with kind `CLIENT` is created for each outgoing request made via
+`fetch()` or undici's `request()`. Error status, `exception` event behavior,
+and span end timing are the same as for the HTTP client spans above. Requests
+that fail before they are dispatched, such as a refused connection, produce
+no span: the undici channels only fire once the request is underway. The
+injected `traceparent` replaces any caller-supplied value rather than
+appending to it, and a stale `tracestate` is removed when the span has none
+to inherit.
 
 Attributes set on undici/fetch client spans:
 
@@ -152,9 +155,8 @@ fresh trace is started instead. The `tracestate` header, when present, is
 attached to the span and forwarded verbatim to outgoing requests. It is never
 parsed or modified.
 * **Outgoing requests**: The `traceparent` header is injected into outgoing
-HTTP requests, together with the stored `tracestate` when one
-is present, enabling distributed tracing across services. Undici and
-fetch requests are covered once that instrumentation lands.
+HTTP and undici/fetch requests, together with the stored `tracestate` when
+one is present, enabling distributed tracing across services.
 
 ## Benchmarks
 
 
@@ -206,22 +206,73 @@ function onClientRequestError({ request, error }) {
   failClientSpan(span, error);
 }
 
-// TODO(bengl): undici/fetch instrumentation. Subscribing to the
-// undici:request:create, undici:request:headers, undici:request:trailers,
-// and undici:request:error channels yields one CLIENT span per fetch()
-// or request() call, with the same shape as the node:http client spans
-// above: http.request.method and url.full (origin + path) attributes,
-// server.address set to the origin, server.port derived from it, and the
-// span ended on trailers, which fire once the response body completes.
-// The headers channel applies the response status; the error channel
-// reuses failClientSpan. Trace context injection must replace rather than
-// append headers: undici's addHeader appends, so a caller-supplied
-// traceparent has to be updated in place or it arrives duplicated. The
-// filter values node:undici and node:fetch enable it, as an alias pair.
-// A working implementation, including its test, exists in
-// nodejs/node#66587 (lib/internal/otel/instrumentations.js and
-// test/parallel/test-otel-undici.js); the PR's diff stays viewable even
-// once it is closed.
+// Undici's addHeader appends rather than replaces; update an existing
+// header in place so a caller-supplied value is overridden, not
+// duplicated.
+function setUndiciHeader(request, name, value) {
+  const headers = request.headers;
+  for (let i = 0; i < headers.length; i += 2) {
+    if (headers[i] === name) {
+      headers[i + 1] = value;
+      return;
+    }
+  }
+  request.addHeader(name, value);
+}
+
+function removeUndiciHeader(request, name) {
+  const headers = request.headers;
+  for (let i = headers.length - 2; i >= 0; i -= 2) {
+    if (headers[i] === name) headers.splice(i, 2);
+  }
+}
+
+function onUndiciRequestCreate({ request }) {
+  const parent = spanStorage.getStore();
+
+  const method = request.method || 'GET';
+  const span = new Span(method, SPAN_KIND_CLIENT, { parent });
+
+  span.setAttribute('http.request.method', method);
+
+  const { origin } = request;
+  const path = request.path;
+  if (origin) {
+    span.setAttribute('server.address', origin);
+    // The origin omits the port when it is the scheme default.
+    const parsed = new URL(origin);
+    span.setAttribute(
+      'server.port',
+      Number(parsed.port) || (parsed.protocol === 'https:' ? 443 : 80),
+    );
+  }
+  span.setAttribute('url.full', `${origin}${path}`);
+
+  request[kSpan] = span;
+
+  if (request.addHeader) {
+    setUndiciHeader(request, 'traceparent', span.toTraceparent());
+    if (span.tracestate)
+      setUndiciHeader(request, 'tracestate', span.tracestate);
+    else removeUndiciHeader(request, 'tracestate');
+  }
+}
+
+function onUndiciRequestHeaders({ request, response }) {
+  const span = request[kSpan];
+  if (span == null) return;
+
+  applyClientResponse(span, response.statusCode);
+}
+
+// undici:request:trailers fires when the response completes, including its
+// body, so the span duration covers the full transfer.
+function onUndiciRequestTrailers({ request }) {
+  const span = request[kSpan];
+  if (span == null) return;
+
+  span.end();
+}
 
 function enableInstrumentations() {
   spanStorage = getSpanStorage();
@@ -236,6 +287,15 @@ function enableInstrumentations() {
     dc.subscribe('http.client.response.finish', onHttpClientResponseFinish);
     dc.subscribe('http.client.request.error', onClientRequestError);
   }
+
+  // node:fetch is an alias for node:undici: both enable the same
+  // channels, which cover every undici request, fetch() included.
+  if (isModuleEnabled('node:undici') || isModuleEnabled('node:fetch')) {
+    dc.subscribe('undici:request:create', onUndiciRequestCreate);
+    dc.subscribe('undici:request:headers', onUndiciRequestHeaders);
+    dc.subscribe('undici:request:trailers', onUndiciRequestTrailers);
+    dc.subscribe('undici:request:error', onClientRequestError);
+  }
 }
 
 module.exports = {
 
@@ -187,6 +187,30 @@ describe('otel init activation', () => {
     );
   });
 
+  it('filter accepts node:fetch as an alias for undici', async () => {
+    const { code, stdout } = await spawnMicrotel([
+      fixture('state-child.js'),
+      JSON.stringify([
+        {
+          endpoint: 'http://127.0.0.1:1',
+          filter: 'node:fetch',
+        },
+      ]),
+      'node:fetch,node:http',
+    ]);
+
+    assert.strictEqual(code, 0);
+    const lines = stdout.trim().split('\n');
+    assert.match(
+      lines.find((l) => l.startsWith('node:fetch:')),
+      /node:fetch:true/,
+    );
+    assert.match(
+      lines.find((l) => l.startsWith('node:http:')),
+      /node:http:false/,
+    );
+  });
+
   it('exclude disables modules without touching the rest', async () => {
     const { code, stdout } = await spawnMicrotel([
       fixture('state-child.js'),
 
@@ -0,0 +1,188 @@
+'use strict';
+
+const assert = require('node:assert');
+const { after, before, describe, it } = require('node:test');
+const {
+  startTracing,
+  startHTTPServer,
+  flushAndCollect,
+  getClientSpan,
+  rawRequest,
+  spanAttrs,
+  closeAll,
+} = require('./common/otel');
+
+describe('otel undici spans', () => {
+  let collector;
+
+  before(async () => {
+    collector = await startTracing({
+      filter: 'node:http,node:undici',
+    });
+  });
+
+  after(() => collector.close());
+
+  it(
+    'creates client spans for fetch requests and overrides existing ' +
+      'trace context headers',
+    async () => {
+      const receivedTraceparent = {};
+      const target = await startHTTPServer((req, res) => {
+        receivedTraceparent[req.url] = req.headers.traceparent;
+        res.writeHead(200);
+        res.end('ok');
+      });
+
+      try {
+        const res = await fetch(`http://127.0.0.1:${target.port}/fetch-test`);
+        await res.text();
+      } catch {}
+
+      // A caller-supplied traceparent must be overridden, not duplicated:
+      // undici's addHeader appends, so a duplicate would arrive as a single
+      // comma-separated value.
+      try {
+        const res = await fetch(
+          `http://127.0.0.1:${target.port}/override-test`,
+          {
+            headers: {
+              traceparent:
+                '00-00000000000000000000000000000000-' + '0000000000000000-00',
+            },
+          },
+        );
+        await res.text();
+      } catch {}
+
+      const spans = await flushAndCollect(collector);
+      await target.close();
+
+      assert.ok(spans.length > 0, 'Expected at least one span from fetch');
+      const clientSpan = getClientSpan(spans, '/fetch-test');
+      assert.ok(clientSpan, 'Expected a CLIENT span from fetch');
+
+      assert.partialDeepStrictEqual(spanAttrs(clientSpan), {
+        'http.request.method': 'GET',
+        'server.address': `http://127.0.0.1:${target.port}`,
+        'server.port': String(target.port),
+        'url.full': `http://127.0.0.1:${target.port}/fetch-test`,
+        'http.response.status_code': '200',
+      });
+
+      const overridden = receivedTraceparent['/override-test'];
+      assert.ok(overridden, 'Expected a traceparent on the override request');
+      assert.match(overridden, /^00-[0-9a-f]{32}-[0-9a-f]{16}-0[01]$/);
+      assert.ok(
+        !overridden.includes(','),
+        'traceparent must be overridden, not duplicated',
+      );
+    },
+  );
+
+  it('creates an error span for a failed fetch', async () => {
+    // A response that dies mid-transfer must end the span with the error
+    // status and an exception event, through the undici:request:error
+    // channel. A fetch that fails to connect never dispatches, so it
+    // produces no span at all.
+    const target = await startHTTPServer((req, res) => {
+      res.writeHead(200);
+      res.write('partial');
+      setTimeout(() => res.destroy(), 50);
+    });
+
+    await fetch(`http://127.0.0.1:${target.port}/destroy`)
+      .then((res) => res.text())
+      .catch(() => {});
+
+    const spans = await flushAndCollect(collector);
+    await target.close();
+
+    const errorSpan = getClientSpan(spans, '/destroy');
+    assert.ok(errorSpan, 'Expected an error client span for the failed fetch');
+
+    assert.ok(errorSpan.status, 'Error span should have status');
+    assert.strictEqual(errorSpan.status.code, 2); // STATUS_ERROR
+    assert.ok(
+      errorSpan.events?.some((e) => e.name === 'exception'),
+      'Should have an exception event',
+    );
+  });
+
+  it('overrides a stale tracestate with the inherited one', async () => {
+    const incomingTraceId = 'a'.repeat(32);
+    const incomingTracestate = 'vendor=inherited';
+
+    let seen;
+    // The fetch target records the trace context headers it receives.
+    const fetchTarget = await startHTTPServer((req, res) => {
+      seen = {
+        traceparent: req.headers.traceparent,
+        tracestate: req.headers.tracestate,
+      };
+      res.writeHead(200);
+      res.end('ok');
+    });
+
+    // A fetch made while handling a traced request inherits the incoming
+    // tracestate and must override the caller-supplied stale one.
+    const frontend = await startHTTPServer((req, res) => {
+      fetch(`http://127.0.0.1:${fetchTarget.port}/down`, {
+        headers: { tracestate: 'stale=caller-value' },
+      }).then((fr) => {
+        fr.text().then(() => {
+          res.writeHead(200);
+          res.end('ok');
+        });
+      });
+    });
+
+    await rawRequest(frontend.port, '/up', {
+      traceparent: `00-${incomingTraceId}-0123456789abcdef-01`,
+      tracestate: incomingTracestate,
+    });
+
+    await flushAndCollect(collector);
+    await closeAll(fetchTarget, frontend);
+
+    assert.ok(seen?.traceparent, 'Expected a traceparent on the fetch');
+    assert.ok(
+      seen.traceparent.includes(incomingTraceId),
+      'The fetch must carry the incoming trace ID',
+    );
+    assert.strictEqual(
+      seen.tracestate,
+      incomingTracestate,
+      'The inherited tracestate must win over the stale one',
+    );
+  });
+
+  it('removes a stale tracestate when the span has none', async () => {
+    let receivedTracestate = 'sentinel';
+    const target = await startHTTPServer((req, res) => {
+      receivedTracestate = req.headers.tracestate;
+      res.writeHead(200);
+      res.end('ok');
+    });
+
+    try {
+      const res = await fetch(`http://127.0.0.1:${target.port}/stale`, {
+        headers: { tracestate: 'vendor=state-for-an-old-trace' },
+      });
+      await res.text();
+    } catch {}
+
+    const spans = await flushAndCollect(collector);
+    await target.close();
+
+    assert.ok(getClientSpan(spans, '/stale'), 'Expected a client span');
+
+    // A stale tracestate from an unrelated trace must not travel with
+    // the fresh trace context, even though the span has none to inject.
+    assert.strictEqual(
+      receivedTracestate,
+      undefined,
+      'stale tracestate must not be forwarded',
+    );
+  });
+});