Fix undici usage on node 26.

.github/workflows/main.yaml

@@ -27,7 +27,7 @@ jobs:
     timeout-minutes: 10
     strategy:
       matrix:
-        node-version: [22.x, 24.x]
+        node-version: [22.x, 24.x, 26.x]
     steps:
     - uses: actions/checkout@v7
       with:

lib/agentCompatibility.js

@@ -1,7 +1,7 @@
 /*!
  * Copyright (c) 2022 Digital Bazaar, Inc. All rights reserved.
  */
-import {Agent} from 'undici';
+import {Agent, fetch as undiciFetch} from 'undici';
 
 // as long as an agent has a reference to it, its associated dispatcher will
 // be kept in this cache for reuse
@@ -34,10 +34,39 @@ export function convertAgent(options) {
 
 // create fetch override uses custom `dispatcher`; since `ky` does not pass
 // the dispatcher option through to `fetch`, we must use this override
+//
+// this uses undici's own `fetch` rather than `globalThis.fetch`. a dispatcher
+// is only usable by the undici that created it: node bundles its own undici
+// (node 22: 6.x, node 24: 7.x, node 26: 8.x) and the handler interface changed
+// between majors, so handing an installed-undici dispatcher to node's built-in
+// `fetch` throws `UND_ERR_INVALID_ARG` ("invalid onError method" on node 26).
+// pairing the dispatcher with the `fetch` from the same module keeps the two
+// in sync on any node version, whichever undici is installed
 function createFetch(dispatcher) {
   return function fetch(...args) {
     dispatcher = (args[1] && args[1].dispatcher) || dispatcher;
     args[1] = {...args[1], dispatcher};
-    return globalThis.fetch(...args);
+    // `ky` builds a global `Request`, which undici's `fetch` does not accept
+    // as one of its own; unpack it into a url and init pair instead
+    if(args[0] instanceof globalThis.Request) {
+      args[1] = {..._requestToInit(args[0]), ...args[1]};
+      args[0] = args[0].url;
+    }
+    return undiciFetch(...args);
+  };
+}
+
+// converts a `Request` into an equivalent `fetch` init object
+function _requestToInit(request) {
+  const {body} = request;
+  return {
+    method: request.method,
+    // an entry list is understood by every `Headers` implementation
+    headers: [...request.headers],
+    body,
+    // undici requires `duplex` whenever a stream body is sent
+    duplex: body ? 'half' : undefined,
+    signal: request.signal,
+    redirect: request.redirect
   };
 }