diff --git a/.github/workflows/main.yaml b/.github/workflows/main.yaml
index 40079ca..926cc02 100644
--- a/.github/workflows/main.yaml
+++ b/.github/workflows/main.yaml
@@ -27,7 +27,7 @@ jobs:
     timeout-minutes: 10
     strategy:
       matrix:
-        node-version: [18.x, 20.x, 22.x, 24.x]
+        node-version: [18.x, 20.x, 22.x, 24.x, 26.x]
     steps:
     - uses: actions/checkout@v7
       with:
diff --git a/CHANGELOG.md b/CHANGELOG.md
index f07a15e..7981edb 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,21 @@
 # @digitalbazaar/http-client ChangeLog
 
+## 4.4.0 - 2026-07-xx
+
+### Changed
+- Update minor dependencies.
+  - `ky@1.14.3`.
+  - `undici@6.28.0`.
+  - All dev dependencies.
+
+### Fixed
+- Keep the platform `fetch` on the agent path when the platform undici and
+  the installed undici share a major version, handing the dispatcher to `ky`
+  to forward. Only fall back to the installed undici's own `fetch` when the
+  majors differ (e.g. node 26's platform undici 8 rejecting an installed
+  undici 6 dispatcher). Fixes the agent path on node 26 without regressing
+  `fetch` performance on compatible runtimes.
+
 ## 4.3.0 - 2026-01-15
 
 ### Changed
diff --git a/lib/agentCompatibility.js b/lib/agentCompatibility.js
index edf24c1..fb339cc 100644
--- a/lib/agentCompatibility.js
+++ b/lib/agentCompatibility.js
@@ -1,9 +1,21 @@
 /*!
  * Copyright (c) 2022 Digital Bazaar, Inc. All rights reserved.
  */
-import {Agent} from 'undici';
+import {Agent, fetch as undiciFetch, Request as UndiciRequest} from 'undici';
+import undiciPkg from 'undici/package.json' with {type: 'json'};
 import {versions} from 'node:process';
 
+// Background: node ships its own copy of undici in the platform but does
+// not expose it (there is no `node:undici`), so this package installs its
+// own. A dispatcher only works with the undici that created it -- the
+// handler contract changed across majors, so handing an installed v6
+// dispatcher to a platform v7 or v8 `fetch` fails with "invalid onError
+// method". Which major the platform provides varies by release line (node
+// 22 has 6, node 24 has 7, node 26 has 8), so no single installed version
+// matches every supported runtime -- with undici 6 installed, both node 24
+// and node 26 take the fallback path below. See
+// digitalbazaar/http-client#43.
+
 // as long as an agent has a reference to it, its associated dispatcher will
 // be kept in this cache for reuse
 const AGENT_CACHE = new WeakMap();
@@ -12,6 +24,22 @@ const AGENT_CACHE = new WeakMap();
 const [major, minor] = versions.node.split('.').map(v => parseInt(v, 10));
 const canConvert = (major > 18) || (major === 18 && minor >= 2);
 
+// true when the installed and platform undici majors match, meaning their
+// dispatchers are interchangeable. Both reads are guarded: a future undici
+// could hide `package.json` behind an `exports` map, and `versions.undici`
+// may be absent. Either way fall back to `false` and use the installed
+// undici's own fetch -- the always-safe path -- rather than throwing at
+// module load and breaking `import` for every consumer.
+const platformFetchCompatible = (() => {
+  try {
+    const installedMajor = parseInt(undiciPkg.version, 10);
+    const platformMajor = parseInt(versions.undici, 10);
+    return platformMajor === installedMajor;
+  } catch{
+    return false;
+  }
+})();
+
 // converts `agent`/`httpsAgent` option to a dispatcher option
 export function convertAgent(options) {
   if(!canConvert) {
@@ -29,24 +57,53 @@ export function convertAgent(options) {
     return options;
   }
 
-  // use custom fetch if agent has already been converted
-  let fetch = AGENT_CACHE.get(agent);
+  // reuse the dispatcher built for this agent
+  let dispatcher = AGENT_CACHE.get(agent);
+  if(!dispatcher) {
+    dispatcher = new Agent({connect: agent.options});
+    AGENT_CACHE.set(agent, dispatcher);
+  }
+
+  // drop the converted legacy options so they are not forwarded to `fetch`
+  const rest = {...options};
+  delete rest.agent;
+  delete rest.httpsAgent;
+
+  // majors match: hand the dispatcher to `ky`, which forwards it to the
+  // platform `fetch` (`ky` deliberately keeps `dispatcher` out of its
+  // request-option registry so it reaches fetch) -- no wrapper needed
+  if(platformFetchCompatible) {
+    return {...rest, dispatcher};
+  }
+
+  // incompatible platform `fetch` that rejects this dispatcher, so route
+  // through the installed undici's own fetch via an override
+  let fetch = AGENT_CACHE.get(dispatcher);
   if(!fetch) {
-    const dispatcher = new Agent({connect: agent.options});
     fetch = createFetch(dispatcher);
     fetch._httpClientCustomFetch = true;
-    AGENT_CACHE.set(agent, fetch);
+    AGENT_CACHE.set(dispatcher, fetch);
   }
-
-  return {...options, fetch};
+  return {...rest, fetch};
 }
 
-// create fetch override uses custom `dispatcher`; since `ky` does not pass
-// the dispatcher option through to `fetch`, we must use this override
-function createFetch(dispatcher) {
-  return function fetch(...args) {
-    dispatcher = (args[1] && args[1].dispatcher) || dispatcher;
-    args[1] = {...args[1], dispatcher};
-    return globalThis.fetch(...args);
+// create fetch override uses custom `dispatcher`; when incompatible, the
+// platform `Request` that `ky` creates cannot be consumed by the installed
+// undici's fetch directly, so it is rebuilt as the installed undici's own
+// `Request` here.
+// Passing the platform `Request` as undici's `Request` *init* (its second
+// constructor argument) works because undici's own `Request` constructor
+// performs its own `RequestInit` dictionary conversion -- it reads exactly
+// the fields its own implementation understands directly off the object it's
+// given, duck-typed rather than `instanceof`-checked, so it stays correct
+// automatically as undici's own supported fields evolve. No manual
+// allow/deny-list of `RequestInit` fields is needed or maintained here.
+function createFetch(defaultDispatcher) {
+  return function fetch(input, init) {
+    const dispatcher = init?.dispatcher || defaultDispatcher;
+    if(input && typeof input === 'object' && typeof input.url === 'string') {
+      input = new UndiciRequest(input.url, input);
+    }
+    return undiciFetch(input, {...init, dispatcher});
   };
 }
diff --git a/package.json b/package.json
index 8ea6b09..007cbd2 100644
--- a/package.json
+++ b/package.json
@@ -37,17 +37,17 @@
     "dist/*"
   ],
   "dependencies": {
-    "ky": "^1.14.2",
-    "undici": "^6.23.0"
+    "ky": "^1.14.3",
+    "undici": "^6.28.0"
   },
   "devDependencies": {
-    "@digitalbazaar/eslint-config": "^7.0.0",
+    "@digitalbazaar/eslint-config": "^7.0.1",
     "c8": "^10.1.3",
     "chai": "^4.5.0",
-    "cors": "^2.8.5",
+    "cors": "^2.8.6",
     "cross-env": "^10.1.0",
     "detect-node": "^2.1.0",
-    "eslint": "^9.39.2",
+    "eslint": "^9.39.5",
     "express": "^5.2.1",
     "karma": "^6.4.4",
     "karma-chai": "^0.1.0",
@@ -56,10 +56,10 @@
     "karma-mocha-reporter": "^2.2.5",
     "karma-sourcemap-loader": "^0.4.0",
     "karma-webpack": "^5.0.1",
-    "mocha": "^11.7.5",
-    "rimraf": "^6.1.2",
-    "rollup": "^4.55.1",
-    "webpack": "^5.104.1"
+    "mocha": "^11.7.6",
+    "rimraf": "^6.1.3",
+    "rollup": "^4.62.3",
+    "webpack": "^5.109.2"
   },
   "repository": {
     "type": "git",
diff --git a/tests/10-client-api.spec.common.cjs b/tests/10-client-api.spec.common.cjs
index 02b4ebc..090b0ed 100644
--- a/tests/10-client-api.spec.common.cjs
+++ b/tests/10-client-api.spec.common.cjs
@@ -91,6 +91,30 @@ describe('http-client API', () => {
       should.exist(response.data);
       response.status.should.equal(200);
     });
+
+    // exercises the agent path with a request body: on an incompatible
+    // runtime the body + headers must survive the Request -> (url, init)
+    // decomposition, on a compatible one it rides the native dispatcher path
+    it('can POST a body over an HTTPS agent', async () => {
+      let err;
+      let response;
+      const url = `https://${httpsHost}/echo`;
+      const payload = {hello: 'world', n: 42, nested: {ok: true}};
+      try {
+        const agent = utils.makeAgent({
+          rejectUnauthorized: false
+        });
+        response = await httpClient.post(url, {agent, json: payload});
+      } catch(e) {
+        err = e;
+      }
+      should.not.exist(err);
+      should.exist(response);
+      response.status.should.equal(200);
+      should.exist(response.data);
+      should.exist(response.data.echo);
+      response.data.echo.should.deep.equal(payload);
+    });
   }
 
   it('handles a get not found error', async () => {
diff --git a/tests/utils.cjs b/tests/utils.cjs
index c988f1e..334c633 100644
--- a/tests/utils.cjs
+++ b/tests/utils.cjs
@@ -111,5 +111,11 @@ function createApp() {
     });
   });
 
+  app.post('/echo', cors(), express.json(), (req, res) => {
+    res.json({
+      echo: req.body
+    });
+  });
+
   return app;
 }