diff --git a/.changeset/proxy-fetch-undici-version-mismatch.md b/.changeset/proxy-fetch-undici-version-mismatch.md
new file mode 100644
index 00000000..df5719ff
--- /dev/null
+++ b/.changeset/proxy-fetch-undici-version-mismatch.md
@@ -0,0 +1,5 @@
+---
+"@slack/slack-github-action": patch
+---
+
+fix: proxy input always fails due to an undici version mismatch
diff --git a/src/proxies.js b/src/proxies.js
index d3b64e0e..5296dbbd 100644
--- a/src/proxies.js
+++ b/src/proxies.js
@@ -1,19 +1,22 @@
-import { ProxyAgent } from "undici";
+import { ProxyAgent, fetch as undiciFetch } from "undici";
 import SlackError from "./errors.js";
 
 /**
  * Return a fetch function that routes requests through a configured proxy.
  *
+ * Uses undici's own fetch, not the global fetch, since Node may bundle a
+ * different (incompatible) undici version for the latter.
+ *
  * @param {import("./config.js").default} config
  * @param {string?} [destination] - A provided request destination.
- * @returns {typeof globalThis.fetch | undefined}
+ * @returns {typeof undiciFetch | undefined}
  */
 export function fetch(config, destination) {
   const dispatcher = proxies(config, destination);
   if (!dispatcher) {
     return undefined;
   }
-  return (url, init) => globalThis.fetch(url, { ...init, dispatcher });
+  return (url, init) => undiciFetch(url, { ...init, dispatcher });
 }
 
 /**
diff --git a/test/proxies.spec.js b/test/proxies.spec.js
index f4ae9227..4d5c84bd 100644
--- a/test/proxies.spec.js
+++ b/test/proxies.spec.js
@@ -1,5 +1,6 @@
 import assert from "node:assert";
 import { beforeEach, describe, it } from "node:test";
+import { errors as undiciErrors } from "undici";
 import Config from "../src/config.js";
 import SlackError from "../src/errors.js";
 import { fetch, proxies } from "../src/proxies.js";
@@ -20,6 +21,24 @@ describe("proxies", () => {
       assert.strictEqual(typeof fetchFn, "function");
     });
 
+    it("dispatches requests through the proxy without a request handler mismatch", async () => {
+      mocks.core.getInput.withArgs("method").returns("chat.postMessage");
+      mocks.core.getInput.withArgs("token").returns("xoxb-example");
+      mocks.core.getInput.withArgs("proxy").returns("http://127.0.0.1:1");
+      const config = new Config(mocks.core);
+      const fetchFn = fetch(config);
+      await assert.rejects(
+        // ".invalid" is reserved by RFC 2606 to never resolve.
+        fetchFn("https://slack-github-action.invalid/api/chat.postMessage", {
+          method: "POST",
+        }),
+        (err) => {
+          assert.ok(!(err.cause instanceof undiciErrors.InvalidArgumentError));
+          return true;
+        },
+      );
+    });
+
     it("returns undefined when no proxy is configured", async () => {
       mocks.core.getInput.withArgs("method").returns("chat.postMessage");
       mocks.core.getInput.withArgs("token").returns("xoxb-example");